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Message-ID: <4393838.cQWhzXzUcj@aspire.rjw.lan>
Date: Tue, 22 May 2018 12:50:46 +0200
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Joel Fernandes (Google.)" <joelaf@...gle.com>,
linux-kernel@...r.kernel.org,
"Joel Fernandes (Google)" <joel@...lfernandes.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>,
Todd Kjos <tkjos@...gle.com>, claudio@...dence.eu.com,
kernel-team@...roid.com, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked
On Tuesday, May 22, 2018 12:50:06 PM CEST Viresh Kumar wrote:
> On 22-05-18, 16:04, Viresh Kumar wrote:
> > Okay, me and Rafael were discussing this patch, locking and races around this.
> >
> > On 18-05-18, 11:55, Joel Fernandes (Google.) wrote:
> > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > index e13df951aca7..5c482ec38610 100644
> > > --- a/kernel/sched/cpufreq_schedutil.c
> > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > @@ -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;
> > > -
> > > if (unlikely(sg_policy->need_freq_update)) {
> > > sg_policy->need_freq_update = false;
> > > /*
> > > @@ -128,7 +125,7 @@ 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 {
> > > + } else if (!sg_policy->work_in_progress) {
> > > sg_policy->work_in_progress = true;
> > > irq_work_queue(&sg_policy->irq_work);
> > > }
> > > @@ -291,6 +288,13 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> > >
> > > ignore_dl_rate_limit(sg_cpu, sg_policy);
> > >
> > > + /*
> > > + * For slow-switch systems, single policy requests can't run at the
> > > + * moment if update is in progress, unless we acquire update_lock.
> > > + */
> > > + if (sg_policy->work_in_progress)
> > > + return;
> > > +
> > > if (!sugov_should_update_freq(sg_policy, time))
> > > return;
> > >
> > > @@ -382,13 +386,27 @@ 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;
> > > + unsigned long flags;
> > > +
> > > + /*
> > > + * Hold sg_policy->update_lock shortly to handle the case where:
> > > + * incase sg_policy->next_freq is read here, and then updated by
> > > + * sugov_update_shared just before work_in_progress is set to false
> > > + * here, we may miss queueing the new update.
> > > + *
> > > + * Note: If a work was queued after the update_lock is released,
> > > + * sugov_work will just be called again by kthread_work code; and the
> > > + * request will be proceed before the sugov thread sleeps.
> > > + */
> > > + raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
> > > + freq = sg_policy->next_freq;
> > > + sg_policy->work_in_progress = false;
> > > + raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
> > >
> > > mutex_lock(&sg_policy->work_lock);
> > > - __cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
> > > - CPUFREQ_RELATION_L);
> > > + __cpufreq_driver_target(sg_policy->policy, freq, CPUFREQ_RELATION_L);
> > > mutex_unlock(&sg_policy->work_lock);
> > > -
> > > - sg_policy->work_in_progress = false;
> > > }
> >
> > And I do see a race here for single policy systems doing slow switching.
> >
> > Kthread Sched update
> >
> > sugov_work() sugov_update_single()
> >
> > lock();
> > // The CPU is free to rearrange below
> > // two in any order, so it may clear
> > // the flag first and then read next
> > // freq. Lets assume it does.
> > work_in_progress = false
> >
> > if (work_in_progress)
> > return;
> >
> > sg_policy->next_freq = 0;
> > freq = sg_policy->next_freq;
> > sg_policy->next_freq = real-next-freq;
> > unlock();
> >
> >
> >
> > Is the above theory right or am I day dreaming ? :)
>
> And here comes the ugly fix:
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 35826f4ec43c..1665da31862e 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -283,6 +283,9 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>
> ignore_dl_rate_limit(sg_cpu, sg_policy);
>
> + if (!policy->fast_switch_enabled)
> + raw_spin_lock(&sg_policy->update_lock);
> +
> /*
> * For slow-switch systems, single policy requests can't run at the
> * moment if update is in progress, unless we acquire update_lock.
> @@ -312,6 +315,9 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> }
>
> sugov_update_commit(sg_policy, time, next_f);
> +
> + if (!policy->fast_switch_enabled)
> + raw_spin_unlock(&sg_policy->update_lock);
> }
>
> static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
>
>
>
Ugly indeed.
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