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Message-ID: <a3cf769c-5f71-17fb-eeee-6e7e17ffa78a@evidence.eu.com>
Date: Tue, 8 May 2018 14:32:27 +0200
From: Claudio Scordino <claudio@...dence.eu.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: linux-kernel@...r.kernel.org,
"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Patrick Bellasi <patrick.bellasi@....com>,
Juri Lelli <juri.lelli@...hat.com>,
Luca Abeni <luca.abeni@...tannapisa.it>,
Joel Fernandes <joelaf@...gle.com>, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency
requests
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@...dence.eu.com>
>> CC: Viresh Kumar <viresh.kumar@...aro.org>
>> CC: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> CC: Peter Zijlstra <peterz@...radead.org>
>> CC: Ingo Molnar <mingo@...hat.com>
>> CC: Patrick Bellasi <patrick.bellasi@....com>
>> CC: Juri Lelli <juri.lelli@...hat.com>
>> Cc: Luca Abeni <luca.abeni@...tannapisa.it>
>> CC: Joel Fernandes <joelaf@...gle.com>
>> CC: linux-pm@...r.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
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