linux-kernel - Re: [sched next] overflowed cpu time for kernel threads in /proc/PID/stat

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Message-ID: <20130902142431.GC2368@swordfish.minsk.epam.com>
Date:	Mon, 2 Sep 2013 17:24:31 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Stanislaw Gruszka <sgruszka@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: Re: [sched next] overflowed cpu time for kernel threads in
 /proc/PID/stat

On (09/02/13 16:13), Frederic Weisbecker wrote:
> > On Mon, Sep 02, 2013 at 03:07:45PM +0200, Frederic Weisbecker wrote:
> > > > Hope this may help.
> > > > I've added a silly check to make sure that `stime < rtime'
> > > > 
> > > > @@ -579,6 +582,10 @@ static void cputime_adjust(struct task_cputime *curr,
> > > >         if (total) {
> > > >                 stime = scale_stime((__force u64)stime,
> > > >                                     (__force u64)rtime, (__force u64)total);
> > > > +               if (stime > rtime) {
> > > > +                       printk(KERN_ERR "Ooops: stime:%llu rtime:%llu\n", stime, rtime);
> > > > +                       WARN_ON(1);
> > > > +               }
> > > >                 utime = rtime - stime;
> > > >         } else {
> > > >                 stime = rtime;
> > [snip]
> > 
> > > Thanks a lot Sergey for testing this further!
> > > 
> > > Interesting results, so rtime is always one or two units off stime after scaling.
> > > Stanislaw made the scaling code with Linus and he has a better idea on the math guts
> > > here.
> > 
> > I don't think this is scale issue, but rather at scale_stime() input
> > stime is already bigger then rtime. Sergey, could you verify that
> > by adding check before scale_stime() ?
> 
> Note that having stime > rtime should be fine to handle. This can happen for
> example if the task runs on tiny timeslices but is unlucky enough that all these
> timeslices are interrupted by the tick.
>

even is stime > rtime, scale_stime() fixes it:

	if (stime > rtime)
		swap(rtime, stime);

	-ss
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