[RFC][PATCH 02/12] perf: Fix cgroup event scheduling

[Peter Zijlstra]

There appears to be a problemin __perf_event_task_sched_in() wrt
cgroup event scheduling.

The normal event scheduling order is:

	CPU pinned
	Task pinned
	CPU flexible
	Task flexible

And since perf_cgroup_sched*() only schedules the cpu context, we must
call this _before_ adding the task events.

Note: double check what happens on the ctx switch optimization where
the task ctx isn't scheduled.

Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/events/core.c |   17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2804,6 +2804,16 @@ void __perf_event_task_sched_in(struct t
 	struct perf_event_context *ctx;
 	int ctxn;
 
+	/*
+	 * If cgroup events exist on this CPU, then we need to check if we have
+	 * to switch in PMU state; cgroup event are system-wide mode only.
+	 *
+	 * Since cgroup events are CPU events, we must schedule these in before
+	 * we schedule in the task events.
+	 */
+	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
+		perf_cgroup_sched_in(prev, task);
+
 	for_each_task_context_nr(ctxn) {
 		ctx = task->perf_event_ctxp[ctxn];
 		if (likely(!ctx))
@@ -2811,13 +2821,6 @@ void __perf_event_task_sched_in(struct t
 
 		perf_event_context_sched_in(ctx, task);
 	}
-	/*
-	 * if cgroup events exist on this CPU, then we need
-	 * to check if we have to switch in PMU state.
-	 * cgroup event are system-wide mode only
-	 */
-	if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
-		perf_cgroup_sched_in(prev, task);
 
 	if (atomic_read(&nr_switch_events))
 		perf_event_switch(task, prev, true);