linux-kernel - Re: [tip:sched/core] sched/cputime: Refactor the cputime

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Date:   Fri, 30 Jun 2017 17:46:02 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Frans Klaver <fransklaver@...il.com>
Cc:     Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        hpa@...or.com, torvalds@...ux-foundation.org,
        Thomas Gleixner <tglx@...utronix.de>, wanpeng.li@...mail.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>,
        sgruszka@...hat.com, Peter Zijlstra <peterz@...radead.org>,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust()
 code

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@...hat.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.