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    SubjectRe: [PATCH v8] perf: Sharing PMU counters across compatible events
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    Hi Peter and Jiri,

    > On Dec 6, 2019, at 4:24 PM, 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.
    >
    > A new state PERF_EVENT_STATE_ENABLED is introduced for the master event.
    > This state is used when the slave event is ACTIVE, but the master event
    > is not.
    >
    > 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>
    > Signed-off-by: Song Liu <songliubraving@fb.com>
    >
    > ---
    > Changes in v8:
    > Fix issues with task event (Jiri).
    > Fix issues with event inherit.
    > Fix mmap'ed events, i.e. perf test 4 (kernel test bot).
    >
    > Changes in v7:
    > Major rewrite to avoid allocating extra master event.

    Could you please share your feedbacks on this version?

    Thanks,
    Song

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