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Date: Wed, 9 May 2018 10:30:37 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Juri Lelli <juri.lelli@...hat.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
Claudio Scordino <claudio@...dence.eu.com>,
Linux Kernel Mailing List <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>,
Luca Abeni <luca.abeni@...tannapisa.it>,
Joel Fernandes <joelaf@...gle.com>,
Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests
On Wed, May 9, 2018 at 10:06 AM, Joel Fernandes <joel@...lfernandes.org> wrote:
> On Wed, May 09, 2018 at 08:45:30AM +0200, Juri Lelli wrote:
>> On 08/05/18 21:54, Joel Fernandes wrote:
>>
>> [...]
>>
>> > 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);
>>
>> Isn't this potentially introducing unneeded irq pressure (and doing the
>> whole wakeup the kthread thing), while the already active kthread could
>> simply handle multiple back-to-back requests before going to sleep?
>
> How about this? Will use the latest request, and also doesn't do unnecessary
> irq_work_queue:
>
> (untested)
> -----8<--------
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index d2c6083304b4..6a3e42b01f52 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -38,7 +38,7 @@ struct sugov_policy {
> struct mutex work_lock;
> struct kthread_worker worker;
> struct task_struct *thread;
> - bool work_in_progress;
> + bool work_in_progress; /* Has kthread been kicked */
>
> bool need_freq_update;
> };
> @@ -92,9 +92,6 @@ 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;
> -
Why this change?
Doing the below is rather pointless if work_in_progress is set, isn't it?
You'll drop the results of it on the floor going forward anyway then AFAICS.
> if (unlikely(sg_policy->need_freq_update)) {
> sg_policy->need_freq_update = false;
> /*
> @@ -129,8 +126,11 @@ 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);
> + /* work_in_progress helps us not queue unnecessarily */
> + if (!sg_policy->work_in_progress) {
> + sg_policy->work_in_progress = true;
> + irq_work_queue(&sg_policy->irq_work);
> + }
> }
> }
>
> @@ -381,13 +381,23 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)
> static void sugov_work(struct kthread_work *work)
> {
> struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> + unsigned int freq;
> +
> + /*
> + * Hold sg_policy->update_lock just enough to handle the case where:
> + * if sg_policy->next_freq is updated before work_in_progress is set to
> + * false, we may miss queueing the new update request since
> + * work_in_progress would appear to be true.
> + */
> + raw_spin_lock(&sg_policy->update_lock);
> + freq = sg_policy->next_freq;
> + sg_policy->work_in_progress = false;
> + raw_spin_unlock(&sg_policy->update_lock);
>
> mutex_lock(&sg_policy->work_lock);
> - __cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
> + __cpufreq_driver_target(sg_policy->policy, 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)
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