linux-kernel - Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg

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Message-ID: <20171130160205.GH9903@localhost.localdomain>
Date:   Thu, 30 Nov 2017 17:02:05 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE
 enter

On 30/11/17 15:41, Patrick Bellasi wrote:
> On 30-Nov 14:12, Juri Lelli wrote:
> > Hi,
> > 
> > On 30/11/17 11:47, Patrick Bellasi wrote:
> > 
> > [...]
> > 
> > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > index 2f52ec0f1539..67339ccb5595 100644
> > > --- a/kernel/sched/cpufreq_schedutil.c
> > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > @@ -347,6 +347,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> > >  
> > >  	sg_cpu->util = util;
> > >  	sg_cpu->max = max;
> > > +
> > > +	/* CPU is entering IDLE, reset flags without triggering an update */
> > > +	if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
> > > +		sg_cpu->flags = 0;
> > > +		goto done;
> > > +	}
> > 
> > Looks good for now. I'm just thinking that we will happen for DL, as a
> > CPU that still "has" a sleeping task is not going to be really idle
> > until the 0-lag time.
> 
> AFAIU, for the time being, DL already cannot really rely on this flag
> for its behaviors to be correct. Indeed, flags are reset as soon as
> a FAIR task wakes up and it's enqueued.

Right, and your flags ORing patch should help with this.

> 
> Only once your DL integration patches are in, we do not depends on
> flags anymore since DL will report a ceratain utilization up to the
> 0-lag time, isn't it?

Utilization won't decrease until 0-lag time, correct. I was just
wondering if resetting flags before that time (when a CPU enters idle)
might be an issue.

> 
> If that's the case, I would say that the flags will be used only to
> jump to the max OPP for RT tasks. Thus, this patch should still be valid.
> 
> > I guess we could move this at that point in time?
> 
> Not sure what you mean here. Right now the new SCHED_CPUFREQ_IDLE flag
> is notified only by idle tasks. That's the only code path where we are
> sure the CPU is entering IDLE.
> 

W.r.t. the possible issue above, I was thinking that we might want to
reset flags at 0-lag time for DL (if CPU is still idle). Anyway, two
distinct set of patches. Who gets in last will have to ponder the thing
a little bit more. :)

Best,

Juri