linux-kernel - Re: [PATCH] sched/schedutil: Don't set next_freq to UINT

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Date:   Wed, 9 May 2018 15:00:23 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        "4 . 12+" <stable@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/schedutil: Don't set next_freq to UINT_MAX

On 09-05-18, 11:23, Rafael J. Wysocki wrote:
> On Wed, May 9, 2018 at 11:15 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > On 09-05-18, 10:56, Rafael J. Wysocki wrote:
> >> I'm kind of concerned about updating the limits via sysfs in which
> >> case the cached next frequency may be out of range, so it's better to
> >> invalidate it right away then.
> >
> > That should not be a problem as __cpufreq_driver_target() will anyway
> > clamp the target frequency to be within limits, whatever the cached
> > value of next_freq is.
> 
> The fast switch case doesn't use it, though.

cpufreq_driver_fast_switch() does the same clamping :)

> > And we aren't invalidating the cached next freq immediately currently
> > as well, as we are waiting until the next time the util update handler
> > is called to set sg_policy->next_freq to UINT_MAX.
> >
> >> > What else do you have in mind to solve this problem ?
> >>
> >> Something like the below?
> >>
> >> ---
> >>  kernel/sched/cpufreq_schedutil.c |    3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> >> ===================================================================
> >> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> >> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> >> @@ -305,7 +305,8 @@ static void sugov_update_single(struct u
> >>       * Do not reduce the frequency if the CPU has not been idle
> >>       * recently, as the reduction is likely to be premature then.
> >>       */
> >> -    if (busy && next_f < sg_policy->next_freq) {
> >> +    if (busy && next_f < sg_policy->next_freq &&
> >> +        sg_policy->next_freq != UINT_MAX) {
> >>          next_f = sg_policy->next_freq;
> >>
> >>          /* Reset cached freq as next_freq has changed */
> >
> > This will fix the problem we have identified currently, but adding a
> > special meaning to next_freq == UINT_MAX invites more hidden corner
> > cases like the one we just found. IMHO, using next_freq only for the
> > *real* frequency values makes its usage more transparent and readable.
> > And we already have the need_freq_update flag which we can use for
> > this special purpose, as is done in my patch.
> 
> So I prefer to do the above as a -stable fix and make the UNIT_MAX
> change on top of that.

Okay, that's fine with me. Will send the next version now :)

Just to make sure, you are fine with the "Fixes" tag now (since you
objected to that earlier) ?

-- 
viresh