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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.

       When  pid  is greater than zero, permission to perform this system call
       is governed by a ptrace access mode  PTRACE_MODE_READ_REALCREDS	check;
       see ptrace(2).

       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  tells	 the  event  to	 ignore the group_fd parameter
	      except for the purpose of setting up  output  redirection	 using
	      the PERF_FLAG_FD_OUTPUT flag.

       PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
	      This  flag  re-routes  the  event's sampled output to instead be
	      included in the mmap buffer of the event specified by group_fd.

       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 */
		     use_clockid    :  1,  /* use clockid for time fields */
		     context_switch :  1,  /* context switch data */

		     __reserved_1   : 37;

	       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 */
	       __s32 clockid;		   /* clock to use for time fields */
	       __u64 sample_regs_intr;	   /* regs to dump on samples */
	       __u32 aux_watermark;	   /* aux bytes before wakeup */
	       __u16 sample_max_stack;	   /* max frames in callchain */
	       __u16 __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 2.6.38, 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_ATTR_SIZE_VER3 is 96 cor-
	      responding  to  the  addition  of	 sample_regs_user   and	  sam-
	      ple_stack_user  in Linux 3.7.  PERF_ATTR_SIZE_VER4 is 104 corre-
	      sponding to the addition	of  sample_regs_intr  in  Linux	 3.19.
	      PERF_ATTR_SIZE_VER5  is  112  corresponding  to  the addition of
	      aux_watermark in Linux 4.1.

       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.

	      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 2.6.35,
			  this used the wrong event on AMD processors.

			  Mispredicted branch instructions.

			  Bus  cycles,	which  can  be	different  from	 total

			  Stalled cycles during issue.

			  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 3.1)
			     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"  event is one that generates an overflow notifica-
	      tion every N events, where N is given by sample_period.  A  sam-
	      pling  event  has	 sample_period	> 0.  When an overflow occurs,
	      requested data is recorded in the mmap buffer.  The  sample_type
	      field controls what data is recorded on each overflow.

	      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 available only 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).

		     The 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

	      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

	      PERF_SAMPLE_REGS_INTR (since Linux 3.19)
		     Records  a	 subset	 of  the current CPU register state as
		     specified	 by   sample_regs_intr.	   Unlike    PERF_SAM-
		     PLE_REGS_USER the register values will return kernel reg-
		     ister state if the overflow happened while kernel code is
		     running.  If the CPU supports hardware sampling of regis-
		     ter state (i.e., PEBS on Intel x86) and precise_ip is set
		     higher  than  zero	 then the register values returned are
		     those captured by hardware at the	time  of  the  sampled
		     instruction's retirement.

	      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_format val-
	      ues, 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
	      kernel space.

	      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 an overflow notification happen when we  cross  the
	      wakeup_watermark	boundary.   Otherwise,	overflow notifications
	      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 possible values of this field 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 the description of

       mmap_data (since Linux 2.6.36)
	      This is the counterpart of the mmap field.  This enables genera-
	      tion  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)
	      When conducting measurements that include processes  running  VM
	      instances (i.e., have executed a KVM_RUN ioctl(2)), only measure
	      events happening inside a guest instance.	 This is only meaning-
	      ful  outside  the	 guests;  this	setting does not change counts
	      gathered inside of a guest.  Currently,  this  functionality  is
	      x86 only.

       exclude_guest (since Linux 3.2)
	      When  conducting	measurements that include processes running VM
	      instances (i.e., have executed a KVM_RUN ioctl(2)), do not  mea-
	      sure  events  happening  inside  guest  instances.  This is only
	      meaningful outside the guests;  this  setting  does  not	change
	      counts  gathered inside of a guest.  Currently, this functional-
	      ity is x86 only.

       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-

       use_clockid (since Linux 4.1)
	      This allows selecting which internal Linux  clock	 to  use  when
	      generating  timestamps  via the clockid field.  This can make it
	      easier to correlate perf sample times with timestamps  generated
	      by other tools.

       context_switch (since Linux 4.3)
	      This enables the generation of PERF_RECORD_SWITCH records when a
	      context switch  occurs.	It  also  enables  the	generation  of
	      PERF_RECORD_SWITCH_CPU_WIDE  records  when  sampling in CPU-wide
	      mode.  This functionality is in addition to existing  tracepoint
	      and  software events for measuring context switches.  The advan-
	      tage of this method is that it will give full  information  even
	      with strict perf_event_paranoid settings.

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

	      wakeup_events  counts  only PERF_RECORD_SAMPLE record types.  To
	      receive overflow notification for all PERF_RECORD	 types	choose
	      watermark and set wakeup_watermark to 1.

	      Prior  to	 Linux	3.0, setting wakeup_events to 0 resulted in no
	      overflow notifications; more recent kernels treat 0 the same  as

       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
	      not allowed.

       bp_addr (since Linux 2.6.33)
	      This  is	the  address  of the breakpoint.  For execution break-
	      points, this is the memory address of the instruction of	inter-
	      est; for read and write breakpoints, it is the memory address of
	      the memory 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
	      Linux 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
	      POINT_LEN_8.   For  an  execution	 breakpoint,   set   this   to

       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 values listed below.  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 (includes direct calls,  indirect	calls,
		     and far jumps).

		     Indirect calls.

	      PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
		     Direct calls.

		     Any return branch.

	      PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
		     Indirect jumps.

	      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.
		     PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1) Branch is
		     part  of  a hardware-generated call stack.	 This requires
		     hardware support,	currently  only	 found	on  Intel  x86
		     Haswell or newer.

       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   is	 described   in	  the	kernel	 header	  file

       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.

       clockid (since Linux 4.1)
	      If  use_clockid  is  set, then this field selects which internal
	      Linux timer to use for timestamps.   The	available  timers  are
	      defined	in  linux/time.h,  with	 CLOCK_MONOTONIC,  CLOCK_MONO-
	      rently supported.

       aux_watermark (since Linux 4.1)
	      This   specifies	 how  much  data  is  required	to  trigger  a
	      PERF_RECORD_AUX sample.

       sample_max_stack (since Linux 4.8)
	      When  sample_type	 includes  PERF_SAMPLE_CALLCHAIN,  this	 field
	      specifies	 how  many  stack frames to report when generating the

   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, the error ENOSPC results.

       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 */
		 struct {
		     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.  Available only if
	      PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
	      Total time the event was enabled and  running.   Normally	 these
	      values 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 present
	      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  an
       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 */
	       __u64 data_offset;	/* where the buffer starts */
	       __u64 data_size;		/* data buffer size */
	       __u64 aux_head;
	       __u64 aux_tail;
	       __u64 aux_offset;
	       __u64 aux_size;


       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 Linux 3.12, these are renamed to cap_bit0 and  you
	      should use the 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 & (((u64)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 & (((u64)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.

       data_offset (since Linux 4.1)
	      Contains the offset of the location in  the  mmap	 buffer	 where
	      perf sample data begins.

       data_size (since Linux 4.1)
	      Contains the size of the perf sample region within the mmap buf-

       aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)
	      The AUX region allows mmaping a separate sample buffer for high-
	      bandwidth	 data streams (separate from the main perf sample buf-
	      fer).  An example of  a  high-bandwidth  stream  is  instruction
	      tracing support, as is found in newer Intel processors.

	      To  set up an AUX area, first aux_offset needs to be set with an
	      offset greater than data_offset+data_size and aux_size needs  to
	      be  set to the desired buffer size.  The desired offset and size
	      must be page aligned, and the size  must	be  a  power  of  two.
	      These  values  are  then	passed to mmap in order to map the AUX
	      buffer.  Pages in the AUX buffer are included  as	 part  of  the
	      RLIMIT_MEMLOCK  resource	limit  (see setrlimit(2)), and also as
	      part of the perf_event_mlock_kb allowance.

	      By default, the AUX buffer will be truncated if it will not  fit
	      in the available space in the ring buffer.  If the AUX buffer is
	      mapped as a read only buffer, then it will operate in ring  buf-
	      fer  mode	 where	old data will be overwritten by new.  In over-
	      write mode, it might not be possible to infer where the new data
	      began, and it is the consumer's job to disable measurement while
	      reading to avoid possible data races.

	      The aux_head and aux_tail ring buffer  pointers  have  the  same
	      behavior	and ordering rules as the previous described data_head
	      and data_tail.

       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.  This allows a newer
       perf.data file to be supported  by  older  perf	tools,	with  the  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.

	      PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
		     Sample happened in the guest kernel.

	      PERF_RECORD_MISC_GUEST_USER  (since Linux 2.6.35)
		     Sample happened in guest user code.

	      Since the following three statuses are  generated	 by  different
	      record types, they alias to the same bit:

	      PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
		     This is set when the mapping is not executable; otherwise
		     the mapping is executable.

	      PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
		     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.

	      PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
		     record  is generated, this bit indicates that the context
		     switch is away from the current process (instead of  into
		     the current process).

	      In addition, the following bits can be set:

		     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.

	      PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
		     This  indicates  there  is	 extended data available (cur-
		     rently not used).

		     This bit is not set by the kernel.	 It  is	 reserved  for
		     the    user-space	 perf	utility	  to   indicate	  that
		     /proc/i[pid]/maps parsing was taking  too	long  and  was
		     stopped, and thus the mmap records may be truncated.

       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];
			  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 &&
						     size != 0 */
			  u64	 weight;	  /* if PERF_SAMPLE_WEIGHT */
			  u64	 data_src;	  /* if PERF_SAMPLE_DATA_SRC */
			  u64	 transaction;	  /* if PERF_SAMPLE_TRANSACTION */
			  u64	 abi;		  /* if PERF_SAMPLE_REGS_INTR */
			  u64	 regs[weight(mask)];
						  /* if PERF_SAMPLE_REGS_INTR */

		      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

		      cycles (since Linux 4.3)
			     This reports the number of cycles	elapsed	 since
			     the previous branch stack update.

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

		      Support for mispred, predicted, and cycles is  optional;
		      if not supported, those 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,

		      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
		      size).  Note that dyn_size is omitted if size is 0.

		      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_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_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_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   the	 value

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

		      The  abi	field  is  one	of  PERF_SAMPLE_REGS_ABI_NONE,

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

		  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

	      PERF_RECORD_AUX (since Linux 4.1)

		  This record reports that new data is available in the	 sepa-
		  rate AUX buffer region.

		      struct {
			  struct perf_event_header header;
			  u64	 aux_offset;
			  u64	 aux_size;
			  u64	 flags;
			  struct sample_id sample_id;

			 offset	 in  the  AUX  mmap  region where the new data

			 size of the data made available.

		  flags	 describes the AUX update.

				if set, then the data returned	was  truncated
				to fit the available buffer size.

				if set, then the data returned has overwritten
				previous data.

	      PERF_RECORD_ITRACE_START (since Linux 4.1)

		  This	record	indicates  which  process  has	initiated   an
		  instruction  trace  event, allowing tools to properly corre-
		  late the instruction addresses in the AUX  buffer  with  the
		  proper executable.

		      struct {
			  struct perf_event_header header;
			  u32	 pid;
			  u32	 tid;

		  pid	 process  ID  of  the  thread  starting an instruction

		  tid	 thread ID  of	the  thread  starting  an  instruction

	      PERF_RECORD_LOST_SAMPLES (since Linux 4.2)

		  When	using  hardware	 sampling  (such  as  Intel PEBS) this
		  record indicates some number of samples that may  have  been

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

		  lost	 the number of potentially lost samples.

	      PERF_RECORD_SWITCH (since Linux 4.3)

		  This	record	indicates  a context switch has happened.  The
		  PERF_RECORD_MISC_SWITCH_OUT bit in the misc field  indicates
		  whether  it  was a context switch into or away from the cur-
		  rent process.

		      struct {
			  struct perf_event_header header;
			  struct sample_id sample_id;

	      PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)

		  As with PERF_RECORD_SWITCH this record indicates  a  context
		  switch  has  happened,  but  it only occurs when sampling in
		  CPU-wide mode and provides  additional  information  on  the
		  process	being	    switched	   to/from.	   The
		  PERF_RECORD_MISC_SWITCH_OUT bit in the misc field  indicates
		  whether  it  was a context switch into or away from the cur-
		  rent process.

		      struct {
			  struct perf_event_header header;
			  u32 next_prev_pid;
			  u32 next_prev_tid;
			  struct sample_id sample_id;

			 The process ID of the previous (if switching  in)  or
			 next (if switching out) process on the CPU.

			 The  thread  ID  of the previous (if switching in) or
			 next (if switching out) thread on the CPU.

   Overflow handling
       Events can be set to notify when a threshold is crossed, indicating  an
       overflow.   Overflow conditions can be captured by monitoring the event
       file descriptor with poll(2), select(2), or  epoll(7).	Alternatively,
       the  overflow events can be captured via sa signal handler, by enabling
       I/O signaling on	 the  file  descriptor;	 see  the  discussion  of  the
       F_SETOWN and F_SETSIG operations in fcntl(2).

       Overflows  are  generated  only	by sampling events (sample_period must
       have a nonzero value).

       There are two ways to generate overflow notifications.

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

       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, POLL_IN is indicated, but	once  the  counter  reaches  0
       POLL_HUP is indicated and the underlying event is disabled.

       Refreshing  an event group leader refreshes all siblings and refreshing
       with a parameter of  0  currently  enables  infinite  refreshes;	 these
       behaviors are unsupported and should not be relied on.

       Starting with Linux 3.18, POLL_HUP is indicated if the event being mon-
       itored is attached to a different process and that process exits.

   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.

       Originally,  when rdpmc support was enabled, any process (not just ones
       with an active perf event) could use the rdpmc  instruction  to	access
       the  counters.	Starting with Linux 4.0, rdpmc support is only allowed
       if an event is currently enabled in a process's	context.   To  restore
       the old behavior, write the value 2 to /sys/devices/cpu/rdpmc.

   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.	An  overflow notification with
	      POLL_IN set will happen on each overflow until the count reaches
	      0;  when	that  happens a notification with POLL_HUP set is sent
	      and the event is disabled.  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.

       PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
	      This  allows attaching a Berkeley Packet Filter (BPF) program to
	      an existing kprobe tracepoint  event.   You  need	 CAP_SYS_ADMIN
	      privileges to use this ioctl.

	      The  argument  is a BPF program file descriptor that was created
	      by a previous bpf(2) system call.

   Using prctl(2)
       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   allow  only user-space measurements (default since Linux
		  1   allow both kernel and user measurements (default	before
		      Linux 4.6).
		  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).

		  This	file  sets  the	 maximum  depth of stack frame entries
		  reported when generating a call trace.

		  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/cpu/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.

		  As of Linux 4.0 the behavior has  changed,  so  that	1  now
		  means	 only  allow  access  to  processes  with  active perf
		  events, with 2 indicating the old allow-anyone-access behav-

	   /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.

       EBUSY (since Linux 4.1)
	      Returned	if  another  event already has exclusive access to the

       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-process limit  on  the  number  of
	      open file descriptors will be reached, and no more events can be

       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.

       EOVERFLOW (since Linux 4.8)
	      Returned	if  PERF_SAMPLE_CALLCHAIN  is	requested   and	  sam-
	      ple_max_stack   is   larger   than   the	maximum	 specified  in

       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

       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.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 4.10 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				  2016-12-12		    PERF_EVENT_OPEN(2)