Re: [PATCH v12] perf: Sharing PMU counters across compatible events

[Song Liu <songliubraving@fb.com>]

Hi Peter,

> On Mar 31, 2020, at 12:55 AM, Song Liu <songliubraving@fb.com> wrote:
> 
> This patch tries to enable PMU sharing. When multiple perf_events are
> counting the same metric, they can share the hardware PMU counter. We
> call these events as "compatible events".
> 
> The PMU sharing are limited to events within the same perf_event_context
> (ctx). When a event is installed or enabled, search the ctx for compatible
> events. This is implemented in perf_event_setup_dup(). One of these
> compatible events are picked as the master (stored in event->dup_master).
> Similarly, when the event is removed or disabled, perf_event_remove_dup()
> is used to clean up sharing.
> 
> If the master event is a cgroup event or a event in a group, it is
> possible that some slave events are ACTIVE, but the master event is not.
> To handle this scenario, we introduced PERF_EVENT_STATE_ENABLED. Also,
> since PMU drivers write into event->count, master event needs another
> variable (master_count) for the reading of this event.
> 
> On the critical paths (add, del read), sharing PMU counters doesn't
> increase the complexity. Helper functions event_pmu_[add|del|read]() are
> introduced to cover these cases. All these functions have O(1) time
> complexity.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Tejun Heo <tj@kernel.org>
> Reported-by: kernel test robot <rong.a.chen@intel.com>
> Signed-off-by: Song Liu <songliubraving@fb.com>

Could you please share your comments on this version? 

Thanks,
Song