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perf_counter: documentation update

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Update the documentation to reflect the current state of affairs

[ Impact: documentation update ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090501102533.296727903@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2009-05-01 12:23:19 +02:00 committed by Ingo Molnar
parent 585e3374d9
commit e5791a808a
1 changed files with 215 additions and 49 deletions
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264
Documentation/perf_counter/design.txt
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@ -34,41 +34,47 @@ can be poll()ed.
When creating a new counter fd, 'perf_counter_hw_event' is:
/*
* Event to monitor via a performance monitoring counter:
*/
struct perf_counter_hw_event {
__u64 event_config;
/*
* The MSB 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.
*/
__u64 config;
__u64 irq_period;
__u64 record_type;
__u64 read_format;
__u64 irq_period;
__u32 record_type;
__u32 read_format;
__u64 disabled : 1, /* off by default */
nmi : 1, /* NMI sampling */
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, /* ditto kernel */
exclude_hv : 1, /* ditto hypervisor */
exclude_idle : 1, /* don't count when idle */
__u64 disabled : 1, /* off by default */
nmi : 1, /* NMI sampling */
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, /* ditto kernel */
exclude_hv : 1, /* ditto hypervisor */
exclude_idle : 1, /* don't count when idle */
mmap : 1, /* include mmap data */
munmap : 1, /* include munmap data */
comm : 1, /* include comm data */
__reserved_1 : 55;
__reserved_1 : 52;
__u32 extra_config_len;
__u32 extra_config_len;
__u32 wakeup_events; /* wakeup every n events */
__u32 __reserved_4;
__u64 __reserved_2;
__u64 __reserved_3;
__u64 __reserved_2;
__u64 __reserved_3;
};
The 'event_config' field specifies what the counter should count. It
The 'config' field specifies what the counter should count. It
is divided into 3 bit-fields:
raw_type: 1 bit (most significant bit) 0x8000_0000_0000_0000
type: 7 bits (next most significant) 0x7f00_0000_0000_0000
event_id: 56 bits (least significant) 0x00ff_0000_0000_0000
raw_type: 1 bit (most significant bit) 0x8000_0000_0000_0000
type: 7 bits (next most significant) 0x7f00_0000_0000_0000
event_id: 56 bits (least significant) 0x00ff_ffff_ffff_ffff
If 'raw_type' is 1, then the counter will count a hardware event
specified by the remaining 63 bits of event_config. The encoding is
@ -134,41 +140,56 @@ enum sw_event_ids {
PERF_COUNT_PAGE_FAULTS_MAJ = 6,
};
Counters of the type PERF_TYPE_TRACEPOINT are available when the ftrace event
tracer is available, and event_id values can be obtained from
/debug/tracing/events/*/*/id
Counters come in two flavours: counting counters and sampling
counters. A "counting" counter is one that is used for counting the
number of events that occur, and is characterised by having
irq_period = 0 and record_type = PERF_RECORD_SIMPLE. A read() on a
counting counter simply returns the current value of the counter as
an 8-byte number.
irq_period = 0.
A read() on a counter returns the current value of the counter and possible
additional values as specified by 'read_format', each value is a u64 (8 bytes)
in size.
/*
* Bits that can be set in hw_event.read_format to request that
* reads on the counter should return the indicated quantities,
* in increasing order of bit value, after the counter value.
*/
enum perf_counter_read_format {
PERF_FORMAT_TOTAL_TIME_ENABLED = 1,
PERF_FORMAT_TOTAL_TIME_RUNNING = 2,
};
Using these additional values one can establish the overcommit ratio for a
particular counter allowing one to take the round-robin scheduling effect
into account.
A "sampling" counter is one that is set up to generate an interrupt
every N events, where N is given by 'irq_period'. A sampling counter
has irq_period > 0 and record_type != PERF_RECORD_SIMPLE. The
record_type controls what data is recorded on each interrupt, and the
available values are currently:
has irq_period > 0. The record_type controls what data is recorded on each
interrupt:
/*
* IRQ-notification data record type:
* Bits that can be set in hw_event.record_type to request information
* in the overflow packets.
*/
enum perf_counter_record_type {
PERF_RECORD_SIMPLE = 0,
PERF_RECORD_IRQ = 1,
PERF_RECORD_GROUP = 2,
enum perf_counter_record_format {
PERF_RECORD_IP = 1U << 0,
PERF_RECORD_TID = 1U << 1,
PERF_RECORD_TIME = 1U << 2,
PERF_RECORD_ADDR = 1U << 3,
PERF_RECORD_GROUP = 1U << 4,
PERF_RECORD_CALLCHAIN = 1U << 5,
};
A record_type value of PERF_RECORD_IRQ will record the instruction
pointer (IP) at which the interrupt occurred. A record_type value of
PERF_RECORD_GROUP will record the event_config and counter value of
all of the other counters in the group, and should only be used on a
group leader (see below). Currently these two values are mutually
exclusive, but record_type will become a bit-mask in future and
support other values.
A sampling counter has an event queue, into which an event is placed
on each interrupt. A read() on a sampling counter will read the next
event from the event queue. If the queue is empty, the read() will
either block or return an EAGAIN error, depending on whether the fd
has been set to non-blocking mode or not.
Such (and other) events will be recorded in a ring-buffer, which is
available to user-space using mmap() (see below).
The 'disabled' bit specifies whether the counter starts out disabled
or enabled. If it is initially disabled, it can be enabled by ioctl
@ -206,6 +227,13 @@ The 'exclude_user', 'exclude_kernel' and 'exclude_hv' bits provide a
way to request that counting of events be restricted to times when the
CPU is in user, kernel and/or hypervisor mode.
The 'mmap' and 'munmap' bits allow recording of PROT_EXEC mmap/munmap
operations, these can be used to relate userspace IP addresses to actual
code, even after the mapping (or even the whole process) is gone,
these events are recorded in the ring-buffer (see below).
The 'comm' bit allows tracking of process comm data on process creation.
This too is recorded in the ring-buffer (see below).
The 'pid' parameter to the perf_counter_open() system call allows the
counter to be specific to a task:
@ -250,6 +278,138 @@ 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.
Like stated, 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().
The mmap size should be 1+2^n pages, where the first page is a meta-data page
(struct perf_counter_mmap_page) that contains various bits of information such
as where the ring-buffer head is.
/*
* Structure of the page that can be mapped via mmap
*/
struct perf_counter_mmap_page {
__u32 version; /* version number of this structure */
__u32 compat_version; /* lowest version this is compat with */
/*
* 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);
*
* NOTE: for obvious reason this only works on self-monitoring
* processes.
*/
__u32 lock; /* seqlock for synchronization */
__u32 index; /* hardware counter identifier */
__s64 offset; /* add to hardware counter value */
/*
* Control data for the mmap() data buffer.
*
* User-space reading this value should issue an rmb(), on SMP capable
* platforms, after reading this value -- see perf_counter_wakeup().
*/
__u32 data_head; /* head in the data section */
};
NOTE: the hw-counter userspace bits are arch specific and are currently only
implemented on powerpc.
The following 2^n pages are the ring-buffer which contains events of the form:
#define PERF_EVENT_MISC_KERNEL (1 << 0)
#define PERF_EVENT_MISC_USER (1 << 1)
#define PERF_EVENT_MISC_OVERFLOW (1 << 2)
struct perf_event_header {
__u32 type;
__u16 misc;
__u16 size;
};
enum perf_event_type {
/*
* The MMAP events record the PROT_EXEC mappings so that we can
* correlate userspace 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_EVENT_MMAP = 1,
PERF_EVENT_MUNMAP = 2,
/*
* struct {
* struct perf_event_header header;
*
* u32 pid, tid;
* char comm[];
* };
*/
PERF_EVENT_COMM = 3,
/*
* When header.misc & PERF_EVENT_MISC_OVERFLOW the event_type field
* will be PERF_RECORD_*
*
* struct {
* struct perf_event_header header;
*
* { u64 ip; } && PERF_RECORD_IP
* { u32 pid, tid; } && PERF_RECORD_TID
* { u64 time; } && PERF_RECORD_TIME
* { u64 addr; } && PERF_RECORD_ADDR
*
* { u64 nr;
* { u64 event, val; } cnt[nr]; } && PERF_RECORD_GROUP
*
* { u16 nr,
* hv,
* kernel,
* user;
* u64 ips[nr]; } && PERF_RECORD_CALLCHAIN
* };
*/
};
NOTE: PERF_RECORD_CALLCHAIN is arch specific and currently only implemented
on x86.
Notification of new events is possible through poll()/select()/epoll() and
fcntl() managing signals.
Normally a notification is generated for every page filled, however one can
additionally set perf_counter_hw_event.wakeup_events to generate one every
so many counter overflow events.
Future work will include a splice() interface to the ring-buffer.
Counters can be enabled and disabled in two ways: via ioctl and via
prctl. When a counter is disabled, it doesn't count or generate
events but does continue to exist and maintain its count value.
@ -269,6 +429,12 @@ 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.
Additionally, non-inherited overflow counters can use
ioctl(fd, PERF_COUNTER_IOC_REFRESH, nr);
to enable a counter for 'nr' events, after which it gets disabled again.
A process can enable or disable all the counter groups that are
attached to it, using prctl: