Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()

[Gustavo A. R. Silva]

Quoting Frans Klaver <fransklaver@xxxxxxxxx>:

> On 29 June 2017 01:57:19 CEST, "Gustavo A. R. Silva"  
> <garsilva@xxxxxxxxxxxxxx> 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.

Oh yeah! something like:

if (stime != 0) {
        if (stime != 0 && utime == 0)
                stime = rtime;
        else
                stime = scale_stime(stime, rtime, stime + utime);
}

I'll be right back with the final patch.

Thanks for your time, Frans.
Much appreciated :)
--
Gustavo A. R. Silva