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SubjectRE: [PATCH V2 1/1] perf, core: Use sample period avg as child event's initial period
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Hi Peter,

The patch is month old. I checked that it still apply to current tip.
Could you please take a look?

Thanks,
Kan

>
> From: Kan Liang <kan.liang@intel.com>
>
> For perf record frequency mode, the initial sample_period is 1. That's
> because perf doesn't know what period should be set. It uses the
> minimum period 1 as the first period. It will trigger an interrupt soon. Then
> there will be enough data to calculate the period for the given frequency.
> But too many very short period like 1 may cause various problems and
> increase the overhead. It's better to limit the 1 period to just the first
> several period setting.
>
> However, for some workload, 1 period is frequently set. For example, perf
> record a busy loop for 10 seconds.
>
> perf record ./finity_busy_loop.sh 10
>
> while [ "A" != "B" ]
> do
> date > /dev/null
> done
>
> Period was changed 150503 times in 10 seconds. 22.5% (33861 times) of the
> period is set to 1. That's because, in the inherit_event, the period for child
> event is inherit from parent's parent's event, which is usually the default
> sample_period 1. Each child event has to recalculate the period from 1
> everytime. That brings high overhead.
>
> This patch keeps the sample period average in parent event. Each new
> child event can use it as its initial sample period.
> The avg_sample_period was introduced in struct perf_event to track the
> average sample period information. The information is kept in the parent
> event, since it's the only node everyone knows. For reducing the
> contention of updating parent event, the value is updated every tick.
> We also need to get rid of 1 period as earlier as possible. So the value is
> also updated in the first period adjustment.
> For each new child event, the average sample period of parent event will
> be assigned to the child as its first sample period.
> For each new child event, the parent event refcount++. Parent will not go
> away until all children go away. So it's safe to access its parent.
>
> After applying this patch, the 1 period rate reduces to 0.1%.
>
> Signed-off-by: Kan Liang <kan.liang@intel.com>
>
> ---
>
> Changes since V1
> - The average sample period will be kept in parent event
> - Use atomic64_t to replace local64_t
> - Only update the AVG every tick or the first time
>
> include/linux/perf_event.h | 2 ++
> kernel/events/core.c | 21 +++++++++++++++++++--
> 2 files changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index
> 486e84c..bb886f0 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -403,6 +403,8 @@ struct perf_event {
> struct list_head child_list;
> struct perf_event *parent;
>
> + atomic64_t avg_sample_period;
> +
> int oncpu;
> int cpu;
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c index
> af0a5ba..3404d52 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2796,6 +2796,7 @@ static void perf_adjust_period(struct perf_event
> *event, u64 nsec, u64 count, bo
> struct hw_perf_event *hwc = &event->hw;
> s64 period, sample_period;
> s64 delta;
> + struct perf_event *head_event = (event->parent != NULL) ?
> +event->parent : event;
>
> period = perf_calculate_period(event, nsec, count);
>
> @@ -2809,6 +2810,17 @@ static void perf_adjust_period(struct
> perf_event *event, u64 nsec, u64 count, bo
>
> hwc->sample_period = sample_period;
>
> + /*
> + * Update the AVG sample period in first period adjustment
> + * Then update it every tick to reduce contention.
> + */
> + if (!disable || (atomic64_read(&head_event->avg_sample_period)
> == 1)) {
> + s64 avg_period;
> +
> + avg_period = (atomic64_read(&head_event-
> >avg_sample_period) + sample_period) / 2;
> + atomic64_set(&head_event->avg_sample_period,
> avg_period);
> + }
> +
> if (local64_read(&hwc->period_left) > 8*sample_period) {
> if (disable)
> event->pmu->stop(event, PERF_EF_UPDATE); @@
> -7030,8 +7042,13 @@ perf_event_alloc(struct perf_event_attr *attr, int
> cpu,
>
> hwc = &event->hw;
> hwc->sample_period = attr->sample_period;
> - if (attr->freq && attr->sample_freq)
> + if (attr->freq && attr->sample_freq) {
> hwc->sample_period = 1;
> + if (parent_event)
> + hwc->sample_period =
> atomic64_read(&parent_event->avg_sample_period);
> + else
> + atomic64_set(&event->avg_sample_period, hwc-
> >sample_period);
> + }
> hwc->last_period = hwc->sample_period;
>
> local64_set(&hwc->period_left, hwc->sample_period); @@ -
> 7902,7 +7919,7 @@ inherit_event(struct perf_event *parent_event,
> child_event->state = PERF_EVENT_STATE_OFF;
>
> if (parent_event->attr.freq) {
> - u64 sample_period = parent_event->hw.sample_period;
> + u64 sample_period = atomic64_read(&parent_event-
> >avg_sample_period);
> struct hw_perf_event *hwc = &child_event->hw;
>
> hwc->sample_period = sample_period;
> --
> 1.8.3.2



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