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Date: Thu, 29 Jun 2017 06:51:16 +0200
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Subject: Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()
To: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
CC: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org
From: Frans Klaver <fransklaver@gmail.com>
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On 29 June 2017 01:57:19 CEST, "Gustavo A. R. Silva" <garsilva@embeddedor.com> wrote:
>>>> --- a/kernel/sched/cputime.c
>>>> +++ b/kernel/sched/cputime.c
>>>> @@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime
>*curr,
>>>>          *            = (rtime_i+1 - rtime_i) + utime_i
>>>>          *            >= utime_i
>>>>          */
>>>> -       if (stime < prev->stime)
>>>> +       if (stime < prev->stime) {
>>>>                 stime = prev->stime;
>>>> -       utime = rtime - stime;
>>>> +               utime = rtime - stime;
>>>> +       }
>>>>
>>>>
>>>> If you confirm this, I will send a patch in a full and proper form.
>>>>
>>>> I'd really appreciate your comments.
>>>
>>> If you do that, how would you meet the guarantee made in line 583?
>>>
>
>You are right, I see now.
>
>Then in this case the following patch would be the way to go:
>
>--- a/kernel/sched/cputime.c
>+++ b/kernel/sched/cputime.c
>@@ -615,10 +615,8 @@ 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;
>+       if (stime == 0)
>                 goto update;
>-       }
>
>         if (utime == 0) {
>                 stime = rtime;
>
>
>but I think this one is even better:
>
>
>--- a/kernel/sched/cputime.c
>+++ b/kernel/sched/cputime.c
>@@ -615,19 +615,11 @@ 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 (utime == 0) {
>+       if (stime != 0 && utime == 0)
>                 stime = rtime;
>-               goto update;
>-       }
>-
>-       stime = scale_stime(stime, rtime, stime + utime);
>+       else
>+               stime = scale_stime(stime, rtime, stime + utime);

I don't think it is better. The stime == 0 case is gone now. So scale_time() will be called in that case. This whole if/else block should only be executed if stime != 0.