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Message-ID: <20180522105006.hvntwnhzzvhznij2@vireshk-i7>
Date: Tue, 22 May 2018 16:20:06 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: "Joel Fernandes (Google.)" <joelaf@...gle.com>
Cc: 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 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)
--
viresh
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