[PATCH] drivers: perf: arm: implement CPU_PM notifier

On Wed, Feb 24, 2016 at 09:20:22AM -0700, Mathieu Poirier wrote:

On 23 February 2016 at 11:22, Lorenzo Pieralisi

[...]

quoted

+static int cpu_pm_pmu_notify(struct notifier_block *b, unsigned long cmd,
+                            void *v)
+{
+       struct arm_pmu *armpmu = container_of(b, struct arm_pmu, cpu_pm_nb);
+       struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
+       int enabled = bitmap_weight(hw_events->used_mask, armpmu->num_events);
+
+       if (!cpumask_test_cpu(smp_processor_id(), &armpmu->supported_cpus))
+               return NOTIFY_DONE;
+
+       /*
+        * Always reset the PMU registers on power-up even if
+        * there are no events running.
+        */
+       if (cmd == CPU_PM_EXIT && armpmu->reset)
+               armpmu->reset(armpmu);

I think this patch does the right thing but I can't get the above
reset.  Wouldn't it be better to do the reset as part of the
CPU_PM_EXIT case below?  At this point nothing tells us the CPU won't
go back down before the event is enabled, wasting the cycle needed to
reset the PMU.

The logic goes, if the cpu is woken up and it has no events enabled,
if we do not reset it (mind, ->reset here sets the PMU register values
to a sane default, some of them are architecturally UNKNOWN on reset, it
does NOT reset the PMU) _and_ we subsequently install an event on it we
do have a problem, that's why whenever a core gets out of low-power we
have to reset its pmu.

Does it make sense ?

Thanks,
Lorenzo

Thanks,
Mathieu

quoted

+
+       if (!enabled)
+               return NOTIFY_OK;
+
+       switch (cmd) {
+       case CPU_PM_ENTER:
+               armpmu->stop(armpmu);
+               cpu_pm_pmu_setup(armpmu, cmd);
+               break;
+       case CPU_PM_EXIT:
+               cpu_pm_pmu_setup(armpmu, cmd);
+       case CPU_PM_ENTER_FAILED:
+               armpmu->start(armpmu);
+               break;
+       default:
+               return NOTIFY_DONE;
+       }
+
+       return NOTIFY_OK;
+}
+
+static int cpu_pm_pmu_register(struct arm_pmu *cpu_pmu)
+{
+       cpu_pmu->cpu_pm_nb.notifier_call = cpu_pm_pmu_notify;
+       return cpu_pm_register_notifier(&cpu_pmu->cpu_pm_nb);
+}
+
+static void cpu_pm_pmu_unregister(struct arm_pmu *cpu_pmu)
+{
+       cpu_pm_unregister_notifier(&cpu_pmu->cpu_pm_nb);
+}
+#else
+static inline int cpu_pm_pmu_register(struct arm_pmu *cpu_pmu) { return 0; }
+static inline void cpu_pm_pmu_unregister(struct arm_pmu *cpu_pmu) { }
+#endif
+
 static int cpu_pmu_init(struct arm_pmu *cpu_pmu)
 {
        int err;

@@ -725,6 +813,10 @@ static int cpu_pmu_init(struct arm_pmu *cpu_pmu)
        if (err)
                goto out_hw_events;

+       err = cpu_pm_pmu_register(cpu_pmu);
+       if (err)
+               goto out_unregister;
+
        for_each_possible_cpu(cpu) {
                struct pmu_hw_events *events = per_cpu_ptr(cpu_hw_events, cpu);
                raw_spin_lock_init(&events->pmu_lock);

@@ -746,6 +838,8 @@ static int cpu_pmu_init(struct arm_pmu *cpu_pmu)

        return 0;

+out_unregister:
+       unregister_cpu_notifier(&cpu_pmu->hotplug_nb);
 out_hw_events:
        free_percpu(cpu_hw_events);
        return err;

@@ -753,6 +847,7 @@ out_hw_events:

 static void cpu_pmu_destroy(struct arm_pmu *cpu_pmu)
 {
+       cpu_pm_pmu_unregister(cpu_pmu);
        unregister_cpu_notifier(&cpu_pmu->hotplug_nb);
        free_percpu(cpu_pmu->hw_events);
 }

diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
index 83b5e34..9ffa316 100644
--- a/include/linux/perf/arm_pmu.h
+++ b/include/linux/perf/arm_pmu.h

@@ -107,6 +107,7 @@ struct arm_pmu {
        struct platform_device  *plat_device;
        struct pmu_hw_events    __percpu *hw_events;
        struct notifier_block   hotplug_nb;
+       struct notifier_block   cpu_pm_nb;
 };

 #define to_arm_pmu(p) (container_of(p, struct arm_pmu, pmu))
--

2.5.1