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Date: Wed, 23 May 2018 15:44:18 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: Linux PM <linux-pm@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Joel Fernandes <joel@...lfernandes.org>,
Patrick Bellasi <patrick.bellasi@....com>,
claudio@...dence.eu.com, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH] cpufreq: schedutil: Avoid missing updates for one-CPU
policies
On 23-05-18, 11:47, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Commit 152db033d775 (schedutil: Allow cpufreq requests to be made
> even when kthread kicked) made changes to prevent utilization updates
> from being discarded during processing a previous request, but it
> left a small window in which that still can happen in the one-CPU
> policy case. Namely, updates coming in after setting work_in_progress
> in sugov_update_commit() and clearing it in sugov_work() will still
> be dropped due to the work_in_progress check in sugov_update_single().
>
> To close that window, rearrange the code so as to acquire the update
> lock around the deferred update branch in sugov_update_single()
> and drop the work_in_progress check from it.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> kernel/sched/cpufreq_schedutil.c | 70 ++++++++++++++++++++++++++-------------
> 1 file changed, 47 insertions(+), 23 deletions(-)
>
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -100,25 +100,41 @@ static bool sugov_should_update_freq(str
> return delta_ns >= sg_policy->freq_update_delay_ns;
> }
>
> -static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
> - unsigned int next_freq)
> +static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> + unsigned int next_freq)
> {
> - struct cpufreq_policy *policy = sg_policy->policy;
> -
> if (sg_policy->next_freq == next_freq)
> - return;
> + return false;
>
> sg_policy->next_freq = next_freq;
> sg_policy->last_freq_update_time = time;
>
> - if (policy->fast_switch_enabled) {
> - next_freq = cpufreq_driver_fast_switch(policy, next_freq);
> - if (!next_freq)
> - return;
> + return true;
> +}
> +
> +static void sugov_fast_switch(struct sugov_policy *sg_policy, u64 time,
> + unsigned int next_freq)
> +{
> + struct cpufreq_policy *policy = sg_policy->policy;
> +
> + if (!sugov_update_next_freq(sg_policy, time, next_freq))
> + return;
> +
> + next_freq = cpufreq_driver_fast_switch(policy, next_freq);
> + if (!next_freq)
> + return;
>
> - policy->cur = next_freq;
> - trace_cpu_frequency(next_freq, smp_processor_id());
> - } else if (!sg_policy->work_in_progress) {
> + policy->cur = next_freq;
> + trace_cpu_frequency(next_freq, smp_processor_id());
> +}
> +
> +static void sugov_deferred_update(struct sugov_policy *sg_policy, u64 time,
> + unsigned int next_freq)
> +{
> + if (!sugov_update_next_freq(sg_policy, time, next_freq))
> + return;
> +
> + if (!sg_policy->work_in_progress) {
> sg_policy->work_in_progress = true;
> irq_work_queue(&sg_policy->irq_work);
> }
> @@ -363,13 +379,6 @@ static void sugov_update_single(struct u
>
> 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;
>
> @@ -391,7 +400,18 @@ static void sugov_update_single(struct u
> sg_policy->cached_raw_freq = 0;
> }
>
> - sugov_update_commit(sg_policy, time, next_f);
> + /*
> + * This code runs under rq->lock for the target CPU, so it won't run
> + * concurrently on two different CPUs for the same target and it is not
> + * necessary to acquire the lock in the fast switch case.
> + */
> + if (sg_policy->policy->fast_switch_enabled) {
> + sugov_fast_switch(sg_policy, time, next_f);
> + } else {
> + raw_spin_lock(&sg_policy->update_lock);
> + sugov_deferred_update(sg_policy, time, next_f);
> + raw_spin_unlock(&sg_policy->update_lock);
> + }
> }
>
> static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
> @@ -435,7 +455,11 @@ sugov_update_shared(struct update_util_d
>
> if (sugov_should_update_freq(sg_policy, time)) {
> next_f = sugov_next_freq_shared(sg_cpu, time);
> - sugov_update_commit(sg_policy, time, next_f);
> +
> + if (sg_policy->policy->fast_switch_enabled)
> + sugov_fast_switch(sg_policy, time, next_f);
> + else
> + sugov_deferred_update(sg_policy, time, next_f);
> }
>
> raw_spin_unlock(&sg_policy->update_lock);
> @@ -450,11 +474,11 @@ static void sugov_work(struct kthread_wo
> /*
> * 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
> + * sugov_deferred_update() 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
> + * 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);
Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
--
viresh
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