perf_event_open() manpage

[Vince Weaver <vweaver1-qKp7vQ+Mknf2fBVCVOL8/A@public.gmane.org>]

Hello

I've been maintaining some perf_event syscall related programming info
for a while, and thought it might be better in manpage format.

The most recent git tree of the manpages doesn't seem to have a syscall 
manpage for perf_event_open, so I've included one below.  I apologize for 
my horrible TROFF skills.

The manpage is based on the linux/perf_event.h include file, plus
a lot of information I've learned through bitter experience over the last 
3 years.

Vince
vweaver1-qKp7vQ+Mknf2fBVCVOL8/A@public.gmane.org




.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" This manpage is Copyright (C) 2012 Vince Weaver

.TH PERF_EVENT_OPEN 2 2012-07-10 "Linux" "Linux Programmer's Manual"
.SH NAME
perf_event_open \- setup performance monitoring
.SH SYNOPSIS
.nf
.B #include <linux/perf_event.h>
.sp
.BI "int perf_event_open(struct perf_event_attr *" hw_event ", pid_t " pid ", int " cpu ", int " group_fd ", unsigned long " flags  );
.fi
.SH DESCRIPTION
Given a list of parameters
.BR perf_event_open ()
returns a file descriptor, a small, nonnegative integer
for use in subsequent system calls
.RB ( read "(2), " mmap "(2), " prctl "(2), " fcntl "(2), etc.)."
The file descriptor returned by a successful call will be
the lowest-numbered file descriptor not currently open for the process.
.PP
A call to
.BR 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.
.PP
Events can be enabled and disabled in two ways: via 
.BR ioctl (2)
and via 
.BR prctl (2) . 
When an eventset is disabled it does not count or generate events but does 
continue to exist and maintain its count value.
Events come in two flavors: counting and sampled. 
A 
.I 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 
.BR read (2)
call.
A
.I sampling
event periodically writes measurements to a buffer that can then
be accessed via
.BR  mmap (2) .
.SS Arguments
.P
The argument
.I pid
allows events to be attached to processes in various ways.
If
.I pid
is 
.B 0
measurements happen on the current task, if
.I pid
is 
.B "greater than 0 "
the process indicated by 
.I pid 
is measured, and if
.I pid
is 
.BR "less than 0"
all processes are counted.

The 
.I cpu
argument allows measurements to be specific to a CPU.
If
.I cpu
is 
.BR "grater than or equal to 0"
measurements are restricted to the specified CPU;
if
.I cpu
is 
.BR -1
the events are measured on all CPUs.
.P
Note that the combination of 
.IR pid "==-1"
and 
.IR cpu "==-1"
is not valid.
.P
A 
.IR pid "> 0" 
and 
.IR 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.
.P
A 
.IR pid "== -1"
and 
.IR cpu ">= 0"
event is per-CPU and measures all processes on  the specified CPU. 
Per-CPU events need 
.B CAP_SYS_ADMIN 
privileges. 
.P
The 
.I group_fd 
argument allows counter groups to be set up. 
A counter group has one counter which is the group leader. 
The leader is created first, with 
.IR group_fd "= -1"
in the 
.BR perf_event_open ()
call that creates it. 
The rest of the group members are created subsequently, with 
.IR group_fd 
giving the fd of the group leader. 
(A single counter on its own is created with 
.IR group_fd "= -1"
and is considered to be a group with only 1 member.)
.P
A counter group is scheduled onto the CPU as a unit: it will only 
be put onto the CPU if all of the counters in the group can be put onto 
the CPU. 
This means that the values of the member counters 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. 
.P
The 
.I flags 
argument is not well documented.  It can be passed the values
.BR ERF_FLAG_FD_NO_GROUP , 
.BR PERF_FLAG_FD_OUTPUT ", or"
.BR PERF_FLAG_PID_CGROUP .
.P
The 
.I perf_event_attr 
structure is what is passed into the 
.BR perf_event_open ()
syscall. 
It is large and has a complicated set of dependent fields.

.IR "__u32 type;"
.TP
.B PERF_TYPE_HARDWARE
chooses one of the "generalized" hardware events provided by the kernel. 
See the 
.I config 
field definition for more details.
.TP
.B PERF_TYPE_SOFTWARE
chooses one of the software-defined events provided by the kernel 
(even if no HW support available).
.TP
.B PERF_TYPE_TRACEPOINT
provided by the ftrace infrastructure?
.TP
.B PERF_TYPE_HW_CACHE 
these are hardware events but require a special encoding.
.TP
.B PERF_TYPE_RAW
allows programming a "raw" implementation-specific event in the 
.IE config field.
.TP
.B PERF_TYPE_BREAKPOINT
breakpoint events provided by the kernel?
.TP
.B CUSTOM PMU
It's not documented very well, but as of 2.6.39 perf_event can support 
multiple PMUs. 
Which one is chosen is handled by putting its PMU number in this field. 
A list of available PMUs can be found in a sysfs file somewhere.

.TP
.IR "__u32 size;"
Place in here the size of
.IR perf_event_attr structure
for forward/backward compatibility. 
Set this using sizeof(struct perf_event_attr) to allow the kernel to see 
what size the struct was at compile time; this apparently help provide 
some sort of backward compatibility.

The define 
.B PERF_ATTR_SIZE_VER0 
is set to 64; this was the sizeof the first published struct.

.TP
.IR "__u64 config;"

This specifies exactly which event you want, in conjunction with 
the type field. 
The 
.IR config1 and config2
fields are also taken into account in cases where 64 bits is not enough.

If a CPU is not able to count the selected event, then the system 
call will return 
.BR EINVAL .

The most significant bit (bit 63) of the config word signifies 
if the rest contains cpu specific (raw) counter configuration data;
if unset, the next 7 bits are an event type and the rest of the bits 
are the event identifier. (is this still true?)

.P
for 
.B PERF_TYPE_HARDWARE
.TP
.B PERF_COUNT_HW_CPU_CYCLES 
total cycles? be wary of what happens during cpu frequency scaling
.TP
.B PERF_COUNT_HW_INSTRUCTIONS
retired instructions. Be careful, these can be affected by various 
issues, most notably hardware interrupt counts
.TP
.B PERF_COUNT_HW_CACHE_REFERENCES
in this case Last Level Cache. Unclear if this should count 
prefetches and coherency messages.
.TP
.B PERF_COUNT_HW_CACHE_MISSES
in this case Last Level Cache. Unclear if this should count 
prefetches and coherency messages.
.TP
.B PERF_COUNT_HW_BRANCH_INSTRUCTIONS
.TP
.B PERF_COUNT_HW_BRANCH_MISSES
.TP
.B PERF_COUNT_HW_BUS_CYCLES
.TP
.B PERF_COUNT_HW_STALLED_CYCLES_FRONTEND
.TP
.B PERF_COUNT_HW_STALLED_CYCLES_BACKEND 

.P
for
.B PERF_TYPE_SOFTWARE
.TP
.B PERF_COUNT_SW_CPU_CLOCK
.TP
.B PERF_COUNT_SW_TASK_CLOCK
.TP
.B PERF_COUNT_SW_PAGE_FAULTS
.TP
.B PERF_COUNT_SW_CONTEXT_SWITCHES
.TP
.B PERF_COUNT_SW_CPU_MIGRATIONS
.TP
.B PERF_COUNT_SW_PAGE_FAULTS_MIN
.TP
.B PERF_COUNT_SW_PAGE_FAULTS_MAJ
.TP
.B PERF_COUNT_SW_ALIGNMENT_FAULTS
.TP
.B PERF_COUNT_SW_EMULATION_FAULTS 

.P
for
.B PERF_TYPE_TRACEPOINT
these are available when the ftrace event tracer is available, 
and 
.I config
values can be obtained from 
.I /debug/tracing/events/*/*/id

.P
for
.B PERF_TYPE_HW_CACHE
To calculate the 
.I config 
value for these, take 
(perf_hw_cache_id) | (perf_hw_cache_op_id << 8) | 
(perf_hw_cache_op_result_id << 16)
.P
perf_hw_cache_id
.TP
.B PERF_COUNT_HW_CACHE_L1D
.TP
.B PERF_COUNT_HW_CACHE_L1I
.TP
.B PERF_COUNT_HW_CACHE_LL
.TP
.B PERF_COUNT_HW_CACHE_DTLB
.TP
.B PERF_COUNT_HW_CACHE_ITLB
.TP
.B PERF_COUNT_HW_CACHE_BPU 
.P
perf_hw_cache_op_id
.TP
.B PERF_COUNT_HW_CACHE_OP_READ
.TP
.B PERF_COUNT_HW_CACHE_OP_WRITE
.TP
.B PERF_COUNT_HW_CACHE_OP_PREFETCH 
.P
perf_hw_cache_op_result_id
.TP
.B PERF_COUNT_HW_CACHE_RESULT_ACCESS
.TP            
.B PERF_COUNT_HW_CACHE_RESULT_MISS 
.P
for
.B  PERF_TYPE_RAW
Most CPUs support events that are not covered by the "generalized" events. 
These are implementation defined; see your CPU manual. 
The libpfm4 library can help you translate from the name in the 
architectural manuals to the raw hex value perf_events 
expects in this field.

.P
for
.B PERF_TYPE_BREAKPOINT

.TP
.IR "union { __u64 sample_period; __u64 sample_freq; };"
A "sampling" counter is one that is set up to generate an interrupt 
every N events, where N is given by 
.IR sample_period . 
A sampling counter has 
.IR sample_period "> 0." 
The 
.IR sample_type field 
controls what data is recorded on each interrupt.

.TP
.IR "__u64 sample_type;"
Various bits can be set here to request info in the overflow packets.
.TP
.B PERF_SAMPLE_IP
.TP
.B PERF_SAMPLE_TID
.TP
.B PERF_SAMPLE_TIME
.TP
.B PERF_SAMPLE_ADDR
.TP
.B PERF_SAMPLE_READ
.TP
.B PERF_SAMPLE_CALLCHAIN
.TP
.B PERF_SAMPLE_ID
.TP
.B PERF_SAMPLE_CPU
.TP
.B PERF_SAMPLE_PERIOD
.TP
.B PERF_SAMPLE_STREAM_ID
.TP
.B PERF_SAMPLE_RAW 
Such (and other) events will be recorded in a ring-buffer, 
which is available to user-space using 
.BR mmap (2)

.TP
.IR "__u64 read_format;"
Specifies the format of the data returned by 
.BR read (2) 
on a perf event fd.
.TP
.B PERF_FORMAT_TOTAL_TIME_ENABLED
Adds the 64-bit "time_enabled" field. 
Can be used to calculate estimated totals if multiplexing is happening 
and an event is being scheduled round-robin.
.TP
.B PERF_FORMAT_TOTAL_TIME_RUNNING
Adds the 64-bit "time_running" field. 
Can be used to calculate estimated totals if multiplexing is happening 
and an event is being scheduled round-robin.
.TP
.B PERF_FORMAT_ID
Adds a 64-bit unique value that corresponds to the event-group.
.TP
.B PERF_FORMAT_GROUP
Allows all counter values in an event-group to be read with one read. 

.TP
.IR "__u64 disabled; (bitfield)"
The 
.I disabled
bit specifies whether the counter starts out disabled or enabled
(disabled is the default). 
If disabled, the event can later be enabled by 
.BR ioctl (2)
or 
.BR prctl (2).

.TP
.IR "__u64 inherit; (bitfield)"
The 
.I inherit 
bit specifies that this counter should count events of child
tasks as well as the task specified. 
This only applies 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 all combinations of read_formats, such as 
.BR PERF_FORMAT_GROUP .

.TP
.IR "__u64 pinned; (bitfield)"
The 
.I pinned 
bit specifies that the counter should always be on the CPU if at all 
possible. 
It only applies 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 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. 
.BR read (2) 
returns 0) until the counter is subsequently enabled or disabled.

.TP
.IR "__u64 exclusive; (bitfield)"
The 
.I 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 supply extra 
configuration information via 'extra_config_len' to exploit advanced 
features of the CPU's Performance Monitor Unit (PMU) that are not 
otherwise accessible and that might disrupt other hardware counters.

.TP
.IR "__u64 exclude_user; (bitfield)"
If set the count excludes events that happen in user-space.

.TP
.IR "__u64 exclude_kernel; (bitfield)"
If set the count excludes events that happen in kernel-space.

.TP
.IR "__u64 exclude_hv; (bitfield)"
If 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.

.TP
.IR "__u64 exclude_idle; (bitfield)"
If set don't count when the CPU is idle.

.TP
.IR "__u64 mmap; (bitfield)"
The 
.I mmap
bit allow recording of things like userspace IP addresses to 
a ring-buffer (described below in subsection MMAP).

.TP
.IR "__u64 comm; (bitfield)"
The 
.I comm bit allows tracking of process comm data on process creation. 
This is recorded in the ring-buffer.

.TP
.IR "__u64 freq; (bitfield)"
Use frequency, not period, when sampling.

.TP
.IR "__u64 inherit_stat; (bitfield)"
per task counts???

.TP
.IR "__u64 enable_on_exec; (bitfield)"
next exec enables???

.TP
.IR "__u64 task; (bitfield)"
trace fork/exit???

.TP
.IR "__u64 watermark; (bitfield)"
If set, have a sampling interrupt happen when we cross the wakeup_watermark 
boundary.

.TP
.IR "__u64 precise_ip; (bitfield)"
The values of this are the following:
.TP
0 - SAMPLE_IP can have arbitrary skid
.TP
1 - SAMPLE_IP must have constant skid
.TP
2 - SAMPLE_IP requested to have 0 skid
.TP
3 - SAMPLE_IP must have 0 skid 
See also PERF_RECORD_MISC_EXACT_IP

.TP
.IR "__u64 mmap_data; (bitfield)"
non-exec mmap data???

.TP
.IR "__u64 sample_id_all; (bitfield)"
sample_type all events

.TP
.IR "union { __u32 wakeup_events; __u32 wakeup_watermark; };"
This union sets how many events (wakeup_events) or bytes 
(wakeup_watermark) happen before an overflow signal happens. 
Which one is used is selected by the 
.IR watermark bit.

.TP
.IR "__u32 bp_type;"
Breakpoint code???

.TP
.IR "union {__u64 bp_addr; __u64 config1;}"
.I bp_addr 
probably has to do with the breakpoint code.

.I 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/SandyBridge uses this field 
on 3.3 and later kernels.

.TP
.IR "union { __u64 bp_len; __u64 config2; };"
.I bp_len 
probably has to do with the breakpoint code.

.I config2 
is a further extension of the config register.

.SS "MMAP Layout"

Asynchronous events, like counter overflow or PROT_EXEC mmap tracking 
are logged into a ring-buffer. 
This ring-buffer is created and accessed through 
.BR mmap (2).

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

There is a bug previous to 2.6.39 where you have to allocate a mmap 
ring buffer when sampling even if you do not use it at all.

Structure of the first meta-data mmap page

    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;
       __u64 __reserved[123];
         1k-aligned hole for extension of the self monitor capabilities
       __u64 data_head;       /* head in the data section           */

User-space reading the data_head value should issue an rmb(), 
on SMP capable platforms, after reading this value.

When the mapping is PROT_WRITE the data_tail value should be written by 
userspace to reflect the last read data. 
In this case the kernel will not over-write unread data.

       __u64 data_tail;       /* user-space written tail            */

.\"         * Bits needed to read the hw counters in user-space.
.\"         *
.\"         *   u32 seq;
.\"         *   s64 count;
.\"         *
.\"         *   do {
.\"         *     seq = pc->lock;
.\"         *
.\"         *     barrier()
.\"         *     if (pc->index) {
.\"         *       count = pmc_read(pc->index - 1);
.\"         *       count += pc->offset;
.\"         *     } else
.\"         *       goto regular_read;
.\"         *
.\"         *     barrier();
. \"         *   } while (pc->lock != seq);

Structure of the following 2^n ring-buffer pages

struct perf_event_header {

    __u32 type;
    
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 events record the PROT_EXEC mappings so that we can correlate 
userspace IPs to code. They have the following structure:
        PERF_RECORD_MMAP
        struct {
            struct perf_event_header header;
            u32 pid, tid;
            u64 addr;
            u64 len;
            u64 pgoff;
            char filename[]; 
        };

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

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

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

        PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
        struct {
            struct perf_event_header header;
            u64 time;
            u64 id;
            u64 stream_id; 
        };

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

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

        PERF_RECORD_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 values;
            if PERF_SAMPLE_READ

            u64 nr
            u64 ips[nr]
            if PERF_SAMPLE_CALLCHAIN

            perf_callchain_context { 
                PERF_CONTEXT_HV 
                PERF_CONTEXT_KERNEL 
                PERF_CONTEXT_USER 
                PERF_CONTEXT_GUEST 
                PERF_CONTEXT_GUEST_KERNEL 
                PERF_CONTEXT_GUEST_USER}
            ;

            u32 size;
            char data[size];
            if PERF_SAMPLE_RAW

The RAW record data is opaque wrt the ABI That is, the ABI doesn't make 
any promises wrt to the stability of its content, it may vary depending 
on event, hardware, kernel version and phase of the moon.
        }; 
    };
    __u16 misc;
        PERF_RECORD_MISC_CPUMODE_MASK
        PERF_RECORD_MISC_CPUMODE_UNKNOWN
        PERF_RECORD_MISC_KERNEL
        PERF_RECORD_MISC_USER
        PERF_RECORD_MISC_HYPERVISOR
        PERF_RECORD_MISC_GUEST_KERNEL
        PERF_RECORD_MISC_GUEST_USER
        PERF_RECORD_MISC_EXACT_IP
        
Indicates that the content of PERF_SAMPLE_IP points to the actual 
instruction that triggered the event. See also perf_event_attr::precise_ip. 
    __u16 size; 

}; 

.SS "Signal Overflow"

Counters can be set to signal when a threshold is crossed.  This is set
up using traditional poll()/select()/epoll() and fcntl() syscalls.

Normally a notification is generated for every page filled, however 
one can additionally set perf_event_attr.wakeup_events to generate one 
every so many counter overflow events.

.SS "Reading Results"
Once a perf_event fd has been opened, the values of the events can be 
read from the fd. 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,  an error is returned (prior to 3.1 this was ENOSPC).

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:
    u64 nr;
    The number of events
    u64 time_enabled;
    Only if PERF_FORMAT_ENABLED was specified
    u64 time_running;
    Only if PERF_FORMAT_RUNNING was specified
    { u64 value; u64 id;} cntr[nr];
An array of "nr" entries containing the event counts and an 
optional unique ID for that counter if the PERF_FORMAT_ID value was 
specified. 

If PERF_FORMAT_GROUP was not specified:
    u64 value;
    The value of the event.
    u64 time_enabled;
    Only if PERF_FORMAT_ENABLED was set
    u64 time_running;
    Only if PERF_FORMAT_RUNNING was set
    u64 id;
A unique value for this particular event, only there if 
PERF_FORMAT_ID was set. 

.SS "perf_event ioctl calls"
.PP
Various ioctls act on perf_event fds
.TP
.B PERF_EVENT_IOC_ENABLE
An individual counter or counter group can be enabled

.TP
.B PERF_EVENT_IOC_DISABLE
An individual counter or counter group can be disabled

Enabling or disabling the leader of a group enables or disables the 
whole 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 only 
affects that counter - disabling an non-leader 
stops that counter from counting but doesn't affect any other counter.

.TP
.B PERF_EVENT_IOC_REFRESH
Additionally, non-inherited overflow counters can use
to enable a counter for 'nr' events, after which it gets disabled again.
I think the goal of IOC_REFRESH is not to reload the period but simply to 
adjust the number of events before the next notifications.

.TP
.B PERF_EVENT_IOC_RESET

.TP
.B PERF_EVENT_IOC_PERIOD
IOC_PERIOD is the command to update the period and that's the one that
does not update the current period but instead defers until next.

.TP
.B PERF_EVENT_IOC_SET_OUTPUT

.TP
.B PERF_EVENT_IOC_SET_FILTER

.SH "Using prctl"
A process can enable or disable all the counter groups that are 
attached to it using prctl.
.I  prctl(PR_TASK_PERF_EVENTS_ENABLE)
.I  prctl(PR_TASK_PERF_EVENTS_DISABLE)
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 processes. 
It only enables or disables 
the group leaders, not any other members in the groups. 

.SH "RETURN VALUE"
.BR perf_event_open ()
returns the new file descriptor, or \-1 if an error occurred
(in which case,
.I errno
is set appropriately).
.SH ERRORS
.TP
.B EINVAL
Returned if the specified event is not available.
.TP
.B ENOSPC
Prior to 3.3 if there was no counter room ENOSPC was returned.
Also if you try to read results into a too small buffer.
Linus did not like this.

.SH NOTES
.BR perf_event_open () 
was introduced in 2.6.31 but was called
.BR perf_counter_open () .  
It was renamed in 2.6.32.

The official way of knowing if perf_event support is enabled is checking
for the existence of the file
.I /proc/sys/kernel/perf_event_paranoid 

.SH BUGS

Prior to 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.

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

Prior to 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 eventset works you have to 
.BR perf_event_open ()
, start, then read before you know for sure you
can get value measurements.

Prior to 2.6.35 PERF_FORMAT_GROUP did not work with attached
processes.

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

In older 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 2.6.36 and 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.

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

.SH EXAMPLE
The following is a short example that measures the total
instruction count of the printf routine.
.nf

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

int 
main(int argc, char **argv) {

    struct perf_event_attr pe;
    long long count;

    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<0) 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);

    close(fd);
}
.fi

.SH "SEE ALSO"
.BR fcntl (2),
.BR mmap (2),
.BR open (2),
.BR prctl (2)
.BR read (2)


--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html