linux-kernel - Re: [PATCH 3/4] cputime: Consolidate vtime handling on context switch

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Message-ID: <20120815192813.GE19716@somewhere>
Date:	Wed, 15 Aug 2012 21:28:17 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/4] cputime: Consolidate vtime handling on context switch

On Wed, Aug 15, 2012 at 05:22:19PM +0200, Martin Schwidefsky wrote:
> On Tue, 14 Aug 2012 16:16:49 +0200
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > The archs that implement virtual cputime accounting all
> > flush the cputime of a task when it gets descheduled
> > and sometimes set up some ground initialization for the
> > next task to account its cputime.
> > 
> > These archs all put their own hooks in their context
> > switch callbacks and handle the off-case themselves.
> > 
> > Consolidate this by creating a new account_switch_vtime()
> > callback called in generic code right after a context switch
> > and that these archs must implement to flush the prev task
> > cputime and initialize the next task cputime related state.
> 
> That change requires that the accounting for the previous process
> can be done before finish_arch_switch() completed. With the old
> code the architecture could to the accounting call in the middle
> of finish_arch_switch, that is not possible anymore. Dunno if this
> is relevant or not. For s390 the new code should work fine.

I'm not sure how this could potentially cause a problem. Interrupts are disabled
between while we switch_to() until finish_lock_switch(). So nothing
should be able to mess up with the accounting of the prev task.

I don't really understand what you mean actually.

Thanks.
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