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

[Claudio Scordino <claudio@evidence.eu.com>]

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@evidence.eu.com>
>> CC: Viresh Kumar <viresh.kumar@linaro.org>
>> CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> CC: Peter Zijlstra <peterz@infradead.org>
>> CC: Ingo Molnar <mingo@redhat.com>
>> CC: Patrick Bellasi <patrick.bellasi@arm.com>
>> CC: Juri Lelli <juri.lelli@redhat.com>
>> Cc: Luca Abeni <luca.abeni@santannapisa.it>
>> CC: Joel Fernandes <joelaf@google.com>
>> CC: linux-pm@vger.kernel.org
>> ---
>>   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 ?

Thanks,

            Claudio