Re: perf on 2.6.38-rc4 wedges my box

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-02-11 at 18:53 +0100, Peter Zijlstra wrote:
> On Fri, 2011-02-11 at 09:35 -0700, David Ahern wrote:
> > I'm guessing in your case perf is using hardware cycles for profiling.
> > 
> > I was able to reproduce the lockup in a VM which uses cpu-clock for
> > profiling - like Jeff's case. The VM is running Fedora 14 with
> > 2.6.38-rc4.
> > 
> Ah, indeed, when I use:
> 
>   perf record -gfe task-clock -- ./aio-stress -O -o 0 -r 4 -d 32 -b 16 /dev/sdb
> 
> things did come apart, something like the below cured that problem (but
> did show the pending softirq thing and triggered something iffy in the
> backtrace code -- will have to stare at those still)

So while this doesn't explain these weird things, it should have at
least one race less -- hrtimer_init() on a possible still running timer
didn't seem like a very good idea, also since hrtimers are nsec the
whole freq thing seemed unnecessary.

---
 kernel/perf_event.c |   37 +++++++++++++++++++++++++++++++++----
 1 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 999835b..08428b3 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -5051,6 +5051,10 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer)
 	u64 period;
 
 	event = container_of(hrtimer, struct perf_event, hw.hrtimer);
+
+	if (event->state < PERF_EVENT_STATE_ACTIVE)
+		return HRTIMER_NORESTART;
+
 	event->pmu->read(event);
 
 	perf_sample_data_init(&data, 0);
@@ -5077,9 +5081,6 @@ static void perf_swevent_start_hrtimer(struct perf_event *event)
 	if (!is_sampling_event(event))
 		return;
 
-	hrtimer_init(&hwc->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
-	hwc->hrtimer.function = perf_swevent_hrtimer;
-
 	period = local64_read(&hwc->period_left);
 	if (period) {
 		if (period < 0)
@@ -5102,7 +5103,31 @@ static void perf_swevent_cancel_hrtimer(struct perf_event *event)
 		ktime_t remaining = hrtimer_get_remaining(&hwc->hrtimer);
 		local64_set(&hwc->period_left, ktime_to_ns(remaining));
 
-		hrtimer_cancel(&hwc->hrtimer);
+		hrtimer_try_to_cancel(&hwc->hrtimer);
+	}
+}
+
+static void perf_swevent_init_hrtimer(struct perf_event *event)
+{
+	struct hw_perf_event *hwc = &event->hw;
+
+	if (!is_sampling_event(event))
+		return;
+
+	hrtimer_init(&hwc->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hwc->hrtimer.function = perf_swevent_hrtimer;
+
+	/*
+	 * Since hrtimers have a fixed rate, we can do a static freq->period
+	 * mapping and avoid the whole period adjust feedback stuff.
+	 */
+	if (event->attr.freq) {
+		long freq = event->attr.sample_freq;
+
+		event->attr.sample_period = NSEC_PER_SEC / freq;
+		hwc->sample_period = event->attr.sample_period;
+		local64_set(&hwc->period_left, hwc->sample_period);
+		event->attr.freq = 0;
 	}
 }
 
@@ -5158,6 +5183,8 @@ static int cpu_clock_event_init(struct perf_event *event)
 	if (event->attr.config != PERF_COUNT_SW_CPU_CLOCK)
 		return -ENOENT;
 
+	perf_swevent_init_hrtimer(event);
+
 	return 0;
 }
 
@@ -5235,6 +5262,8 @@ static int task_clock_event_init(struct perf_event *event)
 	if (event->attr.config != PERF_COUNT_SW_TASK_CLOCK)
 		return -ENOENT;
 
+	perf_swevent_init_hrtimer(event);
+
 	return 0;
 }