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PERF_EVENT_OPEN(2)         Linux Programmer's Manual        PERF_EVENT_OPEN(2)

       perf_event_open - set up performance monitoring

       #include <linux/perf_event.h>
       #include <linux/hw_breakpoint.h>

       int perf_event_open(struct perf_event_attr *attr,
                           pid_t pid, int cpu, int group_fd,
                           unsigned long flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

       Given  a  list of parameters, perf_event_open() returns a file descrip-
       tor, for use in subsequent system calls  (read(2),  mmap(2),  prctl(2),
       fcntl(2), etc.).

       A  call to perf_event_open() creates a file descriptor that allows mea-
       suring performance information.  Each file  descriptor  corresponds  to
       one  event  that  is measured; these can be grouped together to measure
       multiple events simultaneously.

       Events can be enabled and disabled in two ways: via  ioctl(2)  and  via
       prctl(2).   When  an  event  is  disabled it does not count or generate
       overflows but does continue to exist and maintain its count value.

       Events come in two flavors: counting and sampled.  A counting event  is
       one  that  is  used  for  counting  the aggregate number of events that
       occur.  In general, counting event results are gathered with a  read(2)
       call.   A  sampling  event periodically writes measurements to a buffer
       that can then be accessed via mmap(2).

       The pid and cpu arguments allow specifying which  process  and  CPU  to

       pid == 0 and cpu == -1
              This measures the calling process/thread on any CPU.

       pid == 0 and cpu >= 0
              This  measures  the  calling process/thread only when running on
              the specified CPU.

       pid > 0 and cpu == -1
              This measures the specified process/thread on any CPU.

       pid > 0 and cpu >= 0
              This measures the specified process/thread only when running  on
              the specified CPU.

       pid == -1 and cpu >= 0
              This  measures all processes/threads on the specified CPU.  This
              requires   CAP_SYS_ADMIN   capability   or   a    /proc/sys/ker-
              nel/perf_event_paranoid value of less than 1.

       pid == -1 and cpu == -1
              This setting is invalid and will return an error.

       The  group_fd  argument  allows  event  groups to be created.  An event
       group has one event which is the group leader.  The leader  is  created
       first,  with  group_fd = -1.  The rest of the group members are created
       with subsequent perf_event_open() calls with group_fd being set to  the
       file  descriptor  of  the  group leader.  (A single event on its own is
       created with group_fd = -1 and is considered to be a group with only  1
       member.)   An  event group is scheduled onto the CPU as a unit: it will
       be put onto the CPU only if all of the events in the group can  be  put
       onto  the  CPU.  This means that the values of the member events can be
       meaningfully compared--added, divided (to get ratios), and so  on--with
       each other, since they have counted events for the same set of executed

       The flags argument is formed by ORing together zero or more of the fol-
       lowing values:

       PERF_FLAG_FD_CLOEXEC (since Linux 3.14).
              This  flag  enables the close-on-exec flag for the created event
              file descriptor, so that the file  descriptor  is  automatically
              closed  on  execve(2).   Setting the close-on-exec flags at cre-
              ation time, rather than later with  fcntl(2),  avoids  potential
              race    conditions    where    the    calling   thread   invokes
              perf_event_open() and fcntl(2)  at  the  same  time  as  another
              thread calls fork(2) then execve(2).

              This flag allows creating an event as part of an event group but
              having no group leader.  It is unclear why this is useful.

              This flag reroutes the output from an event to the group leader.

       PERF_FLAG_PID_CGROUP (since Linux 2.6.39).
              This flag activates  per-container  system-wide  monitoring.   A
              container is an abstraction that isolates a set of resources for
              finer-grained control (CPUs, memory, etc.).  In this  mode,  the
              event  is  measured  only if the thread running on the monitored
              CPU belongs to the designated container (cgroup).  The cgroup is
              identified  by passing a file descriptor opened on its directory
              in the cgroupfs filesystem.  For instance, if the cgroup to mon-
              itor   is   called  test,  then  a  file  descriptor  opened  on
              /dev/cgroup/test (assuming cgroupfs is mounted  on  /dev/cgroup)
              must  be  passed  as  the  pid  parameter.  cgroup monitoring is
              available only for system-wide events and may therefore  require
              extra permissions.

       The  perf_event_attr structure provides detailed configuration informa-
       tion for the event being created.

           struct perf_event_attr {
               __u32 type;         /* Type of event */
               __u32 size;         /* Size of attribute structure */
               __u64 config;       /* Type-specific configuration */

               union {
                   __u64 sample_period;    /* Period of sampling */
                   __u64 sample_freq;      /* Frequency of sampling */

               __u64 sample_type;  /* Specifies values included in sample */
               __u64 read_format;  /* Specifies values returned in read */

               __u64 disabled       : 1,   /* off by default */
                     inherit        : 1,   /* children inherit it */
                     pinned         : 1,   /* must always be on PMU */
                     exclusive      : 1,   /* only group on PMU */
                     exclude_user   : 1,   /* don't count user */
                     exclude_kernel : 1,   /* don't count kernel */
                     exclude_hv     : 1,   /* don't count hypervisor */
                     exclude_idle   : 1,   /* don't count when idle */
                     mmap           : 1,   /* include mmap data */
                     comm           : 1,   /* include comm data */
                     freq           : 1,   /* use freq, not period */
                     inherit_stat   : 1,   /* per task counts */
                     enable_on_exec : 1,   /* next exec enables */
                     task           : 1,   /* trace fork/exit */
                     watermark      : 1,   /* wakeup_watermark */
                     precise_ip     : 2,   /* skid constraint */
                     mmap_data      : 1,   /* non-exec mmap data */
                     sample_id_all  : 1,   /* sample_type all events */
                     exclude_host   : 1,   /* don't count in host */
                     exclude_guest  : 1,   /* don't count in guest */
                     exclude_callchain_kernel : 1,
                                           /* exclude kernel callchains */
                     exclude_callchain_user   : 1,
                                           /* exclude user callchains */
                     mmap2          :  1,  /* include mmap with inode data */
                     comm_exec      :  1,  /* flag comm events that are due to exec */
                     __reserved_1   : 39;

               union {
                   __u32 wakeup_events;    /* wakeup every n events */
                   __u32 wakeup_watermark; /* bytes before wakeup */

               __u32     bp_type;          /* breakpoint type */

               union {
                   __u64 bp_addr;          /* breakpoint address */
                   __u64 config1;          /* extension of config */

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 config2;          /* extension of config1 */
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __u32 __reserved_2;         /* Align to u64 */


       The fields of the  perf_event_attr  structure  are  described  in  more
       detail below:

       type   This  field specifies the overall event type.  It has one of the
              following values:

                     This indicates one of the "generalized"  hardware  events
                     provided  by the kernel.  See the config field definition
                     for more details.

                     This indicates one of the  software-defined  events  pro-
                     vided  by  the  kernel  (even  if  no hardware support is

                     This indicates a tracepoint provided by the kernel trace-
                     point infrastructure.

                     This  indicates  a hardware cache event.  This has a spe-
                     cial encoding, described in the config field definition.

                     This indicates a "raw" implementation-specific  event  in
                     the config field.

              PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
                     This  indicates  a hardware breakpoint as provided by the
                     CPU.   Breakpoints  can  be  read/write  accesses  to  an
                     address as well as execution of an instruction address.

              dynamic PMU
                     Since  Linux 2.6.39, perf_event_open() can support multi-
                     ple PMUs.  To enable this, a value exported by the kernel
                     can  be  used  in the type field to indicate which PMU to
                     use.  The value to use can be found in the sysfs filesys-
                     tem:  there  is  a  subdirectory  per  PMU instance under
                     /sys/bus/event_source/devices.   In   each   subdirectory
                     there is a type file whose content is an integer that can
                     be   used   in   the   type   field.     For    instance,
                     /sys/bus/event_source/devices/cpu/type contains the value
                     for the core CPU PMU, which is usually 4.

       size   The size of the perf_event_attr structure  for  forward/backward
              compatibility.  Set this using sizeof(struct perf_event_attr) to
              allow the kernel to see the struct size at the time of  compila-

              The  related  define  PERF_ATTR_SIZE_VER0 is set to 64; this was
              the size of the first published struct.  PERF_ATTR_SIZE_VER1  is
              72,  corresponding  to  the  addition  of  breakpoints  in Linux
              2.6.33.  PERF_ATTR_SIZE_VER2 is 80 corresponding to the addition
              of  branch sampling in Linux 3.4.  PERF_ATR_SIZE_VER3 is 96 cor-
              responding  to  the  addition  of  sample_regs_user   and   sam-
              ple_stack_user in Linux 3.7.

       config This  specifies  which  event  you want, in conjunction with the
              type field.  The config1 and config2 fields are also taken  into
              account  in  cases  where 64 bits is not enough to fully specify
              the event.  The encoding of these fields are event dependent.

              The most significant bit (bit 63) of config  signifies  CPU-spe-
              cific  (raw) counter configuration data; if the most significant
              bit is unset, the next 7 bits are an event type and the rest  of
              the bits are the event identifier.

              There  are  various ways to set the config field that are depen-
              dent on the value of the previously described type field.   What
              follows  are  various possible settings for config separated out
              by type.

              If type is PERF_TYPE_HARDWARE, we are measuring one of the  gen-
              eralized hardware CPU events.  Not all of these are available on
              all platforms.  Set config to one of the following:

                          Total cycles.  Be wary of what  happens  during  CPU
                          frequency scaling.

                          Retired  instructions.   Be  careful,  these  can be
                          affected by various issues,  most  notably  hardware
                          interrupt counts.

                          Cache  accesses.   Usually this indicates Last Level
                          Cache accesses but this may vary depending  on  your
                          CPU.  This may include prefetches and coherency mes-
                          sages; again this depends on the design of your CPU.

                          Cache misses.  Usually  this  indicates  Last  Level
                          Cache  misses;  this  is intended to be used in con-
                          junction  with  the   PERF_COUNT_HW_CACHE_REFERENCES
                          event to calculate cache miss rates.

                          Retired branch instructions.  Prior to Linux 2.6.34,
                          this used the wrong event on AMD processors.

                          Mispredicted branch instructions.

                          Bus  cycles,  which  can  be  different  from  total

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
                          Stalled cycles during retirement.

                   PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
                          Total cycles; not affected by CPU frequency scaling.

              If  type is PERF_TYPE_SOFTWARE, we are measuring software events
              provided by the kernel.  Set config to one of the following:

                          This reports the CPU clock, a  high-resolution  per-
                          CPU timer.

                          This reports a clock count specific to the task that
                          is running.

                          This reports the number of page faults.

                          This counts context switches.  Until  Linux  2.6.34,
                          these  were all reported as user-space events, after
                          that they are reported as happening in the kernel.

                          This reports the number of  times  the  process  has
                          migrated to a new CPU.

                          This  counts the number of minor page faults.  These
                          did not require disk I/O to handle.

                          This counts the number of major page faults.   These
                          required disk I/O to handle.

                   PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
                          This  counts  the number of alignment faults.  These
                          happen when unaligned memory  accesses  happen;  the
                          kernel  can handle these but it reduces performance.
                          This happens only on some  architectures  (never  on

                   PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
                          This  counts  the  number  of emulation faults.  The
                          kernel sometimes traps on unimplemented instructions
                          and  emulates  them  for user space.  This can nega-
                          tively impact performance.

                   PERF_COUNT_SW_DUMMY (since Linux 3.12)
                          This is a placeholder  event  that  counts  nothing.
                          Informational  sample  record  types such as mmap or
                          comm must be associated with an active event.   This
                          dummy  event  allows  gathering such records without
                          requiring a counting event.

              If type is PERF_TYPE_TRACEPOINT, then we  are  measuring  kernel
              tracepoints.   The  value  to use in config can be obtained from
              under debugfs tracing/events/*/*/id if ftrace is enabled in  the

              If  type is PERF_TYPE_HW_CACHE, then we are measuring a hardware
              CPU cache event.  To calculate the appropriate config value  use
              the following equation:

                      (perf_hw_cache_id) | (perf_hw_cache_op_id << 8) |
                      (perf_hw_cache_op_result_id << 16)

                  where perf_hw_cache_id is one of:

                             for measuring Level 1 Data Cache

                             for measuring Level 1 Instruction Cache

                             for measuring Last-Level Cache

                             for measuring the Data TLB

                             for measuring the Instruction TLB

                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.0)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of

                             for read accesses

                             for write accesses

                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of

                             to measure accesses

                             to measure misses

              If  type  is  PERF_TYPE_RAW, then a custom "raw" config value is
              needed.  Most CPUs support events that are not  covered  by  the
              "generalized"  events.   These  are  implementation defined; see
              your CPU manual (for example the Intel Volume  3B  documentation
              or  the  AMD  BIOS  and  Kernel  Developer  Guide).  The libpfm4
              library can be used to translate from the name in the  architec-
              tural  manuals to the raw hex value perf_event_open() expects in
              this field.

              If type is PERF_TYPE_BREAKPOINT, then leave config set to  zero.
              Its parameters are set in other places.

       sample_period, sample_freq
              A  "sampling" counter is one that generates an interrupt every N
              events, where N is given by sample_period.  A  sampling  counter
              has  sample_period  >  0.   When  an  overflow interrupt occurs,
              requested data is recorded in the mmap buffer.  The  sample_type
              field controls what data is recorded on each interrupt.

              sample_freq can be used if you wish to use frequency rather than
              period.  In this case, you set the freq flag.  The  kernel  will
              adjust  the sampling period to try and achieve the desired rate.
              The rate of adjustment is a timer tick.

              The various bits in this field specify which values  to  include
              in the sample.  They will be recorded in a ring-buffer, which is
              available to user space using mmap(2).  The order in  which  the
              values are saved in the sample are documented in the MMAP Layout
              subsection below; it is not  the  enum  perf_event_sample_format

                     Records instruction pointer.

                     Records the process and thread IDs.

                     Records a timestamp.

                     Records an address, if applicable.

                     Record counter values for all events in a group, not just
                     the group leader.

                     Records the callchain (stack backtrace).

                     Records a unique ID for the opened event's group leader.

                     Records CPU number.

                     Records the current sampling period.

                     Records  a  unique  ID  for  the  opened  event.   Unlike
                     PERF_SAMPLE_ID  the  actual ID is returned, not the group
                     leader.  This ID is the  same  as  the  one  returned  by

                     Records additional data, if applicable.  Usually returned
                     by tracepoint events.

              PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
                     This provides a record of recent branches, as provided by
                     CPU  branch  sampling hardware (such as Intel Last Branch
                     Record).  Not all hardware supports this feature.

                     See the branch_sample_type field for how to filter  which
                     branches are reported.

              PERF_SAMPLE_REGS_USER (since Linux 3.7)
                     Records  the  current  user-level CPU register state (the
                     values in the process before the kernel was called).

              PERF_SAMPLE_STACK_USER (since Linux 3.7)
                     Records the user level stack, allowing stack unwinding.

              PERF_SAMPLE_WEIGHT (since Linux 3.10)
                     Records a hardware provided weight value  that  expresses
                     how  costly the sampled event was.  This allows the hard-
                     ware to highlight expensive events in a profile.

              PERF_SAMPLE_DATA_SRC (since Linux 3.10)
                     Records the data source: where in  the  memory  hierarchy
                     the  data  associated  with  the sampled instruction came
                     from.  This is only available if the underlying  hardware
                     supports this feature.

              PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
                     Places  the  SAMPLE_ID  value  in a fixed position in the
                     record, either at the beginning (for sample events) or at
                     the end (if a non-sample event).

                     This  was  necessary  because  a  sample  stream may have
                     records from various different event sources with differ-
                     ent sample_type settings.  Parsing the event stream prop-
                     erly was not possible because the format  of  the  record
                     was needed to find SAMPLE_ID, but the format could not be
                     found without knowing what event the sample  belonged  to
                     (causing a circular dependency).

                     This  new  PERF_SAMPLE_IDENTIFIER setting makes the event
                     stream always parsable by putting SAMPLE_ID  in  a  fixed
                     location, even though it means having duplicate SAMPLE_ID
                     values in records.

              PERF_SAMPLE_TRANSACTION (Since Linux 3.13)
                     Records reasons for  transactional  memory  abort  events
                     (for  example,  from  Intel TSX transactional memory sup-

                     The precise_ip setting must  be  greater  than  0  and  a
                     transactional  memory  abort event must be measured or no
                     values will be recorded.  Also note that some  perf_event
                     measurements,  such  as sampled cycle counting, may cause
                     extraneous aborts  (by  causing  an  interrupt  during  a

              This  field specifies the format of the data returned by read(2)
              on a perf_event_open() file descriptor.

                     Adds the 64-bit time_enabled field.  This can be used  to
                     calculate  estimated  totals  if the PMU is overcommitted
                     and multiplexing is happening.

                     Adds the 64-bit time_running field.  This can be used  to
                     calculate  estimated  totals  if the PMU is overcommitted
                     and multiplexing is happening.

                     Adds a 64-bit unique value that corresponds to the  event

                     Allows  all  counter  values in an event group to be read
                     with one read.

              The disabled bit specifies whether the counter starts  out  dis-
              abled  or  enabled.  If disabled, the event can later be enabled
              by ioctl(2), prctl(2), or enable_on_exec.

              When creating an event group, typically the group leader is ini-
              tialized  with  disabled  set to 1 and any child events are ini-
              tialized with disabled set to 0.  Despite disabled being 0,  the
              child events will not start until the group leader is enabled.

              The  inherit bit specifies that this counter should count events
              of child tasks as well as the task specified.  This applies only
              to  new  children,  not to any existing children at the time the
              counter is created (nor to any new children  of  existing  chil-

              Inherit  does  not  work  for some combinations of read_formats,
              such as PERF_FORMAT_GROUP.

       pinned The pinned bit specifies that the counter should  always  be  on
              the  CPU  if at all possible.  It applies only to hardware coun-
              ters and only to group leaders.  If a pinned counter  cannot  be
              put  onto  the  CPU (e.g., because there are not enough hardware
              counters or because of a conflict with some other  event),  then
              the  counter goes into an 'error' state, where reads return end-
              of-file (i.e., read(2) returns 0) until the  counter  is  subse-
              quently enabled or disabled.

              The exclusive bit specifies that when this counter's group is on
              the CPU, it should be the only group using the  CPU's  counters.
              In  the future this may allow monitoring programs to support PMU
              features that need to run alone so  that  they  do  not  disrupt
              other hardware counters.

              Note that many unexpected situations may prevent events with the
              exclusive bit set from ever running.  This  includes  any  users
              running  a  system-wide measurement as well as any kernel use of
              the performance counters (including  the  commonly  enabled  NMI
              Watchdog Timer interface).

              If  this  bit  is  set, the count excludes events that happen in
              user space.

              If this bit is set, the count excludes  events  that  happen  in

              If this bit is set, the count excludes events that happen in the
              hypervisor.  This is mainly for PMUs that have built-in  support
              for  handling this (such as POWER).  Extra support is needed for
              handling hypervisor measurements on most machines.

              If set, don't count when the CPU is idle.

       mmap   The mmap bit enables generation of PERF_RECORD_MMAP samples  for
              every mmap(2) call that has PROT_EXEC set.  This allows tools to
              notice new executable code being mapped into a program  (dynamic
              shared  libraries  for  example) so that addresses can be mapped
              back to the original code.

       comm   The comm bit enables tracking of process command name  as  modi-
              fied  by the exec(2) and prctl(PR_SET_NAME) system calls as well
              as writing to /proc/self/comm.  If the comm_exec  flag  is  also
              successfully set (possible since Linux 3.16), then the misc flag
              PERF_RECORD_MISC_COMM_EXEC can  be  used  to  differentiate  the
              exec(2) case from the others.

       freq   If  this  bit is set, then sample_frequency not sample_period is
              used when setting up the sampling interval.

              This bit enables saving of event counts on  context  switch  for
              inherited  tasks.   This is meaningful only if the inherit field
              is set.

              If this bit is set, a counter is automatically enabled  after  a
              call to exec(2).

       task   If this bit is set, then fork/exit notifications are included in
              the ring buffer.

              If set, have a sampling  interrupt  happen  when  we  cross  the
              wakeup_watermark  boundary.   Otherwise, interrupts happen after
              wakeup_events samples.

       precise_ip (since Linux 2.6.35)
              This controls the amount of skid.  Skid is how many instructions
              execute  between  an  event of interest happening and the kernel
              being able to stop and record the event.  Smaller skid is better
              and allows more accurate reporting of which events correspond to
              which instructions, but hardware is often limited with how small
              this can be.

              The values of this are the following:

              0 -    SAMPLE_IP can have arbitrary skid.

              1 -    SAMPLE_IP must have constant skid.

              2 -    SAMPLE_IP requested to have 0 skid.

              3 -    SAMPLE_IP     must     have    0    skid.     See    also

       mmap_data (since Linux 2.6.36)
              The counterpart of the mmap field.  This enables  generation  of
              PERF_RECORD_MMAP  samples  for  mmap(2)  calls  that do not have
              PROT_EXEC set (for example data and SysV shared memory).

       sample_id_all (since Linux 2.6.38)
              If set, then TID, TIME, ID, STREAM_ID, and CPU can  additionally
              be included in non-PERF_RECORD_SAMPLEs if the corresponding sam-
              ple_type is selected.

              If PERF_SAMPLE_IDENTIFIER is specified, then  an  additional  ID
              value  is  included as the last value to ease parsing the record
              stream.  This may lead to the id value appearing twice.

              The layout is described by this pseudo-structure:
                  struct sample_id {
                      { u32 pid, tid; } /* if PERF_SAMPLE_TID set        */
                      { u64 time;     } /* if PERF_SAMPLE_TIME set       */
                      { u64 id;       } /* if PERF_SAMPLE_ID set         */
                      { u64 stream_id;} /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; } /* if PERF_SAMPLE_CPU set        */
                      { u64 id;       } /* if PERF_SAMPLE_IDENTIFIER set */

       exclude_host (since Linux 3.2)
              Do not measure time spent in VM host.

       exclude_guest (since Linux 3.2)
              Do not measure time spent in VM guest.

       exclude_callchain_kernel (since Linux 3.7)
              Do not include kernel callchains.

       exclude_callchain_user (since Linux 3.7)
              Do not include user callchains.

       mmap2 (since Linux 3.16)
              Generate an extended executable mmap record that contains enough
              additional  information  to  uniquely  identify shared mappings.
              The mmap flag must also be set for this to work.

       comm_exec (since Linux 3.16)
              This is purely a feature-detection flag, it does not change ker-
              nel  behavior.  If this flag can successfully be set, then, when
              comm is enabled, the PERF_RECORD_MISC_COMM_EXEC flag will be set
              in  the  misc  field of a comm record header if the rename event
              being reported was caused by a call  to  exec(2).   This  allows
              tools to distinguish between the various types of process renam-

       wakeup_events, wakeup_watermark
              This union  sets  how  many  samples  (wakeup_events)  or  bytes
              (wakeup_watermark)  happen  before  an  overflow signal happens.
              Which one is used is selected by the watermark bit flag.

              wakeup_events only counts PERF_RECORD_SAMPLE record  types.   To
              receive  a  signal  for  every  incoming  PERF_RECORD  type  set
              wakeup_watermark to 1.

       bp_type (since Linux 2.6.33)
              This chooses the breakpoint type.  It is one of:

                     No breakpoint.

                     Count when we read the memory location.

                     Count when we write the memory location.

                     Count when we read or write the memory location.

                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the combination
              of  HW_BREAKPOINT_R  or  HW_BREAKPOINT_W with HW_BREAKPOINT_X is
              not allowed.

       bp_addr (since Linux 2.6.33)
              bp_addr address of the breakpoint.   For  execution  breakpoints
              this  is  the memory address of the instruction of interest; for
              read and write breakpoints it is the memory address of the  mem-
              ory location of interest.

       config1 (since Linux 2.6.39)
              config1  is  used for setting events that need an extra register
              or otherwise do not fit in the regular config field.   Raw  OFF-
              CORE_EVENTS  on  Nehalem/Westmere/SandyBridge  use this field on
              3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len is the length of the breakpoint being measured if type is
              PERF_TYPE_BREAKPOINT.     Options    are    HW_BREAKPOINT_LEN_1,
              For an execution breakpoint, set this to sizeof(long).

       config2 (since Linux 2.6.39)

              config2 is a further extension of the config1 field.

       branch_sample_type (since Linux 3.4)
              If PERF_SAMPLE_BRANCH_STACK is enabled, then this specifies what
              branches to include in the branch record.

              The first part of the value is the privilege level, which  is  a
              combination  of  one  of the following values.  If the user does
              not set privilege level explicitly,  the  kernel  will  use  the
              event's  privilege  level.  Event and branch privilege levels do
              not have to match.

                     Branch target is in user space.

                     Branch target is in kernel space.

                     Branch target is in hypervisor.

                     A convenience value that is the  three  preceding  values
                     ORed together.

              In  addition to the privilege value, at least one or more of the
              following bits must be set.

                     Any branch type.

                     Any call branch.

                     Any return branch.

                     Indirect calls.

              PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
                     Conditional branches.

              PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
                     Transactional memory aborts.

              PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
                     Branch in transactional memory transaction.

              PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
                     Branch not in transactional memory transaction.

       sample_regs_user (since Linux 3.7)
              This bit mask defines the set of user CPU registers to  dump  on
              samples.   The  layout of the register mask is architecture-spe-
              cific     and     described     in     the     kernel     header

       sample_stack_user (since Linux 3.7)
              This  defines  the  size  of the user stack to dump if PERF_SAM-
              PLE_STACK_USER is specified.

   Reading results
       Once a perf_event_open() file descriptor has been opened, the values of
       the  events  can be read from the file descriptor.  The values that are
       there are specified by the read_format field in the attr  structure  at
       open time.

       If you attempt to read into a buffer that is not big enough to hold the
       data ENOSPC is returned

       Here is the layout of the data returned by a read:

       * If PERF_FORMAT_GROUP was specified to allow reading all events  in  a
         group at once:

             struct read_format {
                 u64 nr;            /* The number of events */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                     u64 value;     /* The value of the event */
                     u64 id;        /* if PERF_FORMAT_ID */
                 } values[nr];

       * If PERF_FORMAT_GROUP was not specified:

             struct read_format {
                 u64 value;         /* The value of the event */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 u64 id;            /* if PERF_FORMAT_ID */

       The values read are as follows:

       nr     The number of events in this file descriptor.  Only available if
              PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total time the event was enabled and  running.   Normally  these
              are  the  same.   If  more  events  are  started, then available
              counter slots on the PMU, then multiplexing happens  and  events
              run  only  part of the time.  In that case, the time_enabled and
              time running values can be used to scale an estimated value  for
              the count.

       value  An unsigned 64-bit value containing the counter result.

       id     A globally unique value for this particular event, only there if
              PERF_FORMAT_ID was specified in read_format.

   MMAP layout
       When using perf_event_open() in sampled mode, asynchronous events (like
       counter  overflow  or  PROT_EXEC mmap tracking) are logged into a ring-
       buffer.  This ring-buffer is created and accessed through mmap(2).

       The mmap size should be 1+2^n pages, where the first page is a metadata
       page (struct perf_event_mmap_page) that contains various bits of infor-
       mation such as where the ring-buffer head is.

       Before kernel 2.6.39, there is a bug that means  you  must  allocate  a
       mmap ring buffer when sampling even if you do not plan to access it.

       The structure of the first metadata mmap page is as follows:

           struct perf_event_mmap_page {
               __u32 version;        /* version number of this structure */
               __u32 compat_version; /* lowest version this is compat with */
               __u32 lock;           /* seqlock for synchronization */
               __u32 index;          /* hardware counter identifier */
               __s64 offset;         /* add to hardware counter value */
               __u64 time_enabled;   /* time event active */
               __u64 time_running;   /* time event on CPU */
               union {
                   __u64   capabilities;
                   struct {
                       __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
                             cap_bit0_is_deprecated : 1,
                             cap_user_rdpmc         : 1,
                             cap_user_time          : 1,
                             cap_user_time_zero     : 1,
               __u16 pmc_width;
               __u16 time_shift;
               __u32 time_mult;
               __u64 time_offset;
               __u64 __reserved[120];   /* Pad to 1k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */

       The  following  list  describes  the fields in the perf_event_mmap_page
       structure in more detail:

              Version number of this structure.

              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset When using rdpmc for reads this offset value must  be  added  to
              the one returned by rdpmc to get the current total event count.

              Time the event was active.

              Time the event was running.

       cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
              There   was   a  bug  in  the  definition  of  cap_usr_time  and
              cap_usr_rdpmc from Linux 3.4 until Linux 3.11.  Both  bits  were
              defined  to  point to the same location, so it was impossible to
              know if cap_usr_time or cap_usr_rdpmc were actually set.

              Starting with 3.12 these are renamed to cap_bit0 and you  should
              use the new cap_user_time and cap_user_rdpmc fields instead.

       cap_bit0_is_deprecated (since Linux 3.12)
              If set, this bit indicates that the kernel supports the properly
              separated cap_user_time and cap_user_rdpmc bits.

              If not-set, it indicates an older kernel where cap_usr_time  and
              cap_usr_rdpmc  map to the same bit and thus both features should
              be used with caution.

       cap_user_rdpmc (since Linux 3.12)
              If the hardware supports user-space read of performance counters
              without  syscall  (this is the "rdpmc" instruction on x86), then
              the following code can be used to do a read:

                  u32 seq, time_mult, time_shift, idx, width;
                  u64 count, enabled, running;
                  u64 cyc, time_offset;

                  do {
                      seq = pc->lock;
                      enabled = pc->time_enabled;
                      running = pc->time_running;

                      if (pc->cap_usr_time && enabled != running) {
                          cyc = rdtsc();
                          time_offset = pc->time_offset;
                          time_mult   = pc->time_mult;
                          time_shift  = pc->time_shift;

                      idx = pc->index;
                      count = pc->offset;

                      if (pc->cap_usr_rdpmc && idx) {
                          width = pc->pmc_width;
                          count += rdpmc(idx - 1);

                  } while (pc->lock != seq);

       cap_user_time  (since Linux 3.12)
              This bit indicates the hardware has a  constant,  nonstop  time-
              stamp counter (TSC on x86).

       cap_user_time_zero (since Linux 3.12)
              Indicates  the  presence of time_zero which allows mapping time-
              stamp values to the hardware clock.

              If cap_usr_rdpmc, this field provides the bit-width of the value
              read  using  the  rdpmc  or equivalent instruction.  This can be
              used to sign extend the result like:

                  pmc <<= 64 - pmc_width;
                  pmc >>= 64 - pmc_width; // signed shift right
                  count += pmc;

       time_shift, time_mult, time_offset

              If cap_usr_time, these fields can be used to  compute  the  time
              delta  since  time_enabled (in nanoseconds) using rdtsc or simi-

                  u64 quot, rem;
                  u64 delta;
                  quot = (cyc >> time_shift);
                  rem = cyc & ((1 << time_shift) - 1);
                  delta = time_offset + quot * time_mult +
                          ((rem * time_mult) >> time_shift);

              Where time_offset, time_mult, time_shift, and cyc  are  read  in
              the seqcount loop described above.  This delta can then be added
              to enabled and possible running (if idx), improving the scaling:

                  enabled += delta;
                  if (idx)
                      running += delta;
                  quot = count / running;
                  rem  = count % running;
                  count = quot * enabled + (rem * enabled) / running;

       time_zero (since Linux 3.12)

              If cap_usr_time_zero is set, then the hardware  clock  (the  TSC
              timestamp  counter on x86) can be calculated from the time_zero,
              time_mult, and time_shift values:

                  time = timestamp - time_zero;
                  quot = time / time_mult;
                  rem  = time % time_mult;
                  cyc = (quot << time_shift) + (rem << time_shift) / time_mult;

              And vice versa:

                  quot = cyc >> time_shift;
                  rem  = cyc & ((1 << time_shift) - 1);
                  timestamp = time_zero + quot * time_mult +
                      ((rem * time_mult) >> time_shift);

              This points to the head of the data section.  The value continu-
              ously  increases, it does not wrap.  The value needs to be manu-
              ally wrapped by the size of the mmap buffer before accessing the

              On  SMP-capable  platforms,  after  reading the data_head value,
              user space should issue an rmb().

              When the mapping is PROT_WRITE, the data_tail  value  should  be
              written  by  user  space to reflect the last read data.  In this
              case, the kernel will not overwrite unread data.

       The following 2^n ring-buffer pages have the layout described below.

       If perf_event_attr.sample_id_all is set, then all event types will have
       the  sample_type  selected  fields  related to where/when (identity) an
       event  took  place  (TID,  TIME,  ID,  CPU,  STREAM_ID)  described   in
       PERF_RECORD_SAMPLE   below,   it   will   be  stashed  just  after  the
       perf_event_header and the  fields  already  present  for  the  existing
       fields, that is, at the end of the payload.  That way a newer
       file will be supported by older perf tools,  with  these  new  optional
       fields being ignored.

       The mmap values start with a header:

           struct perf_event_header {
               __u32   type;
               __u16   misc;
               __u16   size;

       Below,  we  describe  the perf_event_header fields in more detail.  For
       ease of reading, the fields with  shorter  descriptions  are  presented

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sample.

              The  CPU  mode can be determined from this value by masking with
              PERF_RECORD_MISC_CPUMODE_MASK and looking for one of the follow-
              ing  (note  these  are  not  bit masks, only one can be set at a

                     Unknown CPU mode.

                     Sample happened in the kernel.

                     Sample happened in user code.

                     Sample happened in the hypervisor.

                     Sample happened in the guest kernel.

                     Sample happened in guest user code.

              In addition, one of the following bits can be set:

                     This is set when the mapping is not executable; otherwise
                     the mapping is executable.

                     This is set for a PERF_RECORD_COMM record on kernels more
                     recent than Linux 3.16  if  a  process  name  change  was
                     caused  by  an  exec(2)  system call.  It is an alias for
                     PERF_RECORD_MISC_MMAP_DATA since the two values would not
                     be set in the same record.

                     This  indicates that the content of PERF_SAMPLE_IP points
                     to the actual instruction that triggered the event.   See
                     also perf_event_attr.precise_ip.

                     This  indicates  there  is  extended data available (cur-
                     rently not used).

       type   The type value is one of the below.  The values  in  the  corre-
              sponding  record  (that  follows  the header) depend on the type
              selected as shown.

                  The MMAP events record the PROT_EXEC mappings so that we can
                  correlate  user-space  IPs to code.  They have the following

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          char   filename[];

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.  len  is  the
                         length  of  the  allocated memory.  pgoff is the page
                         offset of the allocated memory.  filename is a string
                         describing the backing of the allocated memory.

                  This record indicates when events are lost.

                      struct {
                          struct perf_event_header header;
                          u64 id;
                          u64 lost;
                          struct sample_id sample_id;

                  id     is  the  unique  event  ID  for the samples that were

                  lost   is the number of events that were lost.

                  This record indicates a change in the process name.

                      struct {
                          struct perf_event_header header;
                          u32 pid;
                          u32 tid;
                          char comm[];
                          struct sample_id sample_id;

                  pid    is the process ID.

                  tid    is the thread ID.

                  comm   is a string containing the new name of the process.

                  This record indicates a process exit event.

                      struct {
                          struct perf_event_header header;
                          u32 pid, ppid;
                          u32 tid, ptid;
                          u64 time;
                          struct sample_id sample_id;

                  This record indicates a throttle/unthrottle event.

                      struct {
                          struct perf_event_header header;
                          u64 time;
                          u64 id;
                          u64 stream_id;
                          struct sample_id sample_id;

                  This record indicates a fork event.

                      struct {
                          struct perf_event_header header;
                          u32 pid, ppid;
                          u32 tid, ptid;
                          u64 time;
                          struct sample_id sample_id;

                  This record indicates a read event.

                      struct {
                          struct perf_event_header header;
                          u32 pid, tid;
                          struct read_format values;
                          struct sample_id sample_id;

                  This record indicates a sample.

                      struct {
                          struct perf_event_header header;
                          u64   sample_id;  /* if PERF_SAMPLE_IDENTIFIER */
                          u64   ip;         /* if PERF_SAMPLE_IP */
                          u32   pid, tid;   /* if PERF_SAMPLE_TID */
                          u64   time;       /* if PERF_SAMPLE_TIME */
                          u64   addr;       /* if PERF_SAMPLE_ADDR */
                          u64   id;         /* if PERF_SAMPLE_ID */
                          u64   stream_id;  /* if PERF_SAMPLE_STREAM_ID */
                          u32   cpu, res;   /* if PERF_SAMPLE_CPU */
                          u64   period;     /* if PERF_SAMPLE_PERIOD */
                          struct read_format v; /* if PERF_SAMPLE_READ */
                          u64   nr;         /* if PERF_SAMPLE_CALLCHAIN */
                          u64   ips[nr];    /* if PERF_SAMPLE_CALLCHAIN */
                          u32   size;       /* if PERF_SAMPLE_RAW */
                          char  data[size]; /* if PERF_SAMPLE_RAW */
                          u64   bnr;        /* if PERF_SAMPLE_BRANCH_STACK */
                          struct perf_branch_entry lbr[bnr];
                                            /* if PERF_SAMPLE_BRANCH_STACK */
                          u64   abi;        /* if PERF_SAMPLE_REGS_USER */
                          u64   regs[weight(mask)];
                                            /* if PERF_SAMPLE_REGS_USER */
                          u64   size;       /* if PERF_SAMPLE_STACK_USER */
                          char  data[size]; /* if PERF_SAMPLE_STACK_USER */
                          u64   dyn_size;   /* if PERF_SAMPLE_STACK_USER */
                          u64   weight;     /* if PERF_SAMPLE_WEIGHT */
                          u64   data_src;   /* if PERF_SAMPLE_DATA_SRC */
                          u64   transaction;/* if PERF_SAMPLE_TRANSACTION */

                      If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit unique ID
                      is  included.   This  is  a duplication of the PERF_SAM-
                      PLE_ID id value, but included at the  beginning  of  the
                      sample so parsers can easily obtain the value.

                  ip  If  PERF_SAMPLE_IP is enabled, then a 64-bit instruction
                      pointer value is included.

                  pid, tid
                      If PERF_SAMPLE_TID is enabled, then a 32-bit process  ID
                      and 32-bit thread ID are included.

                      If  PERF_SAMPLE_TIME is enabled, then a 64-bit timestamp
                      is included.  This is obtained via  local_clock()  which
                      is  a  hardware  timestamp  if available and the jiffies
                      value if not.

                      If PERF_SAMPLE_ADDR is enabled, then a 64-bit address is
                      included.   This is usually the address of a tracepoint,
                      breakpoint, or software event; otherwise the value is 0.

                  id  If PERF_SAMPLE_ID is enabled,  a  64-bit  unique  ID  is
                      included.   If  the event is a member of an event group,
                      the group leader ID is returned.  This ID is the same as
                      the one returned by PERF_FORMAT_ID.

                      If  PERF_SAMPLE_STREAM_ID is enabled, a 64-bit unique ID
                      is included.  Unlike PERF_SAMPLE_ID  the  actual  ID  is
                      returned,  not the group leader.  This ID is the same as
                      the one returned by PERF_FORMAT_ID.

                  cpu, res
                      If PERF_SAMPLE_CPU is enabled, this is  a  32-bit  value
                      indicating  which  CPU  was being used, in addition to a
                      reserved (unused) 32-bit value.

                      If PERF_SAMPLE_PERIOD is enabled, a 64-bit  value  indi-
                      cating the current sampling period is written.

                  v   If  PERF_SAMPLE_READ  is  enabled,  a  structure of type
                      read_format is included which has values for all  events
                      in  the  event group.  The values included depend on the
                      read_format value used at perf_event_open() time.

                  nr, ips[nr]
                      If PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit  num-
                      ber  is  included  which  indicates  how  many following
                      64-bit instruction pointers will follow.   This  is  the
                      current callchain.

                  size, data[size]
                      If PERF_SAMPLE_RAW is enabled, then a 32-bit value indi-
                      cating size is included followed by an  array  of  8-bit
                      values  of length size.  The values are padded with 0 to
                      have 64-bit alignment.

                      This RAW record data is opaque with respect to the  ABI.
                      The  ABI  doesn't  make any promises with respect to the
                      stability of its  content,  it  may  vary  depending  on
                      event, hardware, and kernel version.

                  bnr, lbr[bnr]
                      If  PERF_SAMPLE_BRANCH_STACK  is  enabled, then a 64-bit
                      value indicating the number of records is included, fol-
                      lowed  by  bnr  perf_branch_entry  structures which each
                      include the fields:

                      from   This indicates the source instruction (may not be
                             a branch).

                      to     The branch target.

                             The branch target was mispredicted.

                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory transac-

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional memory

                      The  entries are from most to least recent, so the first
                      entry has the most recent branch.

                      Support for mispred and predicted is  optional;  if  not
                      supported, both values will be 0.

                      The  type  of  branches  recorded  is  specified  by the
                      branch_sample_type field.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_USER is enabled, then the  user  CPU
                      registers are recorded.

                      The  abi  field  is  one  of  PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32 or PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of  the  CPU  registers  that
                      were  specified by the sample_regs_user attr field.  The
                      number of values is the number of bits set in  the  sam-
                      ple_regs_user bit mask.

                  size, data[size], dyn_size
                      If  PERF_SAMPLE_STACK_USER  is  enabled,  then  the user
                      stack is recorded.  This can be used to  generate  stack
                      backtraces.   size  is the size requested by the user in
                      sample_stack_user or else the maximum record size.  data
                      is  the  stack data (a raw dump of the memory pointed to
                      by the stack pointer at the time of sampling).  dyn_size
                      is  the amount of data actually dumped (can be less than

                      If PERF_SAMPLE_WEIGHT is enabled, then  a  64-bit  value
                      provided  by the hardware is recorded that indicates how
                      costly the event was.  This allows expensive  events  to
                      stand out more clearly in profiles.

                      If  PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit value
                      is recorded that is made up of the following fields:

                          Type of opcode, a bitwise combination of:

                          PERF_MEM_OP_NA          Not available
                          PERF_MEM_OP_LOAD        Load instruction
                          PERF_MEM_OP_STORE       Store instruction
                          PERF_MEM_OP_PFETCH      Prefetch
                          PERF_MEM_OP_EXEC        Executable code

                          Memory hierarchy level hit or miss, a bitwise combi-
                          nation   of   the   following,   shifted   left   by

                          PERF_MEM_LVL_NA         Not available
                          PERF_MEM_LVL_HIT        Hit
                          PERF_MEM_LVL_MISS       Miss
                          PERF_MEM_LVL_L1         Level 1 cache
                          PERF_MEM_LVL_LFB        Line fill buffer
                          PERF_MEM_LVL_L2         Level 2 cache
                          PERF_MEM_LVL_L3         Level 3 cache
                          PERF_MEM_LVL_LOC_RAM    Local DRAM
                          PERF_MEM_LVL_REM_RAM1   Remote DRAM 1 hop
                          PERF_MEM_LVL_REM_RAM2   Remote DRAM 2 hops
                          PERF_MEM_LVL_REM_CCE1   Remote cache 1 hop
                          PERF_MEM_LVL_REM_CCE2   Remote cache 2 hops
                          PERF_MEM_LVL_IO         I/O memory
                          PERF_MEM_LVL_UNC        Uncached memory

                          Snoop mode, a bitwise combination of the  following,
                          shifted left by PERF_MEM_SNOOP_SHIFT:

                          PERF_MEM_SNOOP_NA       Not available
                          PERF_MEM_SNOOP_NONE     No snoop
                          PERF_MEM_SNOOP_HIT      Snoop hit
                          PERF_MEM_SNOOP_MISS     Snoop miss
                          PERF_MEM_SNOOP_HITM     Snoop hit modified

                          Lock  instruction, a bitwise combination of the fol-
                          lowing, shifted left by PERF_MEM_LOCK_SHIFT:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                          TLB access hit or miss, a bitwise combination of the
                          following, shifted left by PERF_MEM_TLB_SHIFT:

                          PERF_MEM_TLB_NA         Not available
                          PERF_MEM_TLB_HIT        Hit
                          PERF_MEM_TLB_MISS       Miss
                          PERF_MEM_TLB_L1         Level 1 TLB
                          PERF_MEM_TLB_L2         Level 2 TLB
                          PERF_MEM_TLB_WK         Hardware walker
                          PERF_MEM_TLB_OS         OS fault handler

                      If  the  PERF_SAMPLE_TRANSACTION  flag  is  set,  then a
                      64-bit field is recorded describing the sources  of  any
                      transactional memory aborts.

                      The field is a bitwise combination of the following val-

                             Abort from an elision  type  transaction  (Intel-

                             Abort from a generic transaction.

                             Synchronous   abort   (related  to  the  reported

                             Asynchronous abort (not related to  the  reported

                             Retryable  abort  (retrying  the  transaction may
                             have succeeded).

                             Abort due to memory conflicts with other threads.

                             Abort due to write capacity overflow.

                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be obtained
                      from  the high 32 bits of the field by shifting right by
                      PERF_TXN_ABORT_SHIFT       and       masking        with

                  This  record  includes extended information on mmap(2) calls
                  returning executable mappings.  The  format  is  similar  to
                  that of the PERF_RECORD_MMAP record, but includes extra val-
                  ues that allow uniquely identifying shared mappings.

                      struct {
                          struct perf_event_header header;
                          u32 pid;
                          u32 tid;
                          u64 addr;
                          u64 len;
                          u64 pgoff;
                          u32 maj;
                          u32 min;
                          u64 ino;
                          u64 ino_generation;
                          u32 prot;
                          u32 flags;
                          char filename[];
                          struct sample_id sample_id;

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.

                  len    is the length of the allocated memory.

                  pgoff  is the page offset of the allocated memory.

                  maj    is the major ID of the underlying device.

                  min    is the minor ID of the underlying device.

                  ino    is the inode number.

                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                         is a string describing the backing of  the  allocated

   Signal overflow
       Events can be set to deliver a signal when a threshold is crossed.  The
       signal handler is set up using the  poll(2),  select(2),  epoll(2)  and
       fcntl(2), system calls.

       To  generate signals, sampling must be enabled (sample_period must have
       a nonzero value).

       There are two ways to generate signals.

       The first is to set a wakeup_events or wakeup_watermark value that will
       generate  a  signal  if  a certain number of samples or bytes have been
       written to the mmap ring buffer.   In  this  case,  a  signal  of  type
       POLL_IN is sent.

       The  other  way  is  by  use of the PERF_EVENT_IOC_REFRESH ioctl.  This
       ioctl adds to a counter that decrements each time the event  overflows.
       When  nonzero, a POLL_IN signal is sent on overflow, but once the value
       reaches 0, a signal is sent of type POLL_HUP and the  underlying  event
       is disabled.

       Note: on newer kernels (since at least as early as Linux 3.2), a signal
       is provided for every overflow, even if wakeup_events is not set.

   rdpmc instruction
       Starting with Linux 3.4 on x86, you can use the  rdpmc  instruction  to
       get  low-latency  reads  without having to enter the kernel.  Note that
       using rdpmc is not necessarily faster than other  methods  for  reading
       event values.

       Support  for  this  can be detected with the cap_usr_rdpmc field in the
       mmap page; documentation on how to calculate event values can be  found
       in that section.

   perf_event ioctl calls
       Various ioctls act on perf_event_open() file descriptors:

              This  enables  the  individual event or event group specified by
              the file descriptor argument.

              If the PERF_IOC_FLAG_GROUP bit is set  in  the  ioctl  argument,
              then all events in a group are enabled, even if the event speci-
              fied is not the group leader (but see BUGS).

              This disables the individual counter or event group specified by
              the file descriptor argument.

              Enabling  or disabling the leader of a group enables or disables
              the entire group; that is, while the group leader  is  disabled,
              none  of the counters in the group will count.  Enabling or dis-
              abling a member of a group other than the  leader  affects  only
              that  counter;  disabling  a  non-leader stops that counter from
              counting but doesn't affect any other counter.

              If the PERF_IOC_FLAG_GROUP bit is set  in  the  ioctl  argument,
              then all events in a group are disabled, even if the event spec-
              ified is not the group leader (but see BUGS).

              Non-inherited overflow counters can use this to enable a counter
              for a number of overflows specified by the argument, after which
              it is disabled.  Subsequent calls of this ioctl add the argument
              value to the current count.  A signal with POLL_IN set will hap-
              pen on each overflow until the count reaches 0; when  that  hap-
              pens  a  signal  with POLL_HUP set is sent and the event is dis-
              abled.  Using an argument of 0 is considered undefined behavior.

              Reset the event count specified by the file descriptor  argument
              to  zero.  This resets only the counts; there is no way to reset
              the multiplexing time_enabled or time_running values.

              If the PERF_IOC_FLAG_GROUP bit is set  in  the  ioctl  argument,
              then  all  events in a group are reset, even if the event speci-
              fied is not the group leader (but see BUGS).

              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux  3.14  (all  other  architec-
              tures),  the new period takes effect immediately.  On older ker-
              nels, the new period did not take effect until  after  the  next

              The  argument  is  a  pointer  to  a 64-bit value containing the
              desired new period.

              Prior to Linux 2.6.36 this ioctl always failed due to a  bug  in
              the kernel.

              This tells the kernel to report event notifications to the spec-
              ified file descriptor rather than the  default  one.   The  file
              descriptors must all be on the same CPU.

              The  argument  specifies  the  desired file descriptor, or -1 if
              output should be ignored.

       PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
              This adds an ftrace filter to this event.

              The argument is a pointer to the desired ftrace filter.

       PERF_EVENT_IOC_ID (since Linux 3.12)
              This returns the  event  ID  value  for  the  given  event  file

              The  argument  is a pointer to a 64-bit unsigned integer to hold
              the result.

   Using prctl
       A process can enable or disable all the event groups that are  attached
       to    it    using    the    prctl(2)   PR_TASK_PERF_EVENTS_ENABLE   and
       PR_TASK_PERF_EVENTS_DISABLE operations.  This applies to  all  counters
       on  the calling process, whether created by this process or by another,
       and does not affect any counters that this process has created on other
       processes.   It  enables  or  disables  only the group leaders, not any
       other members in the groups.

   perf_event related configuration files
       Files in /proc/sys/kernel/


                  The perf_event_paranoid file can be set to  restrict  access
                  to the performance counters.

                  2   only allow user-space measurements.

                  1   allow both kernel and user measurements (default).

                  0   allow access to CPU-specific data but not raw tracepoint

                  -1  no restrictions.

                  The existence of the perf_event_paranoid file is  the  offi-
                  cial   method   for   determining   if   a  kernel  supports


                  This sets the maximum sample rate.  Setting  this  too  high
                  can  allow  users  to  sample at a rate that impacts overall
                  machine performance and potentially  lock  up  the  machine.
                  The default value is 100000 (samples per second).


                  Maximum  number  of pages an unprivileged user can mlock(2).
                  The default is 516 (kB).

       Files in /sys/bus/event_source/devices/
           Since Linux 2.6.34, the kernel supports having multiple PMUs avail-
           able  for monitoring.  Information on how to program these PMUs can
           be found under /sys/bus/event_source/devices/.   Each  subdirectory
           corresponds to a different PMU.

           /sys/bus/event_source/devices/*/type (since Linux 2.6.38)
                  This  contains an integer that can be used in the type field
                  of perf_event_attr to indicate that you  wish  to  use  this

           /sys/bus/event_source/devices/*/rdpmc (since Linux 3.4)
                  If this file is 1, then direct user-space access to the per-
                  formance counter registers is allowed via the rdpmc instruc-
                  tion.  This can be disabled by echoing 0 to the file.

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This  subdirectory contains information on the architecture-
                  specific subfields available  for  programming  the  various
                  config fields in the perf_event_attr struct.

                  The  content  of  each file is the name of the config field,
                  followed by a colon, followed by a  series  of  integer  bit
                  ranges separated by commas.  For example, the file event may
                  contain the value  config1:1,6-10,44  which  indicates  that
                  event  is  an attribute that occupies bits 1,6-10, and 44 of

           /sys/bus/event_source/devices/*/events/ (since Linux 3.4)
                  This subdirectory contains  files  with  predefined  events.
                  The  contents  are  strings  describing  the  event settings
                  expressed in terms of the fields  found  in  the  previously
                  mentioned  ./format/  directory.   These are not necessarily
                  complete lists of all events supported by a PMU, but usually
                  a subset of events deemed useful or interesting.

                  The  content of each file is a list of attribute names sepa-
                  rated by commas.  Each entry has an optional  value  (either
                  hex  or  decimal).   If  no  value  is specified, then it is
                  assumed to be a single-bit field with  a  value  of  1.   An
                  example entry may look like this: event=0x2,inv,ldlat=3.

                  This  file  is  the  standard  kernel  device  interface for
                  injecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The cpumask file contains a comma-separated list of integers
                  that  indicate  a  representative CPU number for each socket
                  (package) on the motherboard.  This is needed  when  setting
                  up  uncore  or  northbridge  events,  as  those PMUs present
                  socket-wide events.

       perf_event_open() returns the new file descriptor, or -1  if  an  error
       occurred (in which case, errno is set appropriately).

       The  errors  returned by perf_event_open() can be inconsistent, and may
       vary across processor architectures and performance monitoring units.

       E2BIG  Returned if the perf_event_attr size value is too small (smaller
              than  PERF_ATTR_SIZE_VER0), too big (larger than the page size),
              or larger than the kernel supports and the extra bytes  are  not
              zero.  When E2BIG is returned, the perf_event_attr size field is
              overwritten by the kernel to be the size of the structure it was

       EACCES Returned when the requested event requires CAP_SYS_ADMIN permis-
              sions (or a more permissive perf_event paranoid setting).   Some
              common  cases  where  an unprivileged process may encounter this
              error: attaching to a process owned by a different  user;  moni-
              toring  all  processes  on a given CPU (i.e., specifying the pid
              argument as -1); and not setting exclude_kernel when  the  para-
              noid setting requires it.

       EBADF  Returned  if  the  group_fd file descriptor is not valid, or, if
              PERF_FLAG_PID_CGROUP is set, the cgroup file descriptor  in  pid
              is not valid.

       EFAULT Returned  if  the  attr  pointer  points  at  an  invalid memory

       EINVAL Returned if the specified event is invalid.  There are many pos-
              sible  reasons  for this.  A not-exhaustive list: sample_freq is
              higher than the maximum setting; the cpu  to  monitor  does  not
              exist; read_format is out of range; sample_type is out of range;
              the flags value is out of range; exclusive or pinned set and the
              event  is not a group leader; the event config values are out of
              range or set reserved bits; the generic event  selected  is  not
              supported;  or  there  is  not  enough  room to add the selected

       EMFILE Each opened event uses one file descriptor.  If a  large  number
              of  events  are opened the per-user file descriptor limit (often
              1024) will be hit and no more events can be created.

       ENODEV Returned when the event involves a feature not supported by  the
              current CPU.

       ENOENT Returned  if  the type setting is not valid.  This error is also
              returned for some unsupported generic events.

       ENOSPC Prior to Linux 3.3, if there was not enough room for the  event,
              ENOSPC  was returned.  In Linux 3.3, this was changed to EINVAL.
              ENOSPC is still returned if  you  try  to  add  more  breakpoint
              events than supported by the hardware.

       ENOSYS Returned  if PERF_SAMPLE_STACK_USER is set in sample_type and it
              is not supported by hardware.

              Returned if an event requiring a specific  hardware  feature  is
              requested  but  there  is  no  hardware  support.  This includes
              requesting low-skid events if not supported, branch  tracing  if
              it  is not available, sampling if no PMU interrupt is available,
              and branch stacks for software events.

       EPERM  Returned on many (but not all) architectures when an unsupported
              exclude_hv,  exclude_idle,  exclude_user, or exclude_kernel set-
              ting is specified.

              It can also happen, as with EACCES,  when  the  requested  event
              requires   CAP_SYS_ADMIN   permissions  (or  a  more  permissive
              perf_event paranoid setting).  This includes  setting  a  break-
              point on a kernel address, and (since Linux 3.13) setting a ker-
              nel function-trace tracepoint.

       ESRCH  Returned if attempting to attach to  a  process  that  does  not

       perf_event_open()  was  introduced  in  Linux  2.6.31  but  was  called
       perf_counter_open().  It was renamed in Linux 2.6.32.

       This perf_event_open() system call Linux- specific and  should  not  be
       used in programs intended to be portable.

       Glibc  does  not  provide a wrapper for this system call; call it using
       syscall(2).  See the example below.

       The official way of knowing if perf_event_open() support is enabled  is
       checking    for    the    existence    of   the   file   /proc/sys/ker-

       The F_SETOWN_EX option to fcntl(2) is needed to properly  get  overflow
       signals in threads.  This was introduced in Linux 2.6.32.

       Prior  to  Linux 2.6.33 (at least for x86), the kernel did not check if
       events could be scheduled together until read time.  The  same  happens
       on all known kernels if the NMI watchdog is enabled.  This means to see
       if a given set of events works you have  to  perf_event_open(),  start,
       then read before you know for sure you can get valid measurements.

       Prior  to Linux 2.6.34, event constraints were not enforced by the ker-
       nel.  In that case, some events would silently return "0" if the kernel
       scheduled them in an improper counter slot.

       Prior  to  Linux  2.6.34,  there  was a bug when multiplexing where the
       wrong results could be returned.

       Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the  kernel
       if "inherit" is enabled and many threads are started.

       Prior  to  Linux  2.6.35,  PERF_FORMAT_GROUP did not work with attached

       In older Linux 2.6 versions, refreshing an event group leader refreshed
       all  siblings,  and  refreshing  with a parameter of 0 enabled infinite
       refresh.  This behavior is unsupported and should not be relied on.

       There is a bug in the kernel code between Linux 2.6.36  and  Linux  3.0
       that  ignores  the  "watermark" field and acts as if a wakeup_event was
       chosen if the union has a nonzero value in it.

       From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP ioctl  argument
       was  broken  and would repeatedly operate on the event specified rather
       than iterating across all sibling events in a group.

       From Linux 3.4 to Linux 3.11, the mmap cap_usr_rdpmc  and  cap_usr_time
       bits  mapped  to  the  same  location.   Code should migrate to the new
       cap_user_rdpmc and cap_user_time fields instead.

       Always double-check your results!  Various generalized events have  had
       wrong  values.   For example, retired branches measured the wrong thing
       on AMD machines until Linux 2.6.35.

       The following is a short example that measures  the  total  instruction
       count of a call to printf(3).

       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <linux/perf_event.h>
       #include <asm/unistd.h>

       static long
       perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                       int cpu, int group_fd, unsigned long flags)
           int ret;

           ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                          group_fd, flags);
           return ret;

       main(int argc, char **argv)
           struct perf_event_attr pe;
           long long count;
           int fd;

           memset(&pe, 0, sizeof(struct perf_event_attr));
           pe.type = PERF_TYPE_HARDWARE;
           pe.size = sizeof(struct perf_event_attr);
           pe.config = PERF_COUNT_HW_INSTRUCTIONS;
           pe.disabled = 1;
           pe.exclude_kernel = 1;
           pe.exclude_hv = 1;

           fd = perf_event_open(&pe, 0, -1, -1, 0);
           if (fd == -1) {
              fprintf(stderr, "Error opening leader %llx\n", pe.config);

           ioctl(fd, PERF_EVENT_IOC_RESET, 0);
           ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

           printf("Measuring instruction count for this printf\n");

           ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
           read(fd, &count, sizeof(long long));

           printf("Used %lld instructions\n", count);


       fcntl(2), mmap(2), open(2), prctl(2), read(2)

       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2014-08-19                PERF_EVENT_OPEN(2)

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