Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked | Date | Tue, 22 May 2018 12:50:46 +0200 |
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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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