linux-kernel - Re: Q: perf_install_in_context/perf_event

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Date:	Thu, 27 Jan 2011 17:57:12 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_install_in_context/perf_event_enable are racy?

On 01/27, Peter Zijlstra wrote:
>
> Right, so the fact of introducing extra scheduling makes me feel
> uncomfortable... the whole purpose is to observe without perturbing (as
> much as possible).

Yes, agreed.

Well, otoh the patch removes the code which sets ->task_ctx from
__perf_install_in_context() and __perf_event_enable(), and perhaps
we could simplify the things further, but anyway I agree.

> Should I think suffice to get the ctx in sync with the task state, we've
> got the following cases:
>  1) task is in the middle of scheduling in
>  2) task is in the middle of scheduling out
>  3) task is running
>
> Without __ARCH_WANT_INTERRUPT_ON_CTXSW everything is boring and works,
> 1: the IPI will be delayed until 3, 2: the IPI finds another task and
> the next schedule in will sort things.
>
> With, however, things are more interesting. 2 seems to be adequately
> covered by the patch I just send, the IPI will bail and the next
> sched-in of the relevant task will pick matters up. 1 otoh doesn't seem
> covered, the IPI will bail, leaving us stranded.

Hmm, yes... Strangely, I missed that when I was thinking about in_ctxsw.

Perhaps, we can change task_oncpu_function_call() so that it returns
-EAGAIN in case it hits in_ctxsw != 0? If the caller sees -EAGAIN, it
should always retry even if !ctx->is_active.

Oleg.

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