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

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Date:   Wed, 20 Dec 2017 15:47:23 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        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 Wednesday, December 20, 2017 2:28:26 PM CET Peter Zijlstra wrote:
> On Wed, Dec 20, 2017 at 12:55:46PM +0000, Patrick Bellasi wrote:
> > On 20-Dec 09:31, Peter Zijlstra wrote:
> 
> > > 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
> 
> Yeah, just found them and actually munged them into my queue; did all
> the modifications you suggested too. Lets see if it comes apart.

Good, because I think that the Juri's patches should go in first.

Then we'll see what's still missing.

> > > > - 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
> 
> Right, I was actually looking for those patches, but I'm searching
> backwards and hit upon Juri's patches first.
> 
> > 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).
> 
> Well, we still need flags for crap like IO-WAIT IIRC. That's sugov
> internal state and not something the scheduler actually already knows.

Not only sugov to be precise, but yes.

Thanks,
Rafael