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charset="utf-8" Adrian found that there is a probability that the number of samples is small, which is caused by the unreasonable large sampling period. # taskset --cpu 0 perf record -F 1000 -e cs -- taskset --cpu 1 ./test [ perf record: Woken up 1 times to write data ] [ perf record: Captured and wrote 0.010 MB perf.data (204 samples) ] # perf script ... test 865 265.377846: 16 cs: ffffffff832e927b schedule+0x2b test 865 265.378900: 15 cs: ffffffff832e927b schedule+0x2b test 865 265.379845: 14 cs: ffffffff832e927b schedule+0x2b test 865 265.380770: 14 cs: ffffffff832e927b schedule+0x2b test 865 265.381647: 15 cs: ffffffff832e927b schedule+0x2b test 865 265.382638: 16 cs: ffffffff832e927b schedule+0x2b test 865 265.383647: 16 cs: ffffffff832e927b schedule+0x2b test 865 265.384704: 15 cs: ffffffff832e927b schedule+0x2b test 865 265.385649: 14 cs: ffffffff832e927b schedule+0x2b test 865 265.386578: 152 cs: ffffffff832e927b schedule+0x2b test 865 265.396383: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.406183: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.415839: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.425445: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.435052: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.444708: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.454314: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.463970: 154 cs: ffffffff832e927b schedule+0x2b test 865 265.473577: 154 cs: ffffffff832e927b schedule+0x2b ... And the reason is perf_adjust_freq_unthr_events() calculates a value that i= s too big because it incorrectly assumes the count has accumulated only since the= last tick, whereas it can have been much longer. To fix this problem, perf can c= alculate the tick interval by itself. For perf_adjust_freq_unthr_events we can use j= iffies to calculate the tick interval more efficiently, as sugguested by Adrian. Signed-off-by: Luo Gengkun --- include/linux/perf_event.h | 1 + kernel/events/core.c | 16 +++++++++++++--- 2 files changed, 14 insertions(+), 3 deletions(-) diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index afb028c54f33..2708f1d0692c 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -265,6 +265,7 @@ struct hw_perf_event { * State for freq target events, see __perf_event_overflow() and * perf_adjust_freq_unthr_context(). */ + u64 freq_tick_stamp; u64 freq_time_stamp; u64 freq_count_stamp; #endif diff --git a/kernel/events/core.c b/kernel/events/core.c index cad50d3439f1..309af5520f52 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -55,6 +55,7 @@ #include #include #include +#include =20 #include "internal.h" =20 @@ -4112,7 +4113,7 @@ perf_adjust_freq_unthr_context(struct perf_event_cont= ext *ctx, bool unthrottle) { struct perf_event *event; struct hw_perf_event *hwc; - u64 now, period =3D TICK_NSEC; + u64 now, period, tick_stamp; s64 delta; =20 /* @@ -4151,6 +4152,10 @@ perf_adjust_freq_unthr_context(struct perf_event_con= text *ctx, bool unthrottle) */ event->pmu->stop(event, PERF_EF_UPDATE); =20 + tick_stamp =3D jiffies64_to_nsecs(get_jiffies_64()); + period =3D tick_stamp - hwc->freq_tick_stamp; + hwc->freq_tick_stamp =3D tick_stamp; + now =3D local64_read(&event->count); delta =3D now - hwc->freq_count_stamp; hwc->freq_count_stamp =3D now; @@ -4162,8 +4167,13 @@ perf_adjust_freq_unthr_context(struct perf_event_con= text *ctx, bool unthrottle) * to perf_adjust_period() to avoid stopping it * twice. */ - if (delta > 0) - perf_adjust_period(event, period, delta, false); + if (delta > 0) { + /* + * we skip first tick adjust period + */ + if (likely(period !=3D tick_stamp)) + perf_adjust_period(event, period, delta, false); + } =20 event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0); next: --=20 2.34.1