Re: [PATCH v3] schedutil: Allow cpufreq requests to be made even when kthread kicked

From: Rafael J. Wysocki
Date: Wed May 23 2018 - 04:48:10 EST


On Wednesday, May 23, 2018 12:55:53 AM CEST Joel Fernandes wrote:
> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
>
> Currently there is a chance of a schedutil cpufreq update request to be
> dropped if there is a pending update request. This pending request can
> be delayed if there is a scheduling delay of the irq_work and the wake
> up of the schedutil governor kthread.
>
> A very bad scenario is when a schedutil request was already just made,
> such as to reduce the CPU frequency, then a newer request to increase
> CPU frequency (even sched deadline urgent frequency increase requests)
> can be dropped, even though the rate limits suggest that its Ok to
> process a request. This is because of the way the work_in_progress flag
> is used.
>
> This patch improves the situation by allowing new requests to happen
> even though the old one is still being processed. Note that in this
> approach, if an irq_work was already issued, we just update next_freq
> and don't bother to queue another request so there's no extra work being
> done to make this happen.
>
> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> CC: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> CC: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Patrick Bellasi <patrick.bellasi@xxxxxxx>
> CC: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
> CC: Todd Kjos <tkjos@xxxxxxxxxx>
> CC: claudio@xxxxxxxxxxxxxxx
> CC: kernel-team@xxxxxxxxxxx
> CC: linux-pm@xxxxxxxxxxxxxxx
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
> Only commit log update, no code change in v2->v3
>
> kernel/sched/cpufreq_schedutil.c | 34 ++++++++++++++++++++++++--------
> 1 file changed, 26 insertions(+), 8 deletions(-)
>
> 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;
> }
>
> static void sugov_irq_work(struct irq_work *irq_work)
>

Applied, thanks!