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

["Doug Smythies" <dsmythies@telus.net>]

On 2019.08.02 02:12 Rafael J. Wysocki wrote:
> On Fri, Aug 2, 2019 at 7:44 AM Viresh Kumar <viresh.kumar@linaro.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@vger.kernel.org> # v4.18+
>> Reported-by: Doug Smythies <doug.smythies@gmail.com>
>> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.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@telus.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