On Wed, Mar 16, 2016 at 4:43 PM, Peter Zijlstra [off-list ref] wrote:

On Wed, Mar 16, 2016 at 03:52:28PM +0100, Rafael J. Wysocki wrote:

quoted

+void cpufreq_enable_fast_switch(struct cpufreq_policy *policy)
+{
+     mutex_lock(&cpufreq_fast_switch_lock);
+     if (policy->fast_switch_possible && cpufreq_fast_switch_count >= 0) {
+             cpufreq_fast_switch_count++;
+             policy->fast_switch_enabled = true;
+     } else {
+             pr_warn("cpufreq: CPU%u: Fast freqnency switching not enabled\n",
+                     policy->cpu);

This happens because there's transition notifiers, right? Would it make
sense to iterate the notifier here and print the notifier function
symbol for each? That way we've got a clue as to where to start looking
when this happens.

OK

quoted

+     }
+     mutex_unlock(&cpufreq_fast_switch_lock);
+}

quoted

@@ -1653,8 +1703,18 @@ int cpufreq_register_notifier(struct not

      switch (list) {
      case CPUFREQ_TRANSITION_NOTIFIER:
+             mutex_lock(&cpufreq_fast_switch_lock);
+
+             if (cpufreq_fast_switch_count > 0) {
+                     mutex_unlock(&cpufreq_fast_switch_lock);

So while theoretically (it has a return code)
cpufreq_register_notifier() could fail, it never actually did. Now we
do. Do we want to add a WARN here?

Like if (WARN_ON(cpufreq_fast_switch_count > 0)) {

That can be done. :-)