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

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Date:   Wed, 9 May 2018 11:23:01 +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 11:15 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.

> 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.