Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

[Rafael J. Wysocki]

On Fri, May 18, 2018 at 11:13 PM, Saravana Kannan
<skannan@xxxxxxxxxxxxxx> wrote:
> On 05/18/2018 11:55 AM, Joel Fernandes (Google.) wrote:
>>
>> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
>>
>> Currently there is a chance of a schedutil cpufreq update request to be
>> dropped if there is a pending update request. This pending request can
>> be delayed if there is a scheduling delay of the irq_work and the wake
>> up of the schedutil governor kthread.
>>
>> A very bad scenario is when a schedutil request was already just made,
>> such as to reduce the CPU frequency, then a newer request to increase
>> CPU frequency (even sched deadline urgent frequency increase requests)
>> can be dropped, even though the rate limits suggest that its Ok to
>> process a request. This is because of the way the work_in_progress flag
>> is used.
>>
>> This patch improves the situation by allowing new requests to happen
>> even though the old one is still being processed. Note that in this
>> approach, if an irq_work was already issued, we just update next_freq
>> and don't bother to queue another request so there's no extra work being
>> done to make this happen.
>>
>> I had brought up this issue at the OSPM conference and Claudio had a
>> discussion RFC with an alternate approach [1]. I prefer the approach as
>> done in the patch below since it doesn't need any new flags and doesn't
>> cause any other extra overhead.
>>
>> [1] https://patchwork.kernel.org/patch/10384261/
>>
>> LGTMed-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>> LGTMed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
>> CC: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>> CC: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> CC: Ingo Molnar <mingo@xxxxxxxxxx>
>> CC: Patrick Bellasi <patrick.bellasi@xxxxxxx>
>> CC: Juri Lelli <juri.lelli@xxxxxxxxxx>
>> Cc: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
>> CC: Joel Fernandes <joelaf@xxxxxxxxxx>
>> CC: Todd Kjos <tkjos@xxxxxxxxxx>
>> CC: claudio@xxxxxxxxxxxxxxx
>> CC: kernel-team@xxxxxxxxxxx
>> CC: linux-pm@xxxxxxxxxxxxxxx
>> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>> ---
>> v1 -> v2: Minor style related changes.
>>
>>   kernel/sched/cpufreq_schedutil.c | 34 ++++++++++++++++++++++++--------
>>   1 file changed, 26 insertions(+), 8 deletions(-)
>>
>> diff --git a/kernel/sched/cpufreq_schedutil.c
>> b/kernel/sched/cpufreq_schedutil.c
>> index e13df951aca7..5c482ec38610 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -92,9 +92,6 @@ static bool sugov_should_update_freq(struct sugov_policy
>> *sg_policy, u64 time)
>>             !cpufreq_can_do_remote_dvfs(sg_policy->policy))
>>                 return false;
>>
>> -       if (sg_policy->work_in_progress)
>> -               return false;
>> -
>>         if (unlikely(sg_policy->need_freq_update)) {
>>                 sg_policy->need_freq_update = false;
>>                 /*
>> @@ -128,7 +125,7 @@ static void sugov_update_commit(struct sugov_policy
>> *sg_policy, u64 time,
>>
>>                 policy->cur = next_freq;
>>                 trace_cpu_frequency(next_freq, smp_processor_id());
>> -       } else {
>> +       } else if (!sg_policy->work_in_progress) {
>
>
> Not really something you added, but if you are modifying it:
> Do we really need this work_in_progress flag? irq_work_queue() already
> checks if the work is pending and then returns true/false.
>
> Wouldn't the issue you are trying to fix be resolved just by dropping this
> flag check entirely?

You've missed the entire discussion on that several days ago, sorry.

Thanks,
Rafael