linux-kernel - Re: [PATCH v6 07/16] sched/core: uclamp: Add system default clamps

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Date:   Wed, 23 Jan 2019 14:19:24 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        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>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v6 07/16] sched/core: uclamp: Add system default clamps

On 23-Jan 10:22, Peter Zijlstra wrote:
> On Tue, Jan 22, 2019 at 03:41:29PM +0000, Patrick Bellasi wrote:
> > On 22-Jan 16:13, Peter Zijlstra wrote:
> > > On Tue, Jan 22, 2019 at 02:43:29PM +0000, Patrick Bellasi wrote:
> 
> > > > Do you think that could be acceptable?
> > > 
> > > Think so, it's a sysctl poke, 'nobody' ever does that.
> > 
> > Cool, so... I'll keep lazy update for system default.
> 
> Ah, I think I misunderstood. I meant to say that since nobody ever pokes
> at sysctl's it doesn't matter if its a little more expensive and iterate
> everything.

Here I was more worried about the code complexity/overhead... for
something actually not very used/useful.

> Also; if you always keep everything up-to-date, you can avoid doing that
> duplicate accounting.

To update everything we will have to walk all the CPUs and update all
the RUNNABLE tasks currently enqueued, which are either RT or CFS.

That's way more expensive both in code and time then what we do for
cgroups, where at least we have a limited scope since the cgroup
already provides a (usually limited) list of tasks to consider.

Do you think it's really worth to have ?

Perhaps we can add it in a second step, once we have the core bits in
and we really see a need for a specific use-case.


-- 
#include <best/regards.h>

Patrick Bellasi