perf_event: Fix event group handling in __perf_event_sched_*()

Paul Mackerras says:

 "Actually, looking at this more closely, it has to be a group
 leader anyway since it's at the top level of ctx->group_list.  In
 fact I see four places where we do:

  list_for_each_entry(event, &ctx->group_list, group_entry) {
	if (event == event->group_leader)
		...

 or the equivalent, three of which appear to have been introduced
 by afedadf2 ("perf_counter: Optimize sched in/out of counters")
 back in May by Peter Z.

 As far as I can see the if () is superfluous in each case (a
 singleton event will be a group of 1 and will have its
 group_leader pointing to itself)."

 [ See: http://marc.info/?l=linux-kernel&m=125361238901442&w=2 ]

And Peter Zijlstra points out this is a bugfix:

 "The intent was to call event_sched_{in,out}() for single event
  groups because that's cheaper than group_sched_{in,out}(),
  however..

  - as you noticed, I got the condition wrong, it should have read:

      list_empty(&event->sibling_list)

  - it failed to call group_can_go_on() which deals with ->exclusive.

  - it also doesn't call hw_perf_group_sched_in() which might break
    power."

 [ See: http://marc.info/?l=linux-kernel&m=125369523318583&w=2 ]

Changelog v1->v2:

 - Fix the title name according to Peter Zijlstra's suggestion

 - Remove the comments and WARN_ON_ONCE() as Peter Zijlstra's
   suggestion
Signed-off-by: NXiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: NPeter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <4ABC5A55.7000208@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -1030,14 +1030,10 @@ void __perf_event_sched_out(struct perf_event_context *ctx,
 	update_context_time(ctx);
 
 	perf_disable();
-	if (ctx->nr_active) {
-		list_for_each_entry(event, &ctx->group_list, group_entry) {
-			if (event != event->group_leader)
-				event_sched_out(event, cpuctx, ctx);
-			else
-				group_sched_out(event, cpuctx, ctx);
-		}
-	}
+	if (ctx->nr_active)
+		list_for_each_entry(event, &ctx->group_list, group_entry)
+			group_sched_out(event, cpuctx, ctx);
+
 	perf_enable();
  out:
 	spin_unlock(&ctx->lock);
@@ -1258,12 +1254,8 @@ __perf_event_sched_in(struct perf_event_context *ctx,
 		if (event->cpu != -1 && event->cpu != cpu)
 			continue;
 
-		if (event != event->group_leader)
-			event_sched_in(event, cpuctx, ctx, cpu);
-		else {
-			if (group_can_go_on(event, cpuctx, 1))
-				group_sched_in(event, cpuctx, ctx, cpu);
-		}
+		if (group_can_go_on(event, cpuctx, 1))
+			group_sched_in(event, cpuctx, ctx, cpu);
 
 		/*
 		 * If this pinned group hasn't been scheduled,
@@ -1291,15 +1283,9 @@ __perf_event_sched_in(struct perf_event_context *ctx,
 		if (event->cpu != -1 && event->cpu != cpu)
 			continue;
 
-		if (event != event->group_leader) {
-			if (event_sched_in(event, cpuctx, ctx, cpu))
+		if (group_can_go_on(event, cpuctx, can_add_hw))
+			if (group_sched_in(event, cpuctx, ctx, cpu))
 				can_add_hw = 0;
-		} else {
-			if (group_can_go_on(event, cpuctx, can_add_hw)) {
-				if (group_sched_in(event, cpuctx, ctx, cpu))
-					can_add_hw = 0;
-			}
-		}
 	}
 	perf_enable();
  out: