[PATCH 1/2] perf: Reset hwc->last_period on sw clock events

Namhyung Kim
2013-03-18T02:42:09Z

From: Namhyung Kim <namhyung.kim@lge.com>

When cpu/task clock events are initialized, their sampling frequencies
are converted to have a fixed value. However it missed to update the
hwc->last_period which was set to 1 for initial sampling frequency
calibration.

Because this hwc->last_period value is used as a period in perf_swevent_
hrtime(), every recorded sample will have an incorrected period of 1.

$ perf record -e task-clock noploop 1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.158 MB perf.data (~6919 samples) ]

$ perf report -n --show-total-period --stdio
# Samples: 4K of event 'task-clock'
# Event count (approx.): 4000
#
# Overhead Samples Period Command Shared Object Symbol
# ........ ............ ............ ....... ............. ..................
#
99.95% 3998 3998 noploop noploop [.] main
0.03% 1 1 noploop libc-2.15.so [.] init_cacheinfo
0.03% 1 1 noploop ld-2.15.so [.] open_verify

Note that it doesn't affect the non-sampling event so that the perf
stat still gets correct value with or without this patch.

$ perf stat -e task-clock noploop 1

Performance counter stats for 'noploop 1':

1000.272525 task-clock # 1.000 CPUs utilized

1.000560605 seconds time elapsed

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
kernel/events/core.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index c38feef0d683..ac418e88b8a9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5659,6 +5659,7 @@ static void perf_swevent_init_hrtimer(struct perf_event *event)
event->attr.sample_period = NSEC_PER_SEC / freq;
hwc->sample_period = event->attr.sample_period;
local64_set(&hwc->period_left, hwc->sample_period);
+ hwc->last_period = hwc->sample_period;
event->attr.freq = 0;
}
}
--
1.7.11.7