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

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Date:   Fri, 30 Jun 2017 16:41:50 +0200
From:   Frans Klaver <fransklaver@...il.com>
To:     Rik van Riel <riel@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, hpa@...or.com,
        Frederic Weisbecker <fweisbec@...il.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 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.