linux-kernel - Re: perf: [patch] regression with PERF_EVENT_IOC

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Date:	Tue, 31 May 2011 09:17:47 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Vince Weaver <vince@...ter.net>
Cc:	Vince Weaver <vweaver1@...s.utk.edu>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, paulus@...ba.org, acme@...hat.com
Subject: Re: perf: [patch] regression with PERF_EVENT_IOC_REFRESH

On Mon, 2011-05-30 at 21:33 -0400, Vince Weaver wrote:
> On Sun, 29 May 2011, Vince Weaver wrote:
> 
> > On Sat, 28 May 2011, Peter Zijlstra wrote:
> > 
> > > On Fri, 2011-05-27 at 23:38 -0400, Vince Weaver wrote:
> > > > on that note (and while trying to document exactly what the ioctls do) it 
> > > > seems that a PERF_EVENT_IOC_REFRESH with an argument of anything higher 
> > > > than one does not work on kernels 2.6.36 and newer.  The behavior acts
> > > > as if 1 was passed, even if you pass in, say, 3.
> > > 
> > > Urgh, no that should definitely work. Thanks for the test-case, I'll
> > > work on that (probably not until Monday though, but who knows).
> > > 
> > 
> > after a painfully long bisection, it turns out that this problem was in
> > theory introduced by the following commit:
> > 
> >   d57e34fdd60be7ffd0b1d86bfa1a553df86b7172
> > 
> >   perf: Simplify the ring-buffer logic: make perf_buffer_alloc() do everything needed
> > 
> > I'll see if I can come up with a patch, but it's a bit non-obvious why
> > this commit is affecting the REFRESH value at all.
> 
> the problem was the mentioned commit tried to optimize the use of 
> watermark and wakeup_watermark without taking into account that 
> wakeup_watermark is a union with wakeup_events.
> 
> The patch below *should* fix it,

Awesome thanks!

>  but something unrelated has broken 
> overflow support between 2.6.39 and 3.0-rc1 which I haven't had time to 
> investigate.  The overflow count is suddenly about 10x what it should be 
> though.  So the below is semi-untested and I possibly need to do another 
> bisect.  *sigh*

Yeah, I noticed, I was hunting that as well..
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