linux-kernel - Re: [PATCH 2/4] sched: cpufreq: Keep track of cpufreq utilization update flags

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Date:   Wed, 20 Dec 2017 12:55:46 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, morten.rasmussen@....com,
        juri.lelli@...hat.com, tkjos@...roid.com, joelaf@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] sched: cpufreq: Keep track of cpufreq utilization
 update flags

On 20-Dec 09:31, Peter Zijlstra wrote:
> On Wed, Dec 20, 2017 at 09:34:46AM +0530, Viresh Kumar wrote:
> > On 19-12-17, 20:25, Peter Zijlstra wrote:
> > > Yeah, not happy about this either; we had code that did the right thing
> > > without this extra tracking I think.
> > 
> > Sure, but how do you suggest we fix the problems we are facing with
> > the current design? Patrick had a completely different proposal for
> > solving those problems, which I didn't like very much. This patchset
> > replaced these patches from Patrick:
> 
> Please use the normal link format:
> 
>   https://lkml.kernel.org/r/$MSGID
> 
> Then I can find them without having to resort to a frigging browser
> thing.
> 
> I'll try and dig through the email I have.
> 
> > > Also, we can look at the rq state if we want to, we don't need to
> > > duplicate that state.
> > 
> > Well that also looks fine to me, and that would mean this:
> > 
> > - We remove SCHED_CPUFREQ_RT and SCHED_CPUFREQ_DL flags, but still
> >   call the utilization callbacks from RT and DL classes.
> 
> Didn't juri have patches to make DL do something sane? But yes, I think
> those flags are part of the problem.

He recently reposted them here:

  https://lkml.kernel.org/r/20171204102325.5110-1-juri.lelli@redhat.com
 
> > - From the utilization handler, we check runqueues of all three sched
> >   classes to see if they have some work pending (this can be done
> >   smartly by checking only RT first and skipping other checks if RT
> >   has some work).
> 
> No that's wrong. DL should provide a minimum required based on existing
> reservations, we can add the expected CFS average on top and request
> that.
> 
> And for RT all we need to know is if current is of that class, otherwise
> we don't care.

So, this:

   https://marc.info/?i=20171130114723.29210-3-patrick.bellasi%40arm.com

was actually going in this direction, although still working on top of
flags to not change the existing interface too much.

IMO, the advantage of flags is that they are a sort-of "pro-active"
approach, where the scheduler notify sensible events to schedutil.
But keep adding flags seems to overkilling to me too.

If we remove flags then we have to query the scheduler classes "on
demand"... but, as Peter suggests, once we have DL bits Juri posted,
the only issue if to know if an RT task is running.
This the patch above can be just good enough, with no flags at all and
with just a check for current being RT (or DL for the time being).


-- 
#include <best/regards.h>

Patrick Bellasi