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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 30 Apr 2021 16:16:58 +0200
Message-ID: <CAKfTPtBNsarv7REyNFeU5e=-K6OiaR2hxPFm6Ttca5UcnWB6Wg@mail.gmail.com>
Subject: Re: [PATCH v2] sched: Fix out-of-bound access in uclamp
To:     Quentin Perret <qperret@google.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Android Kernel Team <kernel-team@android.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Patrick Bellasi <patrick.bellasi@matbug.net>
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On Fri, 30 Apr 2021 at 15:14, Quentin Perret <qperret@google.com> 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