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Date: Tue, 8 May 2018 21:54:25 -0700
From: Joel Fernandes <joel@...lfernandes.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Claudio Scordino <claudio@...dence.eu.com>,
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
On Tue, May 08, 2018 at 12:24:35PM +0530, Viresh Kumar wrote:
> 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.
Just for discussion sake, is there any need for work_in_progress? If we can
queue multiple work say kthread_queue_work can handle it, then just queuing
works whenever they are available should be Ok and the kthread loop can
handle them. __cpufreq_driver_target is also protected by the work lock if
there is any concern that can have races... only thing is rate-limiting of
the requests, but we are doing a rate limiting, just not for the "DL
increased utilization" type requests (which I don't think we are doing at the
moment for urgent DL requests anyway).
Following is an untested diff to show the idea. What do you think?
thanks,
- Joel
----8<---
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index d2c6083304b4..862634ff4bf3 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -38,7 +38,6 @@ struct sugov_policy {
struct mutex work_lock;
struct kthread_worker worker;
struct task_struct *thread;
- bool work_in_progress;
bool need_freq_update;
};
@@ -92,16 +91,8 @@ 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;
- /*
- * This happens when limits change, so forget the previous
- * next_freq value and force an update.
- */
- sg_policy->next_freq = UINT_MAX;
return true;
}
@@ -129,7 +120,6 @@ 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 {
- sg_policy->work_in_progress = true;
irq_work_queue(&sg_policy->irq_work);
}
}
@@ -386,8 +376,6 @@ static void sugov_work(struct kthread_work *work)
__cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
CPUFREQ_RELATION_L);
mutex_unlock(&sg_policy->work_lock);
-
- sg_policy->work_in_progress = false;
}
static void sugov_irq_work(struct irq_work *irq_work)
@@ -671,7 +659,6 @@ static int sugov_start(struct cpufreq_policy *policy)
sg_policy->freq_update_delay_ns = sg_policy->tunables->rate_limit_us * NSEC_PER_USEC;
sg_policy->last_freq_update_time = 0;
sg_policy->next_freq = UINT_MAX;
- sg_policy->work_in_progress = false;
sg_policy->need_freq_update = false;
sg_policy->cached_raw_freq = 0;
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