linux-kernel - Re: [PATCH RFC v4] cpufreq: schedutil: Make iowait boost more energy efficient

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Date:   Tue, 11 Jul 2017 08:14:01 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Andres Oportus <andresoportus@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC v4] cpufreq: schedutil: Make iowait boost more energy
 efficient

On 10/07/17 22:12, Joel Fernandes wrote:
> Hi Juri,
> 
> On Mon, Jul 10, 2017 at 3:55 AM, Juri Lelli <juri.lelli@....com> wrote:
> > Hi Joel,
> >
> > On 09/07/17 10:08, Joel Fernandes wrote:

[...]

> >>  static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, unsigned long *util,
> >> -                            unsigned long *max)
> >> +                            unsigned long *max, unsigned int flags)
> >>  {
> >>       unsigned long boost_util = sg_cpu->iowait_boost;
> >>       unsigned long boost_max = sg_cpu->iowait_boost_max;
> >> @@ -195,7 +222,16 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, unsigned long *util,
> >>               *util = boost_util;
> >>               *max = boost_max;
> >>       }
> >> -     sg_cpu->iowait_boost >>= 1;
> >> +
> >> +     /*
> >> +      * Incase iowait boost just happened on this CPU, don't reduce it right
> >> +      * away since then the iowait boost will never increase on subsequent
> >> +      * in_iowait wakeups.
> >> +      */
> >> +     if (flags & SCHED_CPUFREQ_IOWAIT && this_cpu_ptr(&sugov_cpu) == sg_cpu)
> >> +             return;
> >> +
> >> +     sugov_decay_iowait_boost(sg_cpu);
> >
> > Mmm, do we need to decay even when we are computing frequency requests
> > for a domain?
> >
> > Considering it a per-cpu thing, isn't enough that it gets bumped up or
> > decayed only when a CPU does an update (by using the above from
> > sugov_update_shared)?
> >
> > If we go this way I think we will only need to reset prev_iowait_boost
> > if delta_ns > TICK_NSEC during the for_each_cpu() loop of sugov_next_
> > freq_shared().
> >
> 
> Actually the "decay" was already being done before (without this
> patch), I am just preserving the same old behavior where we do decay.
> Perhaps your proposal can be a separate match? Or did I miss something
> else subtle here?
> 

True, we are currently decaying anyway.

Looking again at this path made me however think if we really need to. I
guess we need currently, as we bump frenquency to max and then decay it.
But, with your changes, I was wondering if we can simplify the thing and
decay only on the per-CPU update path.

The other reason for trying to simplify this is that I don't
particularly like adding and consuming flags argument at this point, but
I guess we could refactor the code if this is really a problem.

Thanks,

- Juri