Subject: Re: [PATCH 1/2 v2] perf_counter: fix for __perf_event_sched_*()
From: Peter Zijlstra <peterz@infradead.org>
To: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@elte.hu>, Paul Mackerras <paulus@samba.org>,
       LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <4ABC5A55.7000208@cn.fujitsu.com>
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Date: Fri, 25 Sep 2009 10:55:37 +0200
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On Fri, 2009-09-25 at 13:51 +0800, Xiao Guangrong wrote:
> 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)."
> [Can be found at http://marc.info/?l=linux-kernel&m=125361238901442&w=2]
> 
> And Peter Zijlstra point out this is an bugfix:
> "The intent was to call event_sched_{in,out}() for single counter 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."
> [Can be found at http://marc.info/?l=linux-kernel&m=125369523318583&w=2]
> 
> Changelog v1->v2:
> - fix the title name as 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>

Thanks,

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
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