linux-kernel - RE: [PATCH V3 1/2] cpufreq: schedutil: Don't skip freq update when limits change

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Date:   Fri, 2 Aug 2019 17:00:52 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rafael@...nel.org>,
        "'Viresh Kumar'" <viresh.kumar@...aro.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>,
        "'v4 . 18+'" <stable@...r.kernel.org>,
        "'Doug Smythies'" <doug.smythies@...il.com>,
        "'Linux Kernel Mailing List'" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V3 1/2] cpufreq: schedutil: Don't skip freq update when limits change

On 2019.08.02 02:12 Rafael J. Wysocki wrote:
> On Fri, Aug 2, 2019 at 7:44 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>
>> To avoid reducing the frequency of a CPU prematurely, we skip reducing
>> the frequency if the CPU had been busy recently.
>>
>> This should not be done when the limits of the policy are changed, for
>> example due to thermal throttling. We should always get the frequency
>> within the new limits as soon as possible.
>>
>> Trying to fix this by using only one flag, i.e. need_freq_update, can
>> lead to a race condition where the flag gets cleared without forcing us
>> to change the frequency at least once. And so this patch introduces
>>  another flag to avoid that race condition.
>>
>> Fixes: ecd288429126 ("cpufreq: schedutil: Don't set next_freq to UINT_MAX")
>> Cc: v4.18+ <stable@...r.kernel.org> # v4.18+
>> Reported-by: Doug Smythies <doug.smythies@...il.com>
>> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
>> ---
>> V2->V3:
>> - Updated commit log.
>>
>> V1->V2:
>> - Fixed the race condition using a different flag.
>>
>> @Doug: I haven't changed the code since you last tested these. Your
>> Tested-by tag can be useful while applying the patches. Thanks.

Tested-by: Doug Smythies <dsmythies@...us.net>
For acpi-cpufreq/schedutil only (which we already know).

I tested including Rafael's suggested change.
I.E.

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 592ff72..ae3ec77 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -441,7 +441,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
        struct sugov_policy *sg_policy = sg_cpu->sg_policy;
        unsigned long util, max;
        unsigned int next_f;
-       bool busy = false;
+       bool busy;

        sugov_iowait_boost(sg_cpu, time, flags);
        sg_cpu->last_update = time;
@@ -452,8 +452,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
                return;

        /* Limits may have changed, don't skip frequency update */
-       if (!sg_policy->need_freq_update)
-               busy = sugov_cpu_is_busy(sg_cpu);
+       busy = !sg_policy->need_freq_update && sugov_cpu_is_busy(sg_cpu);

        util = sugov_get_util(sg_cpu);
        max = sg_cpu->max;

... Doug