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    SubjectRe: [RFC][PATCH] perf: Rewrite core context handling
    On Sat, Oct 13, 2018 at 08:31:37AM +0000, Song Liu wrote:

    > The only suggestion I have right now is on which struct owns which
    > data:
    >
    > 1. perf_cpu_context owns two perf_event_context: ctx and *task_ctx.
    > This is the same as right now.

    > 2. perf_event_context owns multiple perf_event_pmu_context:
    > One perf_event_pmu_context for software groups;
    > One perf_event_pmu_context for each hardware PMU.

    It does now already, right? Through the pmu_ctx_list we can, given an
    perf_event_context, find all associated perf_event_pmu_context's.

    > 3. perf_event_pmu_context owns RB tree of events. Since we don't
    > need rotation across multiple hardware PMUs, the rotation is
    > within same perf_event_pmu_context.

    By keeping the RB trees in perf_event_context, we get bigger trees,
    which is more efficient (log(n+m) < log(n) + log(m))

    Also, specifically, it means we only need a single merge sort /
    iteration to schedule in a full context, instead of (again) doing 'n' of
    them.

    Also, given a context and a pmu, it is cheaper for finding the relevant
    events; this is needed for big.little for instance. Something the
    proposed patch doesn't fully flesh out.

    > 4. perf_cpu_context owns multiple perf_cpu_pmu_context:
    > One perf_cpu_pmu_context for each hardware PMU.

    What would we need that relation for?

    > perf_cpu_pmu_context is tot needed for software only groups(?).

    Yes, that is a very good question; it mostly centers around what we want
    to do with perf_event_attr::exclusive for software events -- which is
    currently dodgy at best.

    Also, allocating the structure and keeping it around is probably less
    code than explicitly not doing it.

    > 5. perf_cpu_pmu_context has two pointers of perf_event_pmu_context.

    Instead of embedding the thing? Yeah, not sure. Either way around we'd
    not want to free the CPU perf_event_pmu_context that is associated with
    the perf_cpu_pmu_context, and embedding it saves a pointer chase.

    Not sure it actually makes a lot of difference either way around.

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