[PATCH] perf_events: improve task_sched_in()

[eranian@google.com]

This patch is an optimization in perf_event_task_sched_in() to avoid scheduling
the events twice in a row. Without it, the perf_disable()/perf_enable() pair
is invoked twice, thereby pinned events counts while scheduling flexible events
and we go throuh hw_perf_enable() twice. By encapsulating, the whole sequence
into perf_disable()/perf_enable() we ensure, hw_perf_enable() is going to be
invoked only once because of the refcount protection.

	Signed-off-by: Stephane Eranian <eranian@google.com>
--
 perf_event.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1382,6 +1382,8 @@ void perf_event_task_sched_in(struct task_struct *task)
 	if (cpuctx->task_ctx == ctx)
 		return;
 
+	perf_disable();
+
 	/*
 	 * We want to keep the following priority order:
 	 * cpu pinned (that don't need to move), task pinned,
@@ -1394,6 +1396,8 @@ void perf_event_task_sched_in(struct task_struct *task)
 	ctx_sched_in(ctx, cpuctx, EVENT_FLEXIBLE);
 
 	cpuctx->task_ctx = ctx;
+
+	perf_enable();
 }
 
 #define MAX_INTERRUPTS (~0ULL)