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

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Message-ID: <20190123191007.GG17749@hirez.programming.kicks-ass.net>
Date:   Wed, 23 Jan 2019 20:10:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
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 Wed, Jan 23, 2019 at 02:19:24PM +0000, Patrick Bellasi wrote:
> 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 ?

Dunno; the whole double bucket thing seems a bit weird to me; but maybe
it will all look better without the mapping stuff.