linux-kernel - Re: perf_event self-monitoring overhead regression

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Date:	Thu, 1 Dec 2011 10:11:05 -0800
From:	stephane eranian <eranian@...glemail.com>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: perf_event self-monitoring overhead regression

Vince,

I take it your test is all about self-monitoring, single event, single thread.
Did you try breaking down the cost using  TSC and rdtsc() to pinpoint where the
regression is coming from in the 3 perf_event syscalls you're using?


On Thu, Dec 1, 2011 at 5:59 AM, Vince Weaver <vweaver1@...s.utk.edu> wrote:
> On Wed, 30 Nov 2011, Linus Torvalds wrote:
>
>> On Wed, Nov 30, 2011 at 3:54 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>>
>> So bisect works really well for clear bugs that are 100% repeatable
>> and have a very clear "did it happen or not" signature, but I'd be
>> very leery indeed of using it with some performance anomaly.
>
> In this case, the bisection case is pretty clean.  I run 1000 of the tests
> and check the median value, and the values are always 7280us for good and
> 7440us for bad.
>
> The problem is when it starts bisecting into the merges it drops from a
> post-3.0 kernel into a much earlier 3.0-rc kernel (due to the ARM merge
> history) and suddenly then the test becomes meaningless as it starts
> returning other values such as 6880us.  This is because the problem I am
> tracking has ben gradually getting worse over time, and so I guess by
> going back to a 3.0-rc it gets earlier than some other change that made
> performance worse, and thus it becomes impossible to know if a commit is
> good or not.
>
>> That said, maybe you can get the timings to be unambiguous enough by
>> using 'rdtsc' in user space, and try the bisect again.
>
> I'll try but I'm guessing your first thought that this might be
> un-bisectabe is probably true.
>
> Thanks,
>
> Vince
>
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