cpufreq: conservative: Reset requested_freq on limits change

A recently reported issue highlighted that the cached requested_freq
is not guaranteed to stay in sync with policy->cur. If the platform
changes the actual CPU frequency after the governor sets one (e.g.
due to platform-specific frequency scaling) and a re-sync occurs
later, policy->cur may diverge from requested_freq.

This can lead to incorrect behavior in the conservative governor.
For example, the governor may assume the CPU is already running at
the maximum frequency and skip further increases even though there
is still headroom.

Avoid this by resetting the cached requested_freq to policy->cur on
detecting a change in policy limits.

Reported-by: Lifeng Zheng <[email protected]>
Tested-by: Lifeng Zheng <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Viresh Kumar <[email protected]>
Reviewed-by: Zhongqiu Han <[email protected]>
Cc: All applicable <[email protected]>
Link: https://patch.msgid.link/d846a141a98ac0482f20560fcd7525c0f0ec2f30.1773999467.git.viresh.kumar@linaro.org
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: vireshk

drivers/cpufreq/cpufreq_conservative.c

@@ -313,6 +313,17 @@ static void cs_start(struct cpufreq_policy *policy)
 	dbs_info->requested_freq = policy->cur;
 }
 
+static void cs_limits(struct cpufreq_policy *policy)
+{
+	struct cs_policy_dbs_info *dbs_info = to_dbs_info(policy->governor_data);
+
+	/*
+	 * The limits have changed, so may have the current frequency. Reset
+	 * requested_freq to avoid any unintended outcomes due to the mismatch.
+	 */
+	dbs_info->requested_freq = policy->cur;
+}
+
 static struct dbs_governor cs_governor = {
 	.gov = CPUFREQ_DBS_GOVERNOR_INITIALIZER("conservative"),
 	.kobj_type = { .default_groups = cs_groups },
@@ -322,6 +333,7 @@ static struct dbs_governor cs_governor = {
 	.init = cs_init,
 	.exit = cs_exit,
 	.start = cs_start,
+	.limits = cs_limits,
 };
 
 #define CPU_FREQ_GOV_CONSERVATIVE	(cs_governor.gov)

drivers/cpufreq/cpufreq_governor.c

@@ -563,6 +563,7 @@ EXPORT_SYMBOL_GPL(cpufreq_dbs_governor_stop);
 
 void cpufreq_dbs_governor_limits(struct cpufreq_policy *policy)
 {
+	struct dbs_governor *gov = dbs_governor_of(policy);
 	struct policy_dbs_info *policy_dbs;
 
 	/* Protect gov->gdbs_data against cpufreq_dbs_governor_exit() */
@@ -574,6 +575,8 @@ void cpufreq_dbs_governor_limits(struct cpufreq_policy *policy)
 	mutex_lock(&policy_dbs->update_mutex);
 	cpufreq_policy_apply_limits(policy);
 	gov_update_sample_delay(policy_dbs, 0);
+	if (gov->limits)
+		gov->limits(policy);
 	mutex_unlock(&policy_dbs->update_mutex);
 
 out:

drivers/cpufreq/cpufreq_governor.h

@@ -138,6 +138,7 @@ struct dbs_governor {
 	int (*init)(struct dbs_data *dbs_data);
 	void (*exit)(struct dbs_data *dbs_data);
 	void (*start)(struct cpufreq_policy *policy);
+	void (*limits)(struct cpufreq_policy *policy);
 };
 
 static inline struct dbs_governor *dbs_governor_of(struct cpufreq_policy *policy)