linux-kernel - Re: [PATCH v5 02/15] sched/core: make sched_setattr able to tune the current policy

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Date:   Wed, 7 Nov 2018 14:58:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...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>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v5 02/15] sched/core: make sched_setattr able to tune the
 current policy

On Wed, Nov 07, 2018 at 01:50:39PM +0000, Patrick Bellasi wrote:
> On 07-Nov 13:11, Peter Zijlstra wrote:
> > On Mon, Oct 29, 2018 at 06:32:56PM +0000, Patrick Bellasi wrote:
> > 
> > > @@ -50,11 +52,13 @@
> > >  #define SCHED_FLAG_RESET_ON_FORK	0x01
> > >  #define SCHED_FLAG_RECLAIM		0x02
> > >  #define SCHED_FLAG_DL_OVERRUN		0x04
> > > -#define SCHED_FLAG_UTIL_CLAMP		0x08
> > > +#define SCHED_FLAG_TUNE_POLICY		0x08
> > > +#define SCHED_FLAG_UTIL_CLAMP		0x10
> > 
> > That seems to suggest you want to do this patch first, but you didn't..
> 
> I've kept it here just to better highlight this change, suggested by
> Juri, since we was not entirely sure you are fine with it...
> 
> If you think it's ok adding a SCHED_FLAG_TUNE_POLICY behavior to the
> sched_setattr syscall, I can certainly squash into the previous patch,
> which gives more context on why we need it.

I'm fine with the idea I think. It's the details I worry about. Which
fields in particular are not updated with this. Are flags?

Also, I'm not too keen on the name; since it explicitly does not modify
the policy and its related parameters, so TUNE_POLICY is actively wrong.

But the thing that confused me most is how fiddled the numbers to fit
this before UTIL_CLAMP.

> Since we are at that, are we supposed to document some{where,how}
> these API changes ?

I'm pretty sure there's a manpage somewhere... SCHED_SETATTR(2) seems to
exist on my machine. So that wants updates.