[tip:perf/urgent] perf: Reset hwc->last_period on sw clock events

tip-bot for Namhyung Kim
2013-03-18T11:08:13Z

Commit-ID: 778141e3cf0bf29f91cd3cb5c314ea477b9402a7
Gitweb: http://git.kernel.org/tip/778141e3cf0bf29f91cd3cb5c314ea477b9402a7
Author: Namhyung Kim <namhyung.kim@lge.com>
AuthorDate: Mon, 18 Mar 2013 11:41:46 +0900
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 18 Mar 2013 09:15:18 +0100

perf: Reset hwc->last_period on sw clock events

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>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1363574507-18808-1-git-send-email-namhyung@kernel.org
Signed-off-by: Ingo Molnar <mingo@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 b0cd865..fa79c37 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5647,6 +5647,7 @@ 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;
}
}