Re: [PATCH v8] perf: Sharing PMU counters across compatible events

Song Liu
2019-12-12T15:37:19Z

> On Dec 12, 2019, at 5:49 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Dec 06, 2019 at 04:24:47PM -0800, Song Liu wrote:
>
>> @@ -750,6 +752,16 @@ struct perf_event {
>> void *security;
>> #endif
>> struct list_head sb_list;
>> +
>> + /* for PMU sharing */
>> + struct perf_event *dup_master;
>> + /* check event_sync_dup_count() for the use of dup_base_* */
>> + u64 dup_base_count;
>> + u64 dup_base_child_count;
>> + /* when this event is master, read from master*count */
>> + local64_t master_count;
>> + atomic64_t master_child_count;
>> + int dup_active_count;
>> #endif /* CONFIG_PERF_EVENTS */
>> };
>
>> +/* PMU sharing aware version of event->pmu->add() */
>> +static int event_pmu_add(struct perf_event *event,
>> + struct perf_event_context *ctx)
>> +{
>> + struct perf_event *master;
>> + int ret;
>> +
>> + /* no sharing, just do event->pmu->add() */
>> + if (!event->dup_master)
>> + return event->pmu->add(event, PERF_EF_START);
>
> Possibly we should look at the location of perf_event::dup_master to be
> in a hot cacheline. Because I'm thinking you just added a guaranteed
> miss here.

I am not quite sure the best location for these. How about:

diff --git i/include/linux/perf_event.h w/include/linux/perf_event.h
index 7d49f9251621..218cc7f75775 100644
--- i/include/linux/perf_event.h
+++ w/include/linux/perf_event.h
@@ -643,6 +643,16 @@ struct perf_event {
local64_t count;
atomic64_t child_count;

+ /* for PMU sharing */
+ struct perf_event *dup_master;
+ /* check event_sync_dup_count() for the use of dup_base_* */
+ u64 dup_base_count;
+ u64 dup_base_child_count;
+ /* when this event is master, read from master*count */
+ local64_t master_count;
+ atomic64_t master_child_count;
+ int dup_active_count;
+
/*
* These are the total time in nanoseconds that the event
* has been enabled (i.e. eligible to run, and the task has

?

>
>> +
>> + master = event->dup_master;
>> +
>> + if (!master->dup_active_count) {
>> + ret = event->pmu->add(master, PERF_EF_START);
>> + if (ret)
>> + return ret;
>> +
>> + if (master != event)
>> + perf_event_set_state(master, PERF_EVENT_STATE_ENABLED);
>> + }
>> +
>> + master->dup_active_count++;
>> + master->pmu->read(master);
>> + event->dup_base_count = local64_read(&master->count);
>> + event->dup_base_child_count = atomic64_read(&master->child_count);
>> + return 0;
>> +}
>
>> +/* PMU sharing aware version of event->pmu->del() */
>> +static void event_pmu_del(struct perf_event *event,
>> + struct perf_event_context *ctx)
>> +{
>> + struct perf_event *master;
>> +
>> + if (event->dup_master == NULL) {
>> + event->pmu->del(event, 0);
>> + return;
>> + }
>
> How about you write it exactly like the add version:
>
> if (!event->dup_master)
> return event->pmu->del(event, 0);
>
> ?

Sure, will fix in v9.

Thanks,
Song
>
>> +
>> + master = event->dup_master;
>> + event_sync_dup_count(event, master);
>> + if (--master->dup_active_count == 0) {
>> + event->pmu->del(master, 0);
>> + perf_event_set_state(master, PERF_EVENT_STATE_INACTIVE);
>> + } else if (master == event) {
>> + perf_event_set_state(master, PERF_EVENT_STATE_ENABLED);
>> + }
>> +}
>> +
>> +/* PMU sharing aware version of event->pmu->read() */
>> +static void event_pmu_read(struct perf_event *event)
>> +{
>> + if (event->dup_master == NULL) {
>> + event->pmu->read(event);
>> + return;
>> + }
>
> And here too.
>
>> + event_sync_dup_count(event, event->dup_master);
>> +}