linux-kernel - Re: [PATCH v6] perf: Sharing PMU counters across compatible events

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Message-ID: <9FD34CD1-E8C4-48EE-90EE-F0ACABEE5909@fb.com>
Date:   Mon, 30 Sep 2019 05:36:55 +0000
From:   Song Liu <songliubraving@...com>
To:     open list <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        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>
Subject: Re: [PATCH v6] perf: Sharing PMU counters across compatible events

Hi Peter,

> On Sep 18, 2019, at 10:23 PM, Song Liu <songliubraving@...com> wrote:
> 
> This patch tries to enable PMU sharing. To make perf event scheduling
> fast, we use special data structures.
> 
> An array of "struct perf_event_dup" is added to the perf_event_context,
> to remember all the duplicated events under this ctx. All the events
> under this ctx has a "dup_id" pointing to its perf_event_dup. Compatible
> events under the same ctx share the same perf_event_dup. The following
> figure shows a simplified version of the data structure.
> 
>      ctx ->  perf_event_dup -> master
>                     ^
>                     |
>         perf_event /|
>                     |
>         perf_event /
> 
> Connection among perf_event and perf_event_dup are built when events are
> added or removed from the ctx. So these are not on the critical path of
> schedule or perf_rotate_context().
> 
> 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.
> 
> We allocate a separate perf_event for perf_event_dup->master. This needs
> extra attention, because perf_event_alloc() may sleep. To allocate the
> master event properly, a new pointer, tmp_master, is added to perf_event.
> tmp_master carries a separate perf_event into list_[add|del]_event().
> The master event has valid ->ctx and holds ctx->refcount.
> 
> Details about the handling of the master event is added to
> include/linux/perf_event.h, before struct perf_event_dup.

Could you please share your comments/suggestions on this work?

Thanks,
Song