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    SubjectRe: [RFC][PATCH] perf: Rewrite core context handling
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    > On Oct 17, 2018, at 11:33 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > On Wed, Oct 17, 2018 at 07:19:55PM +0200, Peter Zijlstra wrote:
    >> On Wed, Oct 17, 2018 at 04:43:27PM +0000, Song Liu wrote:
    >>
    >>>> That makes task and cpu contexts wildly different, which will complicate
    >>>> matters I feel.
    >>>>
    >>>
    >>> I think we only need different logic when adding events to the task/cpu
    >>> contexts. The ctx_sched_in() and ctx_sched_out() will need some extra
    >>> logic to filter out events that are not being scheduled (don't schedule
    >>> events on PMU-a when rotating PMU-b). This logic will be the same for
    >>> task and cpu context. The difference is, the CPU context will not have
    >>> such events, because we never added such event to CPU context.
    >>>
    >>> Does this make sense? I could try draft a RFC to see how difficult it is.
    >>
    >> I'm not sure it saves much, if we have multiple per-cpu contexts we get
    >> to re-introduce the active_ctx_list and loose the simplification for the
    >> online status.
    >>
    >> Plus that fundamental assymetry -- which would bother my OCD forever
    >> more :-)
    >
    > Worse, the whole syscall that installs the events will come apart. The
    > locking for the two cases is different :/

    I agree... I didn't get into details of locking. I just consider these all
    as part of "adding event to context".

    I believe this patch should give close to the optimal performance. However,
    I do feel it makes the logic more complicate. Before this patch, perf_cpu_context
    and perf_event_context don't need to know much about multiple PMUs. With
    this patch, the two extra *_pmu_context are necessary for performance (and
    maybe also for correctness).

    If we take first a baby step, how about adding more perf_event_ctx to
    task_struct->perf_event_ctxp? We need one sw perf_event_ctx and a few hw
    perf_event_ctx (one for each hw PMU). (I haven't checked whether it is OK
    to allocate these when attaching events). (And I guess you don't really
    like this..)

    On the other hand, this patch makes it possible to create groups of events
    from different hw PMUs. I guess that will be useful.

    Thanks,
    Song
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