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SubjectRe: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked
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 ? :)

--
viresh

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