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SubjectRe: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked
On 22-May 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:
> > @@ -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 ? :)

It could happen, but using:

raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
freq = READ_ONCE(sg_policy->next_freq)
WRITE_ONCE(sg_policy->work_in_progress, false);
raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);

if (!READ_ONCE(sg_policy->work_in_progress)) {
WRITE_ONCE(sg_policy->work_in_progress, true);
irq_work_queue(&sg_policy->irq_work);
}

should fix it by enforcing the ordering as well as documenting the
concurrent access.

However, in the "sched update" side, where do we have the sequence:

sg_policy->next_freq = 0;
sg_policy->next_freq = real-next-freq;

AFAICS we always use locals for next_freq and do one single assignment
in sugov_update_commit(), isn't it?

--
#include <best/regards.h>

Patrick Bellasi

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