linux-kernel - [PATCH] cpufreq: schedutil: force frequency update when limits change

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Message-Id: <20200625064614.101183-1-wvw@google.com>
Date:   Wed, 24 Jun 2020 23:46:14 -0700
From:   Wei Wang <wvw@...gle.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     wei.vince.wang@...il.com, dsmythies@...us.net,
        viresh.kumar@...aro.org, Wei Wang <wvw@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] cpufreq: schedutil: force frequency update when limits change

To avoid reducing the frequency of a CPU prematurely, we skip reducing
the frequency if the CPU had been busy recently.

This should not be done when the limits of the policy are changed, for
example due to thermal throttling. We should always get the frequency
within the new limits as soon as possible.

There was a fix in
commit 600f5badb78c ("cpufreq: schedutil: Don't skip freq update when
limits change") upstream which introduced another flag. However, the
fix didn't address the case when next_freq is the same as previously
voted, which is then checked in sugov_update_next_freq. As a result, the
frequency would be stuck at low until the high demanding workload quits.

test trace:
  kworker/u19:0-1872  ( 1872) [002] ....   347.878871: cpu_frequency_limits: min=600000 max=2348000 cpu_id=6
         dhry64-11525 (11525) [007] d.h2   347.880012: sugov_should_update_freq: thermal limit on policy6
         dhry64-11525 (11525) [007] d.h2   347.880012: sugov_deferred_update: policy6 skipped update
         dhry64-11525 (11525) [007] d.h2   347.884040: sugov_deferred_update: policy6 skipped update
...

This patch fixes this by skipping the check and forcing an update in
this case. The second flag was kept as the limits_change flag could be
updated in thermal kworker from another CPU.

Fixes: ecd288429126 ("cpufreq: schedutil: Don't set next_freq to UINT_MAX")
Signed-off-by: Wei Wang <wvw@...gle.com>
---
 kernel/sched/cpufreq_schedutil.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 7fbaee24c824..dc2cd768022e 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -102,11 +102,12 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
 static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
 				   unsigned int next_freq)
 {
-	if (sg_policy->next_freq == next_freq)
+	if (!sg_policy->need_freq_update && sg_policy->next_freq == next_freq)
 		return false;
 
 	sg_policy->next_freq = next_freq;
 	sg_policy->last_freq_update_time = time;
+	sg_policy->need_freq_update = false;
 
 	return true;
 }
@@ -178,7 +179,6 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
 	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
 		return sg_policy->next_freq;
 
-	sg_policy->need_freq_update = false;
 	sg_policy->cached_raw_freq = freq;
 	return cpufreq_driver_resolve_freq(policy, freq);
 }
-- 
2.27.0.212.ge8ba1cc988-goog