linux-kernel - [PATCH v2 1/6] cpufreq/sched: Fix the usage of CPUFREQ_NEED_UPDATE

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Message-ID: <3010358.e9J7NaK4W3@rjwysocki.net>
Date: Tue, 15 Apr 2025 11:58:08 +0200
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Mario Limonciello <mario.limonciello@....com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Christian Loehle <christian.loehle@....com>,
 Sultan Alsawaf <sultan@...neltoast.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Valentin Schneider <vschneid@...hat.com>, Ingo Molnar <mingo@...hat.com>
Subject:
 [PATCH v2 1/6] cpufreq/sched: Fix the usage of CPUFREQ_NEED_UPDATE_LIMITS

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Commit 8e461a1cb43d ("cpufreq: schedutil: Fix superfluous updates caused
by need_freq_update") modified sugov_should_update_freq() to set the
need_freq_update flag only for drivers with CPUFREQ_NEED_UPDATE_LIMITS
set, but that flag generally needs to be set when the policy limits
change because the driver callback may need to be invoked for the new
limits to take effect.

However, if the return value of cpufreq_driver_resolve_freq() after
applying the new limits is still equal to the previously selected
frequency, the driver callback needs to be invoked only in the case
when CPUFREQ_NEED_UPDATE_LIMITS is set (which means that the driver
specifically wants its callback to be invoked every time the policy
limits change).

Update the code accordingly to avoid missing policy limits changes for
drivers without CPUFREQ_NEED_UPDATE_LIMITS.

Fixes: 8e461a1cb43d ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update")
Closes: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org/
Reported-by: Stephan Gerhold <stephan.gerhold@...aro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---

v1 -> v2:
   * Always set need_freq_update when limits_changed is set.
   * Take CPUFREQ_NEED_UPDATE_LIMITS into account in sugov_update_next_freq().

---
 kernel/sched/cpufreq_schedutil.c |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -83,7 +83,7 @@

 	if (unlikely(sg_policy->limits_changed)) {
 		sg_policy->limits_changed = false;
-		sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
+		sg_policy->need_freq_update = true;
 		return true;
 	}

@@ -95,10 +95,22 @@
 static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
 				   unsigned int next_freq)
 {
-	if (sg_policy->need_freq_update)
+	if (sg_policy->need_freq_update) {
 		sg_policy->need_freq_update = false;
-	else if (sg_policy->next_freq == next_freq)
+		/*
+		 * The policy limits have changed, but if the return value of
+		 * cpufreq_driver_resolve_freq() after applying the new limits
+		 * is still equal to the previously selected frequency, the
+		 * driver callback need not be invoked unless the driver
+		 * specifically wants that to happen on every update of the
+		 * policy limits.
+		 */
+		if (sg_policy->next_freq == next_freq &&
+		    !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
+			return false;
+	} else if (sg_policy->next_freq == next_freq) {
 		return false;
+	}

 	sg_policy->next_freq = next_freq;
 	sg_policy->last_freq_update_time = time;