Re: [PATCH v2] sched: Fix out-of-bound access in uclamp

[Vincent Guittot]

On Fri, 30 Apr 2021 at 15:14, Quentin Perret <qperret@xxxxxxxxxx> wrote:
>
> On Friday 30 Apr 2021 at 15:00:00 (+0200), Dietmar Eggemann wrote:
> > On 30/04/2021 14:03, Vincent Guittot wrote:
> > IMHO, this asks for
> >
> > min_t(unsigned int, clamp_value/UCLAMP_BUCKET_DELTA, UCLAMP_BUCKETS-1);
>
> Yep, that's what I have locally.
>
> > >>  }
> > >>
> > >>  static inline unsigned int uclamp_none(enum uclamp_id clamp_id)
> >
> > Looks like this will fix a lot of possible configs:
> >
> > nbr buckets 1-4, 7-8, 10-12, 14-17, *20*, 26, 29-32 ...
> >
> > We would still introduce larger last buckets, right?
>
> Indeed. The only better alternative I could see was to 'spread' the
> error accross multiple buckets (e.g. make the last few buckets a bit
> bigger instead of having all of it accumulated on the last one), but not
> sure it is worth the overhead.

I don't think it's worth the overhead.

>
> Suggestions are welcome though.
>
> > Examples:
> >
> > nbr_buckets   delta   last bucket size
> >
> > 20            51       +5 = 56
> >
> > 26            39      +10 = 49
> >
> > 29            35       +9 = 44
>
> Yes the error can become worse in the relative sense with a large number
> of buckets, but again the max is 20 so we should be fine?
>
> Thanks,
> Quentin