linux-kernel - Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remote wakeups

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Message-ID: <20110531125621.GA24439@gere.osrc.amd.com>
Date:	Tue, 31 May 2011 14:56:21 +0200
From:	Borislav Petkov <bp@...64.org>
To:	a.p.zijlstra@...llo.nl
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	markus@...ppelsdorf.de, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remote
 wakeups

Hey Peter,

On Tue, May 31, 2011 at 12:31:20PM +0000, tip-bot for Peter Zijlstra wrote:
> Commit-ID:  f01114cb59d670e9b4f2c335930dd57db96e9360
> Gitweb:     http://git.kernel.org/tip/f01114cb59d670e9b4f2c335930dd57db96e9360
> Author:     Peter Zijlstra <a.p.zijlstra@...llo.nl>
> AuthorDate: Tue, 31 May 2011 12:26:55 +0200
> Committer:  Ingo Molnar <mingo@...e.hu>
> CommitDate: Tue, 31 May 2011 14:19:56 +0200
> 
> sched: Fix cross-cpu clock sync on remote wakeups
> 
> Markus reported that commit 317f394160e ("sched: Move the second half
> of ttwu() to the remote cpu") caused some accounting funnies on his AMD
> Phenom II X4, such as weird 'top' results.
> 
> It turns out that this is due to non-synced TSC

this would mean that his machine doesn't pass the TSC sync check at boot
but that's a F10h and they usu. have synchronized TSCs?

I'm confused.

> and the queued remote
> wakeups stopped coupeling the two relevant cpu clocks, which leads to
> wakeups seeing time jumps, which in turn lead to skewed runtime stats.
> 
> Add an explicit call to sched_clock_cpu() to couple the per-cpu clocks
> to restore the normal flow of time.
> 
> Reported-and-tested-by: Markus Trippelsdorf <markus@...ppelsdorf.de>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Link: http://lkml.kernel.org/r/1306835745.2353.3.camel@twins
> Signed-off-by: Ingo Molnar <mingo@...e.hu>

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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Registration: Dornach, Gemeinde Aschheim, Landkreis Muenchen
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