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    SubjectRe: oprofile and ARM A9 hardware counter
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    On Wed, 2012-02-08 at 10:24 +0800, Ming Lei wrote:
    >
    > 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.


    > 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);
    > + }
    > }
    >
    > /*

    So what this patch seems to do is put that filter on period in
    perf_ctx_adjust_freq(). Not making sense.. nor can I see a HZ
    dependency, perf_ctx_adjust_freq() uses TICK_NSEC as time base.


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