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>