linux-kernel - Re: [PATCH] process cputimer is moving faster than its corresponding clock

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Message-ID: <1366957639.7911.42.camel@Wailaba2>
Date:	Fri, 26 Apr 2013 02:27:19 -0400
From:	Olivier Langlois <olivier@...llion01.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, schwidefsky@...ibm.com,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] process cputimer is moving faster than its
 corresponding clock

On Fri, 2013-04-26 at 00:40 -0400, Olivier Langlois wrote:
> On Fri, 2013-04-19 at 11:08 -0700, KOSAKI Motohiro wrote:
> > On Fri, Apr 19, 2013 at 10:38 AM, KOSAKI Motohiro
> > <kosaki.motohiro@...il.com> wrote:
> > >> I feel we are hitting the same issue than this patch:
> > >> https://lkml.org/lkml/2013/4/5/116
> > >>
> > >> I'm adding Kosaki in Cc, who proposed roughly the same fix.
> > >
> > > Thanks to CCing. I'm now sitting LSF and I can't read whole tons emails.
> > > However the fix is definitely same and I definitely agree this approach.
> > >
> > > thank you.
> > 
> > And if I understand correctly, update_gt_cputime() is no longer
> > necessary after this patch because time never makes backward.
> > 
> > What do you think?
> 
> Kosaki, I would tend to say that what you propose is exact. After having
> added the task deltas I was puzzled to see the cputimer still moving
> faster than the process clock. I was seeing it with the the help of
> printk statement inside update_gt_cputime().
> 
> After nailing down the last remaining cause of that inside sched/core.c,
> I have never seen after the cputimer being in advance.
> 
I need to add that I can only confirm that to be true with
sum_exec_runtime.

To affirm it to be true for stime and utime would require more
investigation. I didn't look them at all. I was only concerned with
sum_exec_runtime.

I will prepare a v2 of the patch accounting all the feedbacks that I
received from KOSAKI Motohiro, Frederic Weisbecker and Peter Zijlstra
and send it back here for further discussion.

Thank you very much all!
Olivier




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