linux-kernel - Re: IRQ / SoftIRQ CPU time accounting broken by 457533a7d3402d1d91fbc125c8bd1bd16dcd3cd4

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Message-ID: <20090429100206.6371b093@skybase>
Date:	Wed, 29 Apr 2009 10:02:06 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrew Gallatin <gallatin@...i.com>, linux-kernel@...r.kernel.org,
	rick.jones2@...com, brice@...i.com,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: IRQ / SoftIRQ CPU time accounting broken by
 457533a7d3402d1d91fbc125c8bd1bd16dcd3cd4

On Tue, 28 Apr 2009 16:30:04 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> (cc's added)
> 
> On Thu, 23 Apr 2009 10:19:38 -0400
> Andrew Gallatin <gallatin@...i.com> wrote:
> 
> > When running netperf for some 10GbE tests, I noticed
> > that  IRQ and SOFTIRQ CPU time is no longer reported for an
> > otherwise idle CPU on recent kernels, at least for x86_64.
> > 
> > I spent a few hours git-bisecting until I finally got here:
> > 
> > % git-bisect bad
> > Bisecting: 0 revisions left to test after this
> > [457533a7d3402d1d91fbc125c8bd1bd16dcd3cd4] fix scaled & unscaled cputime 
> > accounting
> > 
> > I have neither CONFIG_NO_HZ, CONFIG_VIRT_CPU_ACCOUNTING, or XEN configured.
> > 
> 
> Thanks for doing the bisection.
> 
> 457533a7d3402d1d91fbc125c8bd1bd16dcd3cd4 was merged late last year, so
> this regression has been around for a while.
> 
> We might have fixed it in more recent kernels - stranger things have
> happened ;)

Hmm, are you sure that this commit caused the problem? I can't see how.
In regard to the accounting of system ticks on x86_64 the only change
is the bug fix that a system tick of a guest cpu has incorrectly been
added to p->stimescaled instead of p->utimescaled.
Without guest cpus in the picture nothing changes. For x86_64
CONFIG_VIRT_CPU_ACCOUNTING=n so account_process_tick is used. Before
the patch accout_process_tick accounted a system tick by calling
account_system_time and account_system_time_scaled. The second function
just added the cputime to p->stimescaled. That is now done in
accout_system_time.

-- 
blue skies,
   Martin.

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

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