Commit 660536db authored Feb 13, 2019 by Raghavendra Rao Ananta Committed by Rishabh Bhatnagar Feb 14, 2019

perf: Prevent waiting for cpu to come out of idle during read



Currently if the CPU is in idle state, the perf_event_read() loops
until it's out of that state. However, the PMU driver updates the
event->count value before going into idle. Hence, the caller of
perf_event_read() can simply return without waiting for the CPU to
come out of idle state and still get an updated value.

Change-Id: If0eb2f9bc64b5b0f2919e37d2c9b4dca5703ba46
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>

parent 0006cb4a

kernel/events/core.c

+1 −6

Original line number	Diff line number	Diff line
		@@ -4073,7 +4073,7 @@ static int perf_event_read(struct perf_event *event, bool group)
		* value in the event structure:
		*/
		preempt_disable();
		again:

		if (state == PERF_EVENT_STATE_ACTIVE) {

		/*
		@@ -4126,11 +4126,6 @@ static int perf_event_read(struct perf_event *event, bool group)
		unsigned long flags;

		raw_spin_lock_irqsave(&ctx->lock, flags);
		if (state != PERF_EVENT_STATE_INACTIVE) {
		raw_spin_unlock_irqrestore(&ctx->lock, flags);
		goto again;
		}

		/*
		* May read while context is not active (e.g., thread is
		* blocked), in that case we cannot update context time