Re: oprofile and ARM A9 hardware counter

[Ming Lei <ming.lei@canonical.com>]

Hi,

CC lkml and perf guys, since looks it is related with perf core.

On Tue, Feb 7, 2012 at 7:59 PM, stephane eranian <eranian@googlemail.com> wrote:
> An easier way to verify we're getting the right number of samples is
> to use perf top:
>
> $ taskset -c 1 noploop 1000 &
> $ sudo perf top
>
> You'll see around 850 irqs/sec, should be closer to 1000.
> But if I drop the rate to 100Hz, then it works:
>
> $ sudo perf top -F 100
>
> That leads me to believe there is too much overhead somewhere.
> Could be in perf_event itself.

Looks like the issue is caused by perf_event itself, but nothing to do
with much overhead
somewhere.

On OMAP4, HZ is 128, and perf_rotate_context may set a new sample period(~8ms),
which is much longer than 1ms in 1000HZ freq mode, so less sample events are
observed. X86 isn't affected since its HZ is 1000.

With patch[1], about 10000 sample events can be generated after running
'perf record -e cycles  ./noploop 10' and 'perf report -D | tail -20'
on panda board.

I am not sure if patch[1] is a right fix, but it can verify the problem.

thanks,
--
Ming Lei

[1], fix adjusting frequency in perf_rotate_context

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 32b48c8..db4faf2 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2300,14 +2300,12 @@ do {					\
 	return div64_u64(dividend, divisor);
 }

-static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
+static void perf_adjust_period(struct perf_event *event, u64 period, u64 count)
 {
 	struct hw_perf_event *hwc = &event->hw;
-	s64 period, sample_period;
+	s64 sample_period;
 	s64 delta;

-	period = perf_calculate_period(event, nsec, count);
-
 	delta = (s64)(period - hwc->sample_period);
 	delta = (delta + 7) / 8; /* low pass filter */

@@ -2363,8 +2361,13 @@ static void perf_ctx_adjust_freq(struct
perf_event_context *ctx, u64 period)
 		delta = now - hwc->freq_count_stamp;
 		hwc->freq_count_stamp = now;

-		if (delta > 0)
+		if (delta > 0) {
+			period = perf_calculate_period(event, period, delta);
+
+			if (period > 4*hwc->sample_period)
+				period = hwc->sample_period;
 			perf_adjust_period(event, period, delta);
+		}
 	}
 }

@@ -4533,8 +4536,10 @@ static int __perf_event_overflow(struct
perf_event *event,

 		hwc->freq_time_stamp = now;

-		if (delta > 0 && delta < 2*TICK_NSEC)
+		if (delta > 0 && delta < 2*TICK_NSEC) {
+			delta = perf_calculate_period(event, delta, hwc->last_period);
 			perf_adjust_period(event, delta, hwc->last_period);
+		}
 	}

 	/*