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 11:31:00 +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 12:27 +0300, Avi Kivity wrote:
> On 07/12/2011 12:18 PM, Peter Zijlstra wrote:
> > >
> > >  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.
> 
> But it means that 'current' is not set to the right value.  If the 
> function depends on it, then it will misbehave.  And in fact 
> preempt_notifier_register(), which is the function we want to call here, 
> does depend on current.
> 
> Of course we need to find more users for this, but I have a feeling this 
> will be generally useful.  The alternative is to keep adding bits to 
> thread_info::flags.

Using TIF_bits sounds like a much better solution for this, wakeups are
really rather expensive and its best to avoid extra if at all possible.