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

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Message-ID: <47BDB3EA-2DF6-4FB1-AF06-F63A342861DF@fb.com>
Date:   Thu, 9 Apr 2020 20:40:25 +0000
From:   Song Liu <songliubraving@...com>
To:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "Tejun Heo" <tj@...nel.org>,
        kernel test robot <rong.a.chen@...el.com>
Subject: Re: [PATCH v12] perf: Sharing PMU counters across compatible events


Hi Peter,

> On Mar 31, 2020, at 12:55 AM, Song Liu <songliubraving@...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@...radead.org>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: Alexey Budankov <alexey.budankov@...ux.intel.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Tejun Heo <tj@...nel.org>
> Reported-by: kernel test robot <rong.a.chen@...el.com>
> Signed-off-by: Song Liu <songliubraving@...com>

Could you please share your comments on this version? 

Thanks,
Song