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Commit 0f5a2601 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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perf: Avoid a useless pmu_disable() in the perf-tick 



Gleb writes:

 > Currently pmu is disabled and re-enabled on each timer interrupt even
 > when no rotation or frequency adjustment is needed. On Intel CPU this
 > results in two writes into PERF_GLOBAL_CTRL MSR per tick. On bare metal
 > it does not cause significant slowdown, but when running perf in a virtual
 > machine it leads to 20% slowdown on my machine.

Cure this by keeping a perf_event_context::nr_freq counter that counts the
number of active events that require frequency adjustments and use this in a
similar fashion to the already existing nr_events != nr_active test in
perf_rotate_context().

By being able to exclude both rotation and frequency adjustments a-priory for
the common case we can avoid the otherwise superfluous PMU disable.

Suggested-by: default avatarGleb Natapov <gleb@redhat.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-515yhoatehd3gza7we9fapaa@git.kernel.org


Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent d6c1c49d

include/linux/perf_event.h

+1 −0
Original line number Diff line number Diff line
@@ -890,6 +890,7 @@ struct perf_event_context { 
	int				nr_active;

	int				is_active;

	int				nr_stat;

	int				nr_freq;

	int				rotate_disable;

	atomic_t			refcount;

	struct task_struct		*task;

kernel/events/core.c

+32 −16
Original line number Diff line number Diff line
@@ -1130,6 +1130,8 @@ event_sched_out(struct perf_event *event, 
	if (!is_software_event(event))

		cpuctx->active_oncpu--;

	ctx->nr_active--;

	if (event->attr.freq && event->attr.sample_freq)

		ctx->nr_freq--;

	if (event->attr.exclusive || !cpuctx->active_oncpu)

		cpuctx->exclusive = 0;

}
@@ -1407,6 +1409,8 @@ event_sched_in(struct perf_event *event, 
	if (!is_software_event(event))

		cpuctx->active_oncpu++;

	ctx->nr_active++;

	if (event->attr.freq && event->attr.sample_freq)

		ctx->nr_freq++;



	if (event->attr.exclusive)

		cpuctx->exclusive = 1;
@@ -2329,6 +2333,9 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx, u64 period) 
	u64 interrupts, now;

	s64 delta;



	if (!ctx->nr_freq)

		return;



	list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {

		if (event->state != PERF_EVENT_STATE_ACTIVE)

			continue;
@@ -2384,12 +2391,14 @@ static void perf_rotate_context(struct perf_cpu_context *cpuctx) 
{

	u64 interval = (u64)cpuctx->jiffies_interval * TICK_NSEC;

	struct perf_event_context *ctx = NULL;

	int rotate = 0, remove = 1;

	int rotate = 0, remove = 1, freq = 0;



	if (cpuctx->ctx.nr_events) {

		remove = 0;

		if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)

			rotate = 1;

		if (cpuctx->ctx.nr_freq)

			freq = 1;

	}



	ctx = cpuctx->task_ctx;
@@ -2397,17 +2406,23 @@ static void perf_rotate_context(struct perf_cpu_context *cpuctx) 
		remove = 0;

		if (ctx->nr_events != ctx->nr_active)

			rotate = 1;

		if (ctx->nr_freq)

			freq = 1;

	}



	if (!rotate && !freq)

		goto done;



	perf_ctx_lock(cpuctx, cpuctx->task_ctx);

	perf_pmu_disable(cpuctx->ctx.pmu);



	if (freq) {

		perf_ctx_adjust_freq(&cpuctx->ctx, interval);

		if (ctx)

			perf_ctx_adjust_freq(ctx, interval);

	}



	if (!rotate)

		goto done;



	if (rotate) {

		cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);

		if (ctx)

			ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
@@ -2417,13 +2432,14 @@ static void perf_rotate_context(struct perf_cpu_context *cpuctx) 
			rotate_ctx(ctx);



		perf_event_sched_in(cpuctx, ctx, current);

	}



	perf_pmu_enable(cpuctx->ctx.pmu);

	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);



done:

	if (remove)

		list_del_init(&cpuctx->rotation_list);



	perf_pmu_enable(cpuctx->ctx.pmu);

	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);

}



void perf_event_task_tick(void)