Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

[Rafael J. Wysocki]

On Tue, May 8, 2018 at 2:32 PM, Claudio Scordino
<claudio@xxxxxxxxxxxxxxx> wrote:
>
>
> Il 08/05/2018 08:54, Viresh Kumar ha scritto:
>>
>> On 07-05-18, 16:43, Claudio Scordino wrote:
>>>
>>> At OSPM, it was mentioned the issue about urgent CPU frequency requests
>>> arriving when a frequency switch is already in progress.
>>>
>>> Besides the various issues (physical time for switching frequency,
>>> on-going kthread activity, etc.) one (minor) issue is the kernel
>>> "forgetting" such request, thus waiting the next switch time for
>>> recomputing the needed frequency and behaving accordingly.
>>>
>>> This patch makes the kthread serve any urgent request occurred during
>>> the previous frequency switch. It introduces a specific flag, only set
>>> when the SCHED_DEADLINE scheduling class increases the CPU utilization,
>>> aiming at decreasing the likelihood of a deadline miss.
>>>
>>> Indeed, some preliminary tests in critical conditions (i.e.
>>> SCHED_DEADLINE tasks with short periods) have shown reductions of more
>>> than 10% of the average number of deadline misses. On the other hand,
>>> the increase in terms of energy consumption when running SCHED_DEADLINE
>>> tasks (not yet measured) is likely to be not negligible (especially in
>>> case of critical scenarios like "ramp up" utilizations).
>>>
>>> The patch is meant as follow-up discussion after OSPM.
>>>
>>> Signed-off-by: Claudio Scordino <claudio@xxxxxxxxxxxxxxx>
>>> 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: linux-pm@xxxxxxxxxxxxxxx
>>> ---
>>>   kernel/sched/cpufreq_schedutil.c | 20 ++++++++++++++++++--
>>>   1 file changed, 18 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/sched/cpufreq_schedutil.c
>>> b/kernel/sched/cpufreq_schedutil.c
>>> index d2c6083..4de06b0 100644
>>> --- a/kernel/sched/cpufreq_schedutil.c
>>> +++ b/kernel/sched/cpufreq_schedutil.c
>>> @@ -41,6 +41,7 @@ struct sugov_policy {
>>>         bool                    work_in_progress;
>>>         bool                    need_freq_update;
>>> +       bool                    urgent_freq_update;
>>>   };
>>>     struct sugov_cpu {
>>> @@ -92,6 +93,14 @@ static bool sugov_should_update_freq(struct
>>> sugov_policy *sg_policy, u64 time)
>>>             !cpufreq_can_do_remote_dvfs(sg_policy->policy))
>>>                 return false;
>>>   +     /*
>>> +        * Continue computing the new frequency. In case of
>>> work_in_progress,
>>> +        * the kthread will resched a change once the current transition
>>> is
>>> +        * finished.
>>> +        */
>>> +       if (sg_policy->urgent_freq_update)
>>> +               return true;
>>> +
>>>         if (sg_policy->work_in_progress)
>>>                 return false;
>>>   @@ -121,6 +130,9 @@ static void sugov_update_commit(struct sugov_policy
>>> *sg_policy, u64 time,
>>>         sg_policy->next_freq = next_freq;
>>>         sg_policy->last_freq_update_time = time;
>>>   +     if (sg_policy->work_in_progress)
>>> +               return;
>>> +
>>>         if (policy->fast_switch_enabled) {
>>>                 next_freq = cpufreq_driver_fast_switch(policy,
>>> next_freq);
>>>                 if (!next_freq)
>>> @@ -274,7 +286,7 @@ static inline bool sugov_cpu_is_busy(struct sugov_cpu
>>> *sg_cpu) { return false; }
>>>   static inline void ignore_dl_rate_limit(struct sugov_cpu *sg_cpu,
>>> struct sugov_policy *sg_policy)
>>>   {
>>>         if (cpu_util_dl(cpu_rq(sg_cpu->cpu)) > sg_cpu->util_dl)
>>> -               sg_policy->need_freq_update = true;
>>> +               sg_policy->urgent_freq_update = true;
>>>   }
>>>     static void sugov_update_single(struct update_util_data *hook, u64
>>> time,
>>> @@ -383,8 +395,11 @@ static void sugov_work(struct kthread_work *work)
>>>         struct sugov_policy *sg_policy = container_of(work, struct
>>> sugov_policy, work);
>>>         mutex_lock(&sg_policy->work_lock);
>>> -       __cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
>>> +       do {
>>> +               sg_policy->urgent_freq_update = false;
>>> +               __cpufreq_driver_target(sg_policy->policy,
>>> sg_policy->next_freq,
>>>                                 CPUFREQ_RELATION_L);
>>
>>
>> If we are going to solve this problem, then maybe instead of the added
>> complexity and a new flag we can look for need_freq_update flag at this
>> location
>> and re-calculate the next frequency if required.
>
>
> I agree.
> Indeed, I've been in doubt if adding a new flag or relying on the existing
> need_freq_update flag (whose name, however, didn't seem to reflect any sense
> of urgency).
> Maybe we can use need_freq_update but change its name to a more meaningful
> string ?

Implicitly, it means "as soon as reasonably possible".