linux-kernel - Re: [PATCH v3 01/14] sched/core: uclamp: extend sched

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Message-ID: <20180813121441.GC2605@e110439-lin>
Date:   Mon, 13 Aug 2018 13:14:41 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...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>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 01/14] sched/core: uclamp: extend sched_setattr to
 support utilization clamping

On 07-Aug 11:59, Juri Lelli wrote:
> Hi,
> 
> Minor comments below.
> 
> On 06/08/18 17:39, Patrick Bellasi wrote:
> 
> [...]
> 
> > + *
> > + * Task Utilization Attributes
> > + * ===========================
> > + *
> > + * A subset of sched_attr attributes allows to specify the utilization which
> > + * should be expected by a task. These attributes allows to inform the
>                                                        ^
> 						     allow
> 
> > + * scheduler about the utilization boundaries within which is safe to schedule
> 
> Isn't all this more about providing hints than safety?

Yes, it's "just" hints... will rephrase to make it more clear.

> > + * the task. These utilization boundaries are valuable information to support
> > + * scheduler decisions on both task placement and frequencies selection.
> > + *
> > + *  @sched_util_min	represents the minimum utilization
> > + *  @sched_util_max	represents the maximum utilization
> > + *
> > + * Utilization is a value in the range [0..SCHED_CAPACITY_SCALE] which
> > + * represents the percentage of CPU time used by a task when running at the
> > + * maximum frequency on the highest capacity CPU of the system. Thus, for
> > + * example, a 20% utilization task is a task running for 2ms every 10ms.
> > + *
> > + * A task with a min utilization value bigger then 0 is more likely to be
> > + * scheduled on a CPU which can provide that bandwidth.
> > + * A task with a max utilization value smaller then 1024 is more likely to be
> > + * scheduled on a CPU which do not provide more then the required bandwidth.
> 
> Isn't s/bandwidth/capacity/ here, above, and in general where you use
> the term "bandwidth" more appropriate? I wonder if overloading this term
> (w.r.t. how is used with DEADLINE) might create confusion. In this case
> we are not providing any sort of guarantees, it's a hint.

Yes, you right... here we are not really granting any bandwidth but
just "improving" the bandwidth provisioning by hinting the scheduler
about a certain min/max capacity required.

The problem related to using capacity is that, from kernel space,
capacity is defined as a static quantity/property of CPUs. Still, I
think it makes sense to argue that util_{min,max} are hints on the
min/max capacity required for a task.

I'll update comments and text to avoid using bandwidth in favour of
capacity.

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi