[PATCH] cpufreq: schedutil: Avoid missing updates for one-CPU policies

From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Linux PM <linux-pm@vger.kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	claudio@evidence.eu.com, Todd Kjos <tkjos@google.com>
Subject: [PATCH] cpufreq: schedutil: Avoid missing updates for one-CPU policies
Date: Wed, 23 May 2018 11:47:45 +0200	[thread overview]
Message-ID: <1672734.JYOlA1IWnU@aspire.rjw.lan> (raw)

From: Rafael J. Wysocki <rafael.j.wysocki@intel.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@intel.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);
