From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 1/3] perf: add context field to perf_event
Date: Tue, 12 Jul 2011 11:18:55 +0200
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Cc: Will Deacon <will.deacon@arm.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Ingo Molnar <mingo@elte.hu>,
	"acme@ghostprotocols.net" <acme@ghostprotocols.net>,
	Jason Wessel <jason.wessel@windriver.com>
To: Avi Kivity <avi@redhat.com>
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On Tue, 2011-07-12 at 12:16 +0300, Avi Kivity wrote:
> On 07/12/2011 12:14 PM, Peter Zijlstra wrote:
> > On Tue, 2011-07-12 at 12:08 +0300, Avi Kivity wrote:
> > >  Similar, but with stronger guarantees: when the function is called,
> > >  current == p, and the task was either sleeping or in userspace.
> >
> > If the task is sleeping, current can never be p.
> 
> The guarantee is that the task was sleeping just before the function is 
> called.  Of course it's woken up to run the function.
> 
> The idea is that you run the function in a known safe point to avoid 
> extra synchronization.
> 

I'd much rather we didn't wake the task and let it sleep, that's usually
a very safe place for tasks to be. All you'd need is a guarantee it
won't be woken up while you're doing your thing.