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Date: Fri, 30 Jun 2017 17:46:02 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Frans Klaver <fransklaver@gmail.com>
Cc: Rik van Riel <riel@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        hpa@zytor.com, torvalds@linux-foundation.org,
        Thomas Gleixner <tglx@linutronix.de>, wanpeng.li@hotmail.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Gustavo A. R. Silva" <garsilva@embeddedor.com>, sgruszka@redhat.com,
        Peter Zijlstra <peterz@infradead.org>,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust()
 code
Message-ID: <20170630154559.GA21730@lerouge>
References: <20170629184128.GA5271@embeddedgus>
 <tip-72298e5c92c50edd8cb7cfda4519483ce65fa166@git.kernel.org>
 <1498831200.6130.13.camel@redhat.com>
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On Fri, Jun 30, 2017 at 04:41:50PM +0200, Frans Klaver wrote:
> On Fri, Jun 30, 2017 at 4:00 PM, Rik van Riel <riel@redhat.com> wrote:
> > On Fri, 2017-06-30 at 06:10 -0700, tip-bot for Gustavo A. R. Silva
> > wrote:
> >
> >> +++ b/kernel/sched/cputime.c
> >> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime
> >> *curr,
> >>        * userspace. Once a task gets some ticks, the monotonicy
> >> code at
> >>        * 'update' will ensure things converge to the observed
> >> ratio.
> >>        */
> >> -     if (stime == 0) {
> >> -             utime = rtime;
> >> -             goto update;
> >> +     if (stime != 0) {
> >> +             if (utime == 0)
> >> +                     stime = rtime;
> >> +             else
> >> +                     stime = scale_stime(stime, rtime, stime +
> >> utime);
> >>       }
> >>
> >> -     if (utime == 0) {
> >> -             stime = rtime;
> >> -             goto update;
> >> -     }
> >> -
> >> -     stime = scale_stime(stime, rtime, stime + utime);
> >> -
> >> -update:
> >
> > Wait, what?
> >
> > This get rid of the utime = rtime assignment, when
> > stime == 0.  That could be a correctness issue.
> 
> The first time utime is used after that assignment, it is overwritten
> with rtime - stime. The utime = rtime assignment is then pointless.

Right, I also got confused first but after starring at the code, the patch looks right.

Thanks.