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

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Message-ID: <20110711210753.GA3582@e102144-lin.cambridge.arm.com>
Date:	Mon, 11 Jul 2011 22:07:53 +0100
From:	Will Deacon <will.deacon@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Avi Kivity <avi@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"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 Mon, Jul 04, 2011 at 03:36:57PM +0100, Frederic Weisbecker wrote:
> On Mon, Jul 04, 2011 at 05:10:20PM +0300, Avi Kivity wrote:
> > On 07/04/2011 04:58 PM, Frederic Weisbecker wrote:
> > >Another thing I would like to do in the even longer term is to not use perf anymore
> > >for ptrace breakpoints, because that involves a heavy dependency and few people are
> > >happy with that. Instead we should just have a generic hook into the sched_switch()
> > >and handle pure ptrace breakpoints there. The central breakpoint API would still be
> > >there to reserve/schedule breakpoint resources between ptrace and perf.
> > >
> > 
> > 'struct preempt_notifier' may be the hook you're looking for.
> 
> Yeah looks like a perfect fit as it's per task.

I had a quick look at this and I think the preempt_notifier stuff needs
slightly extending so that we can register a notifier for a task other than
current [e.g. the child of current on which we are installing breakpoints].

If the task in question is running, it looks like this will introduce a race
condition between notifier registration and rescheduling. For the purposes
of ptrace this shouldn't be a problem as the child will be stopped, but
others might also want to make use of the new functionality.

Any ideas on how this could be achieved, or am I better off just restricting
this to children that are being traced?

Cheers,

Will
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