perf: Fix cgroup event scheduling

There appears to be a problem in __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.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -2806,6 +2806,16 @@ void __perf_event_task_sched_in(struct task_struct *prev,
 	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))
@@ -2813,13 +2823,6 @@ void __perf_event_task_sched_in(struct task_struct *prev,
 
 		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);