[RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

[Claudio Scordino <claudio@evidence.eu.com>]

At OSPM, it was mentioned the issue about urgent CPU frequency requests
arriving when a frequency switch is already in progress.

Besides the various issues (physical time for switching frequency,
on-going kthread activity, etc.) one (minor) issue is the kernel
"forgetting" such request, thus waiting the next switch time for
recomputing the needed frequency and behaving accordingly.

This patch makes the kthread serve any urgent request occurred during
the previous frequency switch. It introduces a specific flag, only set
when the SCHED_DEADLINE scheduling class increases the CPU utilization,
aiming at decreasing the likelihood of a deadline miss.

Indeed, some preliminary tests in critical conditions (i.e.
SCHED_DEADLINE tasks with short periods) have shown reductions of more
than 10% of the average number of deadline misses. On the other hand,
the increase in terms of energy consumption when running SCHED_DEADLINE
tasks (not yet measured) is likely to be not negligible (especially in
case of critical scenarios like "ramp up" utilizations).

The patch is meant as follow-up discussion after OSPM.

Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
CC: Viresh Kumar <viresh.kumar@linaro.org>
CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Patrick Bellasi <patrick.bellasi@arm.com>
CC: Juri Lelli <juri.lelli@redhat.com>
Cc: Luca Abeni <luca.abeni@santannapisa.it>
CC: Joel Fernandes <joelaf@google.com>
CC: linux-pm@vger.kernel.org
---
 kernel/sched/cpufreq_schedutil.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index d2c6083..4de06b0 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -41,6 +41,7 @@ struct sugov_policy {
 	bool			work_in_progress;
 
 	bool			need_freq_update;
+	bool			urgent_freq_update;
 };
 
 struct sugov_cpu {
@@ -92,6 +93,14 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
 	    !cpufreq_can_do_remote_dvfs(sg_policy->policy))
 		return false;
 
+	/*
+	 * Continue computing the new frequency. In case of work_in_progress,
+	 * the kthread will resched a change once the current transition is
+	 * finished.
+	 */
+	if (sg_policy->urgent_freq_update)
+		return true;
+
 	if (sg_policy->work_in_progress)
 		return false;
 
@@ -121,6 +130,9 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
 	sg_policy->next_freq = next_freq;
 	sg_policy->last_freq_update_time = time;
 
+	if (sg_policy->work_in_progress)
+		return;
+
 	if (policy->fast_switch_enabled) {
 		next_freq = cpufreq_driver_fast_switch(policy, next_freq);
 		if (!next_freq)
@@ -274,7 +286,7 @@ static inline bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu) { return false; }
 static inline void ignore_dl_rate_limit(struct sugov_cpu *sg_cpu, struct sugov_policy *sg_policy)
 {
 	if (cpu_util_dl(cpu_rq(sg_cpu->cpu)) > sg_cpu->util_dl)
-		sg_policy->need_freq_update = true;
+		sg_policy->urgent_freq_update = true;
 }
 
 static void sugov_update_single(struct update_util_data *hook, u64 time,
@@ -383,8 +395,11 @@ static void sugov_work(struct kthread_work *work)
 	struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
 
 	mutex_lock(&sg_policy->work_lock);
-	__cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
+	do {
+		sg_policy->urgent_freq_update = false;
+		__cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
 				CPUFREQ_RELATION_L);
+	} while (sg_policy->urgent_freq_update);
 	mutex_unlock(&sg_policy->work_lock);
 
 	sg_policy->work_in_progress = false;
@@ -673,6 +688,7 @@ static int sugov_start(struct cpufreq_policy *policy)
 	sg_policy->next_freq			= UINT_MAX;
 	sg_policy->work_in_progress		= false;
 	sg_policy->need_freq_update		= false;
+	sg_policy->urgent_freq_update		= false;
 	sg_policy->cached_raw_freq		= 0;
 
 	for_each_cpu(cpu, policy->cpus) {
-- 
2.7.4