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

[Claudio Scordino]

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 ?

Thanks,

           Claudio