perf: Do not set task_ctx pointer in cpuctx if there are no events in the context

Do not set task_ctx pointer during sched_in if there are no
events associated with the context.  Otherwise if during task
execution total number of events in the system will become zero
perf_event_context_sched_out() will not be called and cpuctx->task_ctx
will be left with a stale value.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20111023171033.GI17571@redhat.comSigned-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -2173,6 +2173,7 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 
 	perf_event_sched_in(cpuctx, ctx, task);
 
+	if (ctx->nr_events)
 		cpuctx->task_ctx = ctx;
 
 	perf_pmu_enable(ctx->pmu);