linux-kernel - Re: [PATCH] cpufreq: conservative: Fix requested

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Message-ID: <CAHKzcEMx2V-uvkM7_gUXiL1jEiYDwNn3KoGHqneqXAwmUQJ+0w@mail.gmail.com>
Date:   Tue, 9 Oct 2018 18:06:08 +0200
From:   Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...il.com>
To:     rafael@...nel.org
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Waldemar Rymarkiewicz <waldemarx.rymarkiewicz@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Bartholomae, Thomas" <t.bartholomae@...el.com>
Subject: Re: [PATCH] cpufreq: conservative: Fix requested_freq handling

On Tue, 9 Oct 2018 at 09:47, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Mon, Oct 8, 2018 at 5:11 PM Waldemar Rymarkiewicz
> <waldemar.rymarkiewicz@...il.com> wrote:
> >
> > From: Waldemar Rymarkiewicz <waldemarx.rymarkiewicz@...el.com>
> >
> > The governor updates dbs_info->requested_freq only after increasing or
> > decreasing frequency. There is, however, an use case when this is not
> > sufficient.
> >
> > Imagine, external module constraining cpufreq policy in a way that policy->max
>
> Is the "external module" here a utility or a demon running in user space?

No, this is a driver that communicates with a firmware and makes sure
CPU is running at the highest rate in specific time.
It uses verify_within_limits  and update_policy, a standard way to
constraint cpufreq policy limits.

> > @@ -136,10 +135,10 @@ static unsigned int cs_dbs_update(struct cpufreq_policy *policy)
> >                         requested_freq = policy->min;
> >
> >                 __cpufreq_driver_target(policy, requested_freq, CPUFREQ_RELATION_L);
> > -               dbs_info->requested_freq = requested_freq;
> >         }
> >
> >   out:
> > +       dbs_info->requested_freq = requested_freq;
>
> This will have a side effect when requested_freq is updated before the
> thresholds checks due to the policy_dbs->idle_periods < UINT_MAX
> check.
>
> Shouldn't that be avoided?

I would say we should.

A hardware design I use is running 4.9 kernel where the check does not
exist yet, so there is not a problem.
Anyway, the check policy_dbs->idle_periods < UINT_MAX  can change
requested_freq  either to requested_freq = policy->min or
requested_freq -= freq_steps;. The first case will not change anything
for us as policy->max=min=cur. The second, however, will force to
update freq which is definitely not expected when limits are set to
min=max. Simply it will not go out  here:

if (load < cs_tuners->down_threshold) {
      if (requested_freq == policy->min)
           goto out;   <---
...
}

Am I right here? If so, shouldn't we check explicitly

/*
* If requested_freq is out of range, it is likely that the limits
* changed in the meantime, so fall back to current frequency in that
* case.
*/
if (requested_freq > policy->max || requested_freq < policy->min)
       requested_freq = policy->cur;

+/*
+* If the the new limits min,max are equal, there is no point to process further
+*/
+
+if (requested_freq == policy->max  &&  requested_freq == policy->min)
+     goto out;

Thanks,
/Waldek