linux-kernel - Re: [PATCH 1/3] perf: add context field to perf

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Message-ID: <20110721163757.GF8446@e102144-lin.cambridge.arm.com>
Date:	Thu, 21 Jul 2011 17:37:57 +0100
From:	Will Deacon <will.deacon@....com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	"acme@...stprotocols.net" <acme@...stprotocols.net>,
	Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [PATCH 1/3] perf: add context field to perf_event

On Thu, Jul 21, 2011 at 04:59:00PM +0100, Avi Kivity wrote:
> On 07/21/2011 06:46 PM, Will Deacon wrote:
> > >
> > >  This is (and must be) called from a preempt disabled context, no mutexes
> > >  around here.
> >
> > Bah, yes, that is essential if you're dealing with current. Maybe use a
> > spinlock instead?
> 
> Could work.  Not thrilled about adding it to the kvm hot path, but I 
> can't say it will make a measurable impact.

Understood, but at least it doesn't contribute to finish_task_switch. I also
wouldn't expect to have multiple preempt registrations in parallel for the
same task so that lock should rarely (if ever) be contended.

Will
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