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

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Date:   Wed, 9 May 2018 11:32:20 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        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 Wed, May 9, 2018 at 11:30 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> 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) ?

OK, so to be clear, I'm going to queue up the simple patch I posted
with a FIxes: tag.  I'll resend it with a changelog shortly.

Then please send a UINT_MAX change on top of that and it won't be an
urgent fix any more.