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

NAME
       perf_event_open - set up performance monitoring

SYNOPSIS
       #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.

DESCRIPTION
       Given a list of parameters, perf_event_open() returns a file descriptor, for use in subse-
       quent system calls (read(2), mmap(2), prctl(2), fcntl(2), etc.).

       A call to perf_event_open() creates a file descriptor that  allows  measuring  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  measure-
       ments to a buffer that can then be accessed via mmap(2).

   Arguments
       The  argument pid allows events to be attached to processes in various ways.  If pid is 0,
       measurements happen on the current thread, if pid is greater than 0, the process indicated
       by pid is measured, and if pid is -1, all processes are counted.

       The  cpu  argument allows measurements to be specific to a CPU.	If cpu is greater than or
       equal to 0, measurements are restricted to the specified CPU; if cpu is -1, the events are
       measured on all CPUs.

       Note that the combination of pid == -1 and cpu == -1 is not valid.

       A  pid > 0 and cpu == -1 setting measures per-process and follows that process to whatever
       CPU the process gets scheduled to.  Per-process events can be created by any user.

       A pid == -1 and cpu >= 0 setting is per-CPU and measures all processes  on  the	specified
       CPU.    Per-CPU	 events   need	 the   CAP_SYS_ADMIN   capability   or	a  /proc/sys/ker-
       nel/perf_event_paranoid value of less than 1.

       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 fd 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), etc., with each other, since they have counted
       events for the same set of executed instructions.

       The flags argument is formed by ORing together zero or more of the following values:

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

       PERF_FLAG_FD_OUTPUT
	      This flag re-routes 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 grain 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  identi-
	      fied  by passing a file descriptor opened on its directory in the cgroupfs filesys-
	      tem.  For instance, if the cgroup to monitor 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 information 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 */
			 __reserved_1	: 41;

	       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:

	      PERF_TYPE_HARDWARE
		     This indicates one of the "generalized" hardware events provided by the ker-
		     nel.  See the config field definition for more details.

	      PERF_TYPE_SOFTWARE
		     This  indicates  one  of  the software-defined events provided by the kernel
		     (even if no hardware support is available).

	      PERF_TYPE_TRACEPOINT
		     This indicates a tracepoint provided by the  kernel  tracepoint  infrastruc-
		     ture.

	      PERF_TYPE_HW_CACHE
		     This  indicates  a  hardware  cache  event.   This  has  a special encoding,
		     described in the config field definition.

	      PERF_TYPE_RAW
		     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 instruc-
		     tion address.

	      dynamic PMU
		     Since Linux 2.6.39, perf_event_open() can support multiple PMUs.  To  enable
		     this,  a value exported by the kernel can be used in the type field to indi-
		     cate 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 sub-directory 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 compilation.

	      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 addi-
	      tion of branch sampling in Linux 3.4.  PERF_ATR_SIZE_VER3 is  96	corresponding  to
	      the addition of sample_regs_user and sample_stack_user in Linux 3.7.

       config This  specifies which event you want, in conjunction with the type field.  The con-
	      fig1 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 depen-
	      dent.

	      The most significant bit (bit 63) of config signifies  CPU-specific  (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 dependent 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 generalized hardware CPU
	      events.  Not all of these are available on all platforms.  Set config to one of the
	      following:

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

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

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

		   PERF_COUNT_HW_CACHE_MISSES
			  Cache  misses.  Usually this indicates Last Level Cache misses; this is
			  intended to be used in conjunction with the  PERF_COUNT_HW_CACHE_REFER-
			  ENCES event to calculate cache miss rates.

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

		   PERF_COUNT_HW_BRANCH_MISSES
			  Mispredicted branch instructions.

		   PERF_COUNT_HW_BUS_CYCLES
			  Bus cycles, which can be different from total cycles.

		   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:

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

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

		   PERF_COUNT_SW_PAGE_FAULTS
			  This reports the number of page faults.

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

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

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

		   PERF_COUNT_SW_PAGE_FAULTS_MAJ
			  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
			  x86).

		   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 negatively impact performance.

	      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 kernel.

	      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:

		      PERF_COUNT_HW_CACHE_L1D
			     for measuring Level 1 Data Cache

		      PERF_COUNT_HW_CACHE_L1I
			     for measuring Level 1 Instruction Cache

		      PERF_COUNT_HW_CACHE_LL
			     for measuring Last-Level Cache

		      PERF_COUNT_HW_CACHE_DTLB
			     for measuring the Data TLB

		      PERF_COUNT_HW_CACHE_ITLB
			     for measuring the Instruction TLB

		      PERF_COUNT_HW_CACHE_BPU
			     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

		      PERF_COUNT_HW_CACHE_OP_READ
			     for read accesses

		      PERF_COUNT_HW_CACHE_OP_WRITE
			     for write accesses

		      PERF_COUNT_HW_CACHE_OP_PREFETCH
			     for prefetch accesses

		  and perf_hw_cache_op_result_id is one of

		      PERF_COUNT_HW_CACHE_RESULT_ACCESS
			     to measure accesses

		      PERF_COUNT_HW_CACHE_RESULT_MISS
			     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  imple-
	      mentation  defined; see your CPU manual (for example the Intel Volume 3B documenta-
	      tion or the AMD BIOS and Kernel Developer Guide).  The libpfm4 library can be  used
	      to  translate  from  the	name  in  the  architectural 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  over-
	      flow  interrupt  occurs,	requested  data is recorded in the mmap buffer.  The sam-
	      ple_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.

       sample_type
	      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 order.

	      PERF_SAMPLE_IP
		     Records instruction pointer.

	      PERF_SAMPLE_TID
		     Records the process and thread IDs.

	      PERF_SAMPLE_TIME
		     Records a timestamp.

	      PERF_SAMPLE_ADDR
		     Records an address, if applicable.

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

	      PERF_SAMPLE_CALLCHAIN
		     Records the callchain (stack backtrace).

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

	      PERF_SAMPLE_CPU
		     Records CPU number.

	      PERF_SAMPLE_PERIOD
		     Records the current sampling period.

	      PERF_SAMPLE_STREAM_ID
		     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 PERF_FORMAT_ID.

	      PERF_SAMPLE_RAW
		     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  sam-
		     pling  hardware  (such  as Intel Last Branch Record).  Not all hardware sup-
		     ports 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  sam-
		     pled event was.  This allows the hardware 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.

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

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

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

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

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

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

       inherit
	      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
	      children).

	      Inherit  does  not  work	for  some combinations of read_formats, such as PERF_FOR-
	      MAT_GROUP.

       pinned The pinned bit specifies that the counter should always be on the  CPU  if  at  all
	      possible.   It  applies  only to hardware counters and only to group leaders.  If a
	      pinned counter cannot be put onto the CPU (e.g., because there are not enough hard-
	      ware  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 subsequently enabled or disabled.

       exclusive
	      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  monitor-
	      ing  programs  to  support  PMU features that need to run alone so that they do not
	      disrupt other hardware counters.

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

       exclude_kernel
	      If this bit is set, the count excludes events that happen in kernel-space.

       exclude_hv
	      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.

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

       mmap   The mmap bit enables recording of exec mmap events.

       comm   The comm bit enables tracking of process command name as modified  by  the  exec(2)
	      and  prctl(PR_SET_NAME)  system calls.  Unfortunately for tools, there is no way to
	      distinguish one system call versus the other.

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

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

       enable_on_exec
	      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.

       watermark
	      If set, have a sampling interrupt happen when we cross the wakeup_watermark  bound-
	      ary.  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 corre-
	      spond 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 PERF_RECORD_MISC_EXACT_IP.

       mmap_data (Since Linux 2.6.36)
	      The  counterpart	of  the mmap field, but enables including data mmap events in the
	      ring-buffer.

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

       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.

       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
	      bitflag.

	      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:

	      HW_BREAKPOINT_EMPTY
		     no breakpoint

	      HW_BREAKPOINT_R
		     count when we read the memory location

	      HW_BREAKPOINT_W
		     count when we write the memory location

	      HW_BREAKPOINT_RW
		     count when we read or write the memory location

	      HW_BREAKPOINT_X
		     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 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  OFFCORE_EVENTS  on  Nehalem/Westmere/Sandy-
	      Bridge 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_BREAK-
	      POINT.  Options are HW_BREAKPOINT_LEN_1, HW_BREAKPOINT_LEN_2,  HW_BREAKPOINT_LEN_4,
	      HW_BREAKPOINT_LEN_8.  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.  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.  The value is formed by ORing together zero or  more  of  the
	      following values, although PERF_SAMPLE_BRANCH_ANY covers all branch types.

	      PERF_SAMPLE_BRANCH_USER
		     Branch target is in user space

	      PERF_SAMPLE_BRANCH_KERNEL
		     Branch target is in kernel space

	      PERF_SAMPLE_BRANCH_HV
		     Branch target is in hypervisor

	      PERF_SAMPLE_BRANCH_ANY
		     Any branch type.

	      PERF_SAMPLE_BRANCH_ANY_CALL
		     Any call branch

	      PERF_SAMPLE_BRANCH_ANY_RETURN
		     Any return branch

	      PERF_SAMPLE_BRANCH_IND_CALL
		     Indirect calls

	      PERF_SAMPLE_BRANCH_PLM_ALL
		     User, kernel, and hv

       sample_regs_user (Since Linux 3.7)
	      This  bitmask defines the set of user CPU registers to dump on samples.  The layout
	      of the register mask is architecture specific and described in  the  kernel  header
	      arch/ARCH/include/uapi/asm/perf_regs.h.

       sample_stack_user (Since Linux 3.7)
	      This defines the size of the user stack to dump if PERF_SAMPLE_STACK_USER is speci-
	      fied.

   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 */
		 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.  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 than 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 information 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;
		   __u64   cap_usr_time  : 1,
			   cap_usr_rdpmc : 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 looks at the fields in the perf_event_mmap_page structure in more detail:

       version
	      Version number of this structure.

       compat_version
	      The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset Add this to hardware counter value??

       time_enabled
	      Time the event was active.

       time_running
	      Time the event was running.

       cap_usr_time
	      User time capability

       cap_usr_rdpmc
	      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;
		  s64 pmc = 0;

		  do {
		      seq = pc->lock;
		      barrier();
		      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;
			  pmc = rdpmc(idx - 1);
		      }

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

       pmc_width
	      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 similar.

		  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;

       data_head
	      This  points to the head of the data section.  The value continuously increases, it
	      does not wrap.  The value needs to be manually wrapped by the size of the mmap buf-
	      fer before accessing the samples.

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

       data_tail;
	      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 over-write 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, i.e., at the end
       of the payload.	That way a newer perf.data 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.

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

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

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

	      PERF_RECORD_LOST
		  This record indicates when events are lost.

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

		  id	 is the unique event ID for the samples that were lost.

		  lost	 is the number of events that were lost.

	      PERF_RECORD_COMM
		  This record indicates a change in the process name.

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

	      PERF_RECORD_EXIT
		  This record indicates a process exit event.

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

	      PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
		  This record indicates a throttle/unthrottle event.

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

	      PERF_RECORD_FORK
		  This record indicates a fork event.

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

	      PERF_RECORD_READ
		  This record indicates a read event.

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

	      PERF_RECORD_SAMPLE
		  This record indicates a sample.

		      struct {
			  struct perf_event_header header;
			  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 */
		      };

		  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.

		  time	 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.

		  addr	 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.

		  stream_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.

		  period If PERF_SAMPLE_PERIOD is enabled, a 64-bit value indicating 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 number is included
			 which indicates how many following 64-bit instruction pointers will fol-
			 low.  This is the current callchain.

		  size, data[size]
			 If  PERF_SAMPLE_RAW  is  enabled, then a 32-bit value indicating size is
			 included followed by an array of 8-bit values of length size.	The  val-
			 ues 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,  followed  by  bnr	perf_branch_entry
			 structures which each include the fields:

			 from	indicating the source instruction (may not be a branch)

			 to	the branch target

			 mispred
				the branch target was mispredicted

			 predicted
				the branch target was predicted.
		  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.

		  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_SAM-
			 PLE_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 sample_regs_user bitmask.

		  size, data[size], dyn_size
			 If  PERF_SAMPLE_STACK_USER  is  enabled,  then  record the user stack to
			 enable  backtracing.	size  is  the  size  requested	by  the  user  in
			 stack_user_size  or  else  the  maximum  record size.	data is the stack
			 data.	dyn_size is the amount of data actually dumped (can be less  than
			 size).

		  weight 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.

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

			 mem_op 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), and PERF_MEM_OP_EXEC (executable code).

			 mem_lvl
				memory	hierarchy  level  hit  or  miss, a bitwise combination of
				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),  and  PERF_MEM_LVL_UNC
				(uncached memory).

			 mem_snoop
				snoop mode,  a	bitwise  combination  of  PERF_MEM_SNOOP_NA  (not
				available),  PERF_MEM_SNOOP_NONE  (no  snoop), PERF_MEM_SNOOP_HIT
				(snoop	  hit),    PERF_MEM_SNOOP_MISS	  (snoop    miss),    and
				PERF_MEM_SNOOP_HITM (snoop hit modified).

			 mem_lock
				lock  instruction, a bitwise combination of PERF_MEM_LOCK_NA (not
				available) and PERF_MEM_LOCK_LOCKED (locked transaction).

			 mem_dtlb
				tlb access hit or miss, a bitwise combination of  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), and PERF_MEM_TLB_OS (OS
				fault handler).

       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 following (note these are
	      not bit masks, only one can be set at a time):

	      PERF_RECORD_MISC_CPUMODE_UNKNOWN
		     Unknown CPU mode.

	      PERF_RECORD_MISC_KERNEL
		     Sample happened in the kernel.

	      PERF_RECORD_MISC_USER
		     Sample happened in user code.

	      PERF_RECORD_MISC_HYPERVISOR
		     Sample happened in the hypervisor.

	      PERF_RECORD_MISC_GUEST_KERNEL
		     Sample happened in the guest kernel.

	      PERF_RECORD_MISC_GUEST_USER
		     Sample happened in guest user code.

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

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

	      PERF_RECORD_MISC_EXACT_IP
		     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
		     This indicates there is extended data available (currently not used).

       size   This indicates the size of the record.

   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 non-zero 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 non-zero, a POLL_IN signal is sent on
       overflow, but once the value reaches 0, a signal is sent of type POLL_HUP and the underly-
       ing event is disabled.

       Note: on newer kernels (definitely noticed with 3.2) a signal is provided for every  over-
       flow, 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; documenta-
       tion 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

       PERF_EVENT_IOC_ENABLE
	      Enables the individual event or event group specified by the file descriptor  argu-
	      ment.

	      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 specified is not the  group	leader	(but  see
	      BUGS).

       PERF_EVENT_IOC_DISABLE
	      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 disabling 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 specified is not the group leader (but see
	      BUGS).

       PERF_EVENT_IOC_REFRESH
	      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  happen  on  each overflow until the count reaches 0; when that happens a
	      signal with POLL_HUP set is sent and the event is disabled.  Using an argument of 0
	      is considered undefined behavior.

       PERF_EVENT_IOC_RESET
	      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 specified is  not  the  group	leader	(but  see
	      BUGS).

       PERF_EVENT_IOC_PERIOD
	      IOC_PERIOD  is  the  command  to	update the period; it does not update the current
	      period but instead defers until next.

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

       PERF_EVENT_IOC_SET_OUTPUT
	      This tells the kernel to report event notifications to the specified file  descrip-
	      tor 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.

   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 current process, whether created	by  this  process  or  by
       another,  and  does  not  affect  any counters that this process has created on other pro-
       cesses.	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/

	   /proc/sys/kernel/perf_event_paranoid

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

		  2 - only allow user-space measurements

		  1 - (default) allow both kernel and user measurements

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

		  -1 - no restrictions

		  The existence of the perf_event_paranoid file is the official method for deter-
		  mining if a kernel supports perf_event_open().

	   /proc/sys/kernel/perf_event_max_sample_rate

		  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).

	   /proc/sys/kernel/perf_event_mlock_kb

		  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 available 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 you wish to use this PMU.

	   /sys/bus/event_source/devices/*/rdpmc (Since Linux 3.4)
		  If  this  file  is  1, then direct user-space access to the performance counter
		  registers is allowed via the rdpmc instruction.  This can be disabled by  echo-
		  ing 0 to the file.

	   /sys/bus/event_source/devices/*/format/ (Since Linux 3.4)
		  This sub-directory contains information on the architecture-specific sub-fields
		  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
		  perf_event_attr::config1.

	   /sys/bus/event_source/devices/*/events/ (Since Linux 3.4)
		  This	sub-directory  contains  files with pre-defined 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 com-
		  plete 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 separated by commas.
		  Each entry has an optional value (either hex or decimal).  If no value is spec-
		  ified  than it is assumed to be a single-bit field with a value of 1.  An exam-
		  ple entry may look like this: event=0x2,inv,ldlat=3

	   /sys/bus/event_source/devices/*/uevent
		  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.

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

ERRORS
       EINVAL Returned if the specified event is not available.

       ENOSPC Prior to Linux 3.3, if there  was  not  enough  room  for  the  event,  ENOSPC  was
	      returned.   Linus  did  not  like  this, and this was changed to EINVAL.	ENOSPC is
	      still returned if you try to read results into too small of a buffer.

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

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

NOTES
       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/kernel/perf_event_paranoid.

BUGS
       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  measure-
       ments.

       Prior  to  Linux  2.6.34 event constraints were not enforced by the kernel.  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 processes.

       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  non-zero
       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.

       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.

EXAMPLE
       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>

       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;
       }

       int
       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);
	      exit(EXIT_FAILURE);
	   }

	   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);

	   close(fd);
       }

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

COLOPHON
       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-07-16			       PERF_EVENT_OPEN(2)
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