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: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <4ABC5A55.7000208@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 22 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 0f86feb6db0c..e50543db642a 100644
--- a/kernel/perf_event.c
+++ b/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: