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Date: Fri, 18 May 2018 14:13:29 -0700
From: Saravana Kannan <skannan@...eaurora.org>
To: "Joel Fernandes (Google.)" <joelaf@...gle.com>
CC: linux-kernel@...r.kernel.org,
"Joel Fernandes (Google)" <joel@...lfernandes.org>,
Viresh Kumar <viresh.kumar@...aro.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 05/18/2018 11:55 AM, Joel Fernandes (Google.) wrote:
> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
>
> 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.
>
> I had brought up this issue at the OSPM conference and Claudio had a
> discussion RFC with an alternate approach [1]. I prefer the approach as
> done in the patch below since it doesn't need any new flags and doesn't
> cause any other extra overhead.
>
> [1] https://patchwork.kernel.org/patch/10384261/
>
> LGTMed-by: Viresh Kumar <viresh.kumar@...aro.org>
> LGTMed-by: Juri Lelli <juri.lelli@...hat.com>
> CC: Viresh Kumar <viresh.kumar@...aro.org>
> CC: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Ingo Molnar <mingo@...hat.com>
> CC: Patrick Bellasi <patrick.bellasi@....com>
> CC: Juri Lelli <juri.lelli@...hat.com>
> Cc: Luca Abeni <luca.abeni@...tannapisa.it>
> CC: Joel Fernandes <joelaf@...gle.com>
> CC: Todd Kjos <tkjos@...gle.com>
> CC: claudio@...dence.eu.com
> CC: kernel-team@...roid.com
> CC: linux-pm@...r.kernel.org
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
> v1 -> v2: Minor style related changes.
>
> 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) {
Not really something you added, but if you are modifying it:
Do we really need this work_in_progress flag? irq_work_queue() already
checks if the work is pending and then returns true/false.
Wouldn't the issue you are trying to fix be resolved just by dropping
this flag check entirely?
> 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)
>
-Saravana
--
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