linux-kernel - Re: [PATCH v2] sched/uclamp: Fix overzealous type replacement

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Date:   Fri, 15 Nov 2019 21:21:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <Dietmar.Eggemann@....com>,
        Tejun Heo <tj@...nel.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Qais Yousef <qais.yousef@....com>
Subject: Re: [PATCH v2] sched/uclamp: Fix overzealous type replacement

On Fri, Nov 15, 2019 at 06:34:19PM +0100, Vincent Guittot wrote:
> On Fri, 15 Nov 2019 at 18:10, Valentin Schneider
> <valentin.schneider@....com> wrote:
> >
> > On 15/11/2019 14:29, Valentin Schneider wrote:
> > > On 15/11/2019 14:07, Vincent Guittot wrote:
> > >>> -static inline enum uclamp_id uclamp_none(enum uclamp_id clamp_id)
> > >>> +static inline unsigned int uclamp_none(enum uclamp_id clamp_id)
> > >>
> > >> Out of curiosity why uclamp decided to use unsigned int to manipulate
> > >> utilization instead of unsigned long which is the type of util_avg ?
> > >>
> > >
> > > I didn't stare at the discussion much, but I think it stems from the
> > > design choices behind struct uclamp_se: everything is crammed in an unsigned
> > > int bitfield. Let me see if I can find some relevant mails.
> > >
> >
> > So I think a relevant mail is:
> >
> > https://lore.kernel.org/lkml/20180912174236.GB24106@hirez.programming.kicks-ass.net/
> >
> > Other than that, the uclamp_se.value field was 'int' in v1 and has been
> > 'unsigned int' for all following versions. uclamp_bucket.value is a bitfield
> > of an 'unsigned long' just because we want more headroom for the tasks count,
> > AFAICT.
> 
> Thanks for the pointer and deep diving in the email threads

It was all in an effort to minimize the number of cachelines touched to
maintain this data.