linux-kernel - Re: [PATCH] perf_counter: cleanup for __perf_event_sched

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Date:	Tue, 22 Sep 2009 13:17:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf_counter: cleanup for __perf_event_sched_in()

On Tue, 2009-09-22 at 19:39 +1000, Paul Mackerras wrote:
> Xiao Guangrong writes:
> 
> > It must be a group leader if event->attr.pinned is "1"
> 
> 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).  Peter, do you agree or have I missed something?

/me kicks those neurons back to work..

Ah, yes, I think you're right, the second hunk of afedadf2 is a
pessimisation due to the extra branch.


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