[RFC PATCH 1/2] arm64: kernel: perf: add cpu hotplug notifier

[Lorenzo Pieralisi]

When a CPU is taken offline, its PMU registers content is lost
and needs to be reset on power up, since for most of the PMU registers
content is UNKNOWN upon CPU reset. This patch implements a cpu hotplug
notifier and hooks the reset call in the respective notifier callback
function.

Cc: Will Deacon <will.deacon at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
---
 arch/arm64/kernel/perf_event.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 25a5308..83f21d8 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -21,6 +21,7 @@
 #define pr_fmt(fmt) "hw perfevents: " fmt
 
 #include <linux/bitmap.h>
+#include <linux/cpu.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/kernel.h>
@@ -1315,6 +1316,30 @@ static struct pmu_hw_events *armpmu_get_cpu_events(void)
 	return this_cpu_ptr(&cpu_hw_events);
 }
 
+/*
+ * PMU hardware loses all context when a CPU goes offline.
+ * When a CPU is hotplugged back in, since some hardware registers are
+ * UNKNOWN at reset, the PMU must be explicitly reset to avoid reading
+ * junk values out of them.
+ */
+static int cpu_pmu_notifier(struct notifier_block *b, unsigned long action,
+			    void *hcpu)
+{
+	if ((action & ~CPU_TASKS_FROZEN) != CPU_STARTING)
+		return NOTIFY_DONE;
+
+	if (cpu_pmu->reset)
+		cpu_pmu->reset(NULL);
+	else
+		return NOTIFY_DONE;
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block cpu_pmu_notifier_block = {
+	.notifier_call = cpu_pmu_notifier,
+};
+
 static void __init cpu_pmu_init(struct arm_pmu *armpmu)
 {
 	int cpu;
@@ -1325,6 +1350,8 @@ static void __init cpu_pmu_init(struct arm_pmu *armpmu)
 		raw_spin_lock_init(&events->pmu_lock);
 	}
 	armpmu->get_hw_events = armpmu_get_cpu_events;
+
+	register_cpu_notifier(&cpu_pmu_notifier_block);
 }
 
 static int __init init_hw_perf_events(void)
-- 
2.2.1