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

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Message-ID: <20120816155922.5de4f1e3@de.ibm.com>
Date:	Thu, 16 Aug 2012 15:59:22 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 Thu, 16 Aug 2012 14:50:33 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Thu, Aug 16, 2012 at 09:50:32AM +0200, Martin Schwidefsky wrote:
> > On Wed, 15 Aug 2012 21:28:17 +0200
> > Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > 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.
> > 
> > It is more a theoretical consideration. If the finish_arch_switch code
> > updates fields that are required to do the cputime accounting then the
> > order could be important. But then you could move that necessary code
> > from finish_arch_switch to account_switch_vtime.
> > As said that change is fine for s390, so I'm good with it.
> 
> Ah ok. Well like you said this is fine for s390. And it looks also fine
> to me on ia64 and powerpc as it doesn't look like we depend on something
> done in finish_arch_switch() there. They were flush the previous task
> cputime from switch_to() anyway.
> 
> Thanks.
> 
> PS: can I add your ack?

Sure, feel free to add my Acked-by.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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