[PATCH v2 1/2] cpufreq/schedutil: Remove fast_switch_possible flag if driver doesn't set fast_switch

[Wyes Karny]

The set value of `fast_switch_enabled` indicates that fast_switch
callback is set. For some drivers such as amd_pstate and intel_pstate,
the adjust_perf callback is used but it still sets
`fast_switch_possible` flag. This is because this flag also decides
whether schedutil governor selects adjust_perf function for frequency
update. This condition in the schedutil governor forces the scaling
driver to set the `fast_switch_possible` flag.

Remove `fast_switch_enabled` check when schedutil decides to select
adjust_perf function for frequency update. Thus removing this drivers
are now free to remove `fast_switch_possible` flag if they don't use
fast_switch callback.

This issue becomes apparent when aperf/mperf overflow occurs.  When this
happens, kernel disables frequency invariance calculation which causes
schedutil to fallback to sugov_update_single_freq which currently relies
on the fast_switch callback.

Normal flow:
  sugov_update_single_perf
    cpufreq_driver_adjust_perf
      cpufreq_driver->adjust_perf

Error case flow:
  sugov_update_single_perf
    sugov_update_single_freq  <-- This is chosen because the freq invariant is disabled due to aperf/mperf overflow
      cpufreq_driver_fast_switch
         cpufreq_driver->fast_switch <-- Here NULL pointer dereference is happening, because fast_switch is not set

This change fixes this NULL pointer dereference issue.

Fixes: a61dec744745 ("cpufreq: schedutil: Avoid missing updates for one-CPU policies")
Signed-off-by: Wyes Karny <wyes.karny@xxxxxxx>

Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/cpufreq/amd-pstate.c     | 10 +++++++---
 drivers/cpufreq/cpufreq.c        | 20 +++++++++++++++++++-
 drivers/cpufreq/intel_pstate.c   |  3 +--
 include/linux/cpufreq.h          |  1 +
 kernel/sched/cpufreq_schedutil.c |  2 +-
 5 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index 5a3d4aa0f45a..007bfe724a6a 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -671,8 +671,14 @@ static int amd_pstate_cpu_init(struct cpufreq_policy *policy)
 	/* It will be updated by governor */
 	policy->cur = policy->cpuinfo.min_freq;
 
+	/**
+	 * For shared memory system frequency update takes time that's why
+	 * do this in deferred kthread context.
+	 */
 	if (boot_cpu_has(X86_FEATURE_CPPC))
-		policy->fast_switch_possible = true;
+		current_pstate_driver->adjust_perf = amd_pstate_adjust_perf;
+	else
+		current_pstate_driver->adjust_perf = NULL;
 
 	ret = freq_qos_add_request(&policy->constraints, &cpudata->req[0],
 				   FREQ_QOS_MIN, policy->cpuinfo.min_freq);
@@ -697,8 +703,6 @@ static int amd_pstate_cpu_init(struct cpufreq_policy *policy)
 	policy->driver_data = cpudata;
 
 	amd_pstate_boost_init(cpudata);
-	if (!current_pstate_driver->adjust_perf)
-		current_pstate_driver->adjust_perf = amd_pstate_adjust_perf;
 
 	return 0;
 
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 6b52ebe5a890..366747012104 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -501,6 +501,13 @@ void cpufreq_enable_fast_switch(struct cpufreq_policy *policy)
 	if (!policy->fast_switch_possible)
 		return;
 
+	/**
+	 * It's not expected driver's fast_switch callback is not set
+	 * even fast_switch_possible is true.
+	 */
+	if (WARN_ON(!cpufreq_driver_has_fast_switch()))
+		return;
+
 	mutex_lock(&cpufreq_fast_switch_lock);
 	if (cpufreq_fast_switch_count >= 0) {
 		cpufreq_fast_switch_count++;
@@ -2143,6 +2150,17 @@ unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
 }
 EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
 
+/**
+ * cpufreq_driver_has_fast_switch - Check "fast switch" callback.
+ *
+ * Return 'true' if the ->fast_switch callback is present for the
+ * current driver or 'false' otherwise.
+ */
+bool cpufreq_driver_has_fast_switch(void)
+{
+	return !!cpufreq_driver->fast_switch;
+}
+
 /**
  * cpufreq_driver_adjust_perf - Adjust CPU performance level in one go.
  * @cpu: Target CPU.
@@ -2157,7 +2175,7 @@ EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
  * and it is expected to select a suitable performance level equal to or above
  * @min_perf and preferably equal to or below @target_perf.
  *
- * This function must not be called if policy->fast_switch_enabled is unset.
+ * By default this function takes the fast frequency update path.
  *
  * Governors calling this function must guarantee that it will never be invoked
  * twice in parallel for the same CPU and that it will never be called in
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 2548ec92faa2..007893514c87 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -2698,8 +2698,6 @@ static int __intel_pstate_cpu_init(struct cpufreq_policy *policy)
 
 	intel_pstate_init_acpi_perf_limits(policy);
 
-	policy->fast_switch_possible = true;
-
 	return 0;
 }
 
@@ -2955,6 +2953,7 @@ static int intel_cpufreq_cpu_init(struct cpufreq_policy *policy)
 	if (ret)
 		return ret;
 
+	policy->fast_switch_possible = true;
 	policy->cpuinfo.transition_latency = INTEL_CPUFREQ_TRANSITION_LATENCY;
 	/* This reflects the intel_pstate_get_cpu_pstates() setting. */
 	policy->cur = policy->cpuinfo.min_freq;
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 26e2eb399484..7a32cfca26c9 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -604,6 +604,7 @@ struct cpufreq_governor {
 /* Pass a target to the cpufreq driver */
 unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
 					unsigned int target_freq);
+bool cpufreq_driver_has_fast_switch(void);
 void cpufreq_driver_adjust_perf(unsigned int cpu,
 				unsigned long min_perf,
 				unsigned long target_perf,
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index e3211455b203..f993ecf731a9 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -776,7 +776,7 @@ static int sugov_start(struct cpufreq_policy *policy)
 
 	if (policy_is_shared(policy))
 		uu = sugov_update_shared;
-	else if (policy->fast_switch_enabled && cpufreq_driver_has_adjust_perf())
+	else if (cpufreq_driver_has_adjust_perf())
 		uu = sugov_update_single_perf;
 	else
 		uu = sugov_update_single_freq;
-- 
2.34.1