Re: [patch/rfc] perf on raspberry-pi without overflow interrupt

[Vince Weaver]

On Wed, 8 Jan 2014, Peter Zijlstra wrote:

> On Wed, Jan 08, 2014 at 04:28:20PM -0500, Vince Weaver wrote:
> > Should the perf_event interface handle setups like this better and work 
> > fine in aggregate mode but return ENOTSUP if a sampled or overflow event 
> > is attempted?
> 
> Yeah that would be better, we do something similar for P6 class machines
> without lapic IIRC.

You're right.  Something like the following works for me on the rasp-pi,
although maybe if x86 is doing it too things should be moved up into 
generic code?

---

Return EOPNOTSUPP if we have no PMU overflow interrupt but a
sampled event is requested.

Signed-off-by: Vince Weaver <vincent.weaver@xxxxxxxxx>

diff --git a/arch/arm/include/asm/pmu.h b/arch/arm/include/asm/pmu.h
index f24edad..f1f27a3 100644
--- a/arch/arm/include/asm/pmu.h
+++ b/arch/arm/include/asm/pmu.h
@@ -87,6 +87,7 @@ struct arm_pmu {
 	u64		max_period;
 	struct platform_device	*plat_device;
 	struct pmu_hw_events	*(*get_hw_events)(void);
+	int		no_overflow_interrupt;
 };
 
 #define to_arm_pmu(p) (container_of(p, struct arm_pmu, pmu))
diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index bc3f2ef..fa4ecc3 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -397,7 +397,7 @@ __hw_perf_event_init(struct perf_event *event)
 	 */
 	hwc->config_base	    |= (unsigned long)mapping;
 
-	if (!hwc->sample_period) {
+	if (!is_sampling_event(event)) {
 		/*
 		 * For non-sampling runs, limit the sample_period to half
 		 * of the counter width. That way, the new counter value
@@ -407,6 +407,12 @@ __hw_perf_event_init(struct perf_event *event)
 		hwc->sample_period  = armpmu->max_period >> 1;
 		hwc->last_period    = hwc->sample_period;
 		local64_set(&hwc->period_left, hwc->sample_period);
+	} else {
+		/*
+		 * If we have no PMU interrupt we cannot sample
+		 */
+		if (armpmu->no_overflow_interrupt)
+			return -EOPNOTSUPP;
 	}
 
 	if (event->group_leader != event) {
diff --git a/arch/arm/kernel/perf_event_cpu.c b/arch/arm/kernel/perf_event_cpu.c
index d85055c..a74e0cd 100644
--- a/arch/arm/kernel/perf_event_cpu.c
+++ b/arch/arm/kernel/perf_event_cpu.c
@@ -97,8 +97,9 @@ static int cpu_pmu_request_irq(struct arm_pmu *cpu_pmu, irq_handler_t handler)
 
 	irqs = min(pmu_device->num_resources, num_possible_cpus());
 	if (irqs < 1) {
-		pr_err("no irqs for PMUs defined\n");
-		return -ENODEV;
+		printk_once("no irqs for PMUs defined, emulating with hrtimer\n");
+		cpu_pmu->no_overflow_interrupt=1;
+		return 0;
 	}
 
 	for (i = 0; i < irqs; ++i) {
--
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