linux-kernel - Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

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Date:   Thu, 14 Mar 2019 09:39:09 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

On Thu, Mar 14, 2019 at 8:41 AM Patrick Bellasi <patrick.bellasi@....com> wrote:
>
> On 14-Mar 08:29, Suren Baghdasaryan wrote:
> > On Thu, Mar 14, 2019 at 7:46 AM Patrick Bellasi <patrick.bellasi@....com> wrote:
> > > On 13-Mar 14:32, Suren Baghdasaryan wrote:
> > > > On Fri, Feb 8, 2019 at 2:06 AM Patrick Bellasi <patrick.bellasi@....com> wrote:
>
> [...]
>
> > > > > The rq::uclamp::bucket[clamp_id][] array is scanned every time we need
> > > > > to find a new MAX aggregated clamp value for a clamp_id. This operation
> > > > > is required only when we dequeue the last task of a clamp bucket
> > > > > tracking the current MAX aggregated clamp value. In these cases, the CPU
> > > > > is either entering IDLE or going to schedule a less boosted or more
> > > > > clamped task.
>
> The following:
>
> > > > > The expected number of different clamp values, configured at build time,
> > > > > is small enough to fit the full unordered array into a single cache
> > > > > line.
>
> will read:
>
> The expected number of different clamp values, configured at build time,
> is small enough to fit the full unordered array into a single cache
> line for the default UCLAMP_BUCKETS configuration of 7 buckets.

I think keeping default to be 5 is good. As you mentioned it's a nice
round number and keeping it at the minimum also hints that this is not
a free resource and the more buckets you use the more you pay.
Documentation might say "to fit the full unordered array into a single
cache line for configurations of up to 7 buckets".

> [...]
>
> > Got it. From reading the documentation at the beginning my impression
> > was that whatever value I choose within allowed 5-20 range it would
> > still fit in a cache line. To disambiguate it might be worse
> > mentioning that this is true for the default value or for values up to
> > 7. Thanks!
>
> Right, I hope the above proposed change helps to clarify that.
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi