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

[Liang, Kan]

Hi Peter,

Could you please review the patch?

Thanks,
Kan


> 
> 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@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> >
> > ---
> >
> > 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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