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 20:18:49 +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 v7 01/15] sched/core: uclamp: Add CPU's clamp buckets
 refcounting

On Thu, Mar 14, 2019 at 03:07:53PM +0000, Patrick Bellasi wrote:
> On 14-Mar 14:32, Peter Zijlstra wrote:
> > On Thu, Mar 14, 2019 at 12:13:15PM +0000, Patrick Bellasi wrote:
> > > > I'd be most impressed if they pull this off. Check the generated code
> > > > and see I suppose :-)
> > > 
> > > On x86 the code generated looks exactly the same:
> > > 
> > >    https://godbolt.org/z/PjmA7k
> > 
> > Argh, they do mult by inverse to avoid the division, and can do this
> > because its a constant.
> > 
> > And then yes, your arm version looks worse.
> 
> your "arm version" is worst then x86, or "your version" is worse?
> 
> IOW, should I keep the code as the original? Do you prefer your
> version? Or... we don't really care...

Yeah, keep the original, it didn't matter on x86 and arm regressed with
my version.

> > It does what I expected with -Os, but as Rutland said the other day,
> > that stands for Optimize for Sadness.
> 
> Yes, I guess we cannot optimize for all flags... however, just let me
> know what you prefer and I'll put that version in ;)

Yeah, don't bother optimizing for -Os, it generally creates atrocious
crap, hence Rutland calling it 'Optimize for Sadness'.