Commit 8ce939a0 authored by Peter Zijlstra (Intel)'s avatar Peter Zijlstra (Intel) Committed by Peter Zijlstra
Browse files

perf: Avoid the read if the count is already updated 



The event may have been updated in the PMU-specific implementation,
e.g., Intel PEBS counters snapshotting. The common code should not
read and overwrite the value.

The PERF_SAMPLE_READ in the data->sample_type can be used to detect
whether the PMU-specific value is available. If yes, avoid the
pmu->read() in the common code. Add a new flag, skip_read, to track the
case.

Factor out a perf_pmu_read() to clean up the code.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarKan Liang <kan.liang@linux.intel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250121152303.3128733-3-kan.liang@linux.intel.com
parent f9bdf1f9

include/linux/perf_event.h

+7 −1
Original line number Diff line number Diff line
@@ -1062,7 +1062,13 @@ struct perf_output_handle { 
	struct perf_buffer		*rb;

	unsigned long			wakeup;

	unsigned long			size;

	u64				aux_flags;

	union {

		u64			flags;		/* perf_output*() */

		u64			aux_flags;	/* perf_aux_output*() */

		struct {

			u64		skip_read : 1;

		};

	};

	union {

		void			*addr;

		unsigned long		head;

kernel/events/core.c

+16 −17
Original line number Diff line number Diff line
@@ -1191,6 +1191,12 @@ static void perf_assert_pmu_disabled(struct pmu *pmu) 
	WARN_ON_ONCE(*this_cpu_ptr(pmu->pmu_disable_count) == 0);

}



static inline void perf_pmu_read(struct perf_event *event)

{

	if (event->state == PERF_EVENT_STATE_ACTIVE)

		event->pmu->read(event);

}



static void get_ctx(struct perf_event_context *ctx)

{

	refcount_inc(&ctx->refcount);
@@ -3473,8 +3479,7 @@ static void __perf_event_sync_stat(struct perf_event *event, 
	 * we know the event must be on the current CPU, therefore we

	 * don't need to use it.

	 */

	if (event->state == PERF_EVENT_STATE_ACTIVE)

		event->pmu->read(event);

	perf_pmu_read(event);



	perf_event_update_time(event);
@@ -4618,15 +4623,8 @@ static void __perf_event_read(void *info) 


	pmu->read(event);



	for_each_sibling_event(sub, event) {

		if (sub->state == PERF_EVENT_STATE_ACTIVE) {

			/*

			 * Use sibling's PMU rather than @event's since

			 * sibling could be on different (eg: software) PMU.

			 */

			sub->pmu->read(sub);

		}

	}

	for_each_sibling_event(sub, event)

		perf_pmu_read(sub);



	data->ret = pmu->commit_txn(pmu);
@@ -7444,9 +7442,8 @@ static void perf_output_read_group(struct perf_output_handle *handle, 
	if (read_format & PERF_FORMAT_TOTAL_TIME_RUNNING)

		values[n++] = running;



	if ((leader != event) &&

	    (leader->state == PERF_EVENT_STATE_ACTIVE))

		leader->pmu->read(leader);

	if ((leader != event) && !handle->skip_read)

		perf_pmu_read(leader);



	values[n++] = perf_event_count(leader, self);

	if (read_format & PERF_FORMAT_ID)
@@ -7459,9 +7456,8 @@ static void perf_output_read_group(struct perf_output_handle *handle, 
	for_each_sibling_event(sub, leader) {

		n = 0;



		if ((sub != event) &&

		    (sub->state == PERF_EVENT_STATE_ACTIVE))

			sub->pmu->read(sub);

		if ((sub != event) && !handle->skip_read)

			perf_pmu_read(sub);



		values[n++] = perf_event_count(sub, self);

		if (read_format & PERF_FORMAT_ID)
@@ -7520,6 +7516,9 @@ void perf_output_sample(struct perf_output_handle *handle, 
{

	u64 sample_type = data->type;



	if (data->sample_flags & PERF_SAMPLE_READ)

		handle->skip_read = 1;



	perf_output_put(handle, *header);



	if (sample_type & PERF_SAMPLE_IDENTIFIER)

kernel/events/ring_buffer.c

+1 −0
Original line number Diff line number Diff line
@@ -185,6 +185,7 @@ __perf_output_begin(struct perf_output_handle *handle, 


	handle->rb    = rb;

	handle->event = event;

	handle->flags = 0;



	have_lost = local_read(&rb->lost);

	if (unlikely(have_lost)) {