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

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Date:   Wed, 20 Dec 2017 15:52:43 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 3:31:00 PM CET Patrick Bellasi wrote:
> On 20-Dec 14:28, 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.
> > 
> > > > > - 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.
> 
> Right, that flag is set from:
> 
>     core.c::io_schedule_prepare()
> 
> for the current task, which is going to be dequeued soon.
> 
> Once it wakes up the next time, at enqueue time we trigger a boosting
> by passing schedutil that flag.
> 
> Thus, unless we are happy to delay the boosting until the task is
> actually picked for execution (don't think so), then we need to keep
> the flag and signal schedutil at enqueue time.
> 
> However, was wondering one thing: should't we already have a vruntime
> bonus for IO sleeping tasks? Because in that case, the task is likely
> to be on CPU quite soon... and thus, perhaps by removing the flag and
> moving the schedutil notification into core.c at the end of
> __schedule() should be working to detect both RT and FAIR::IOWAIT
> boosted tasks.

schedutil is not the only user of this flag.

Thanks,
Rafael