perf: Do no try to schedule task events if there are none

perf_event_sched_in() shouldn't try to schedule task events if there
are none otherwise task's ctx->is_active will be set and will not be
cleared during sched_out. This will prevent newly added events from
being scheduled into the task context.

Fixes a boo-boo in commit 1d5f003f5a ("perf: Do not set task_ctx
pointer in cpuctx if there are no events in the context").

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20111122140821.GF2557@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -2174,11 +2174,11 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 	 */
 	cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
 
-	perf_event_sched_in(cpuctx, ctx, task);
-
 	if (ctx->nr_events)
 		cpuctx->task_ctx = ctx;
 
+	perf_event_sched_in(cpuctx, cpuctx->task_ctx, task);
+
 	perf_pmu_enable(ctx->pmu);
 	perf_ctx_unlock(cpuctx, ctx);