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    SubjectRe: [RFC 2/2] perf: Sharing PMU counters across compatible events
    On Fri, May 04, 2018 at 04:11:02PM -0700, Song Liu wrote:
    > Connection among perf_event and perf_event_dup are built with function
    > rebuild_event_dup_list(cpuctx). This function is only called when events
    > are added/removed or when a task is scheduled in/out. So it is not on
    > critical path of perf_rotate_context().

    Why is perf_rotate_context() the only critical path? I would say the
    context switch path is rather critical too.

    > @@ -2919,8 +3014,10 @@ static void ctx_sched_out(struct perf_event_context *ctx,
    >
    > if (ctx->task) {
    > WARN_ON_ONCE(cpuctx->task_ctx != ctx);
    > - if (!ctx->is_active)
    > + if (!ctx->is_active) {
    > cpuctx->task_ctx = NULL;
    > + rebuild_event_dup_list(cpuctx);
    > + }
    > }
    >
    > /*

    > +static void rebuild_event_dup_list(struct perf_cpu_context *cpuctx)
    > +{
    > + int dup_count = cpuctx->ctx.nr_events;
    > + struct perf_event_context *ctx = cpuctx->task_ctx;
    > + struct sched_in_data sid = {
    > + .ctx = ctx,
    > + .cpuctx = cpuctx,
    > + .can_add_hw = 1,
    > + };
    > +
    > + if (ctx)
    > + dup_count += ctx->nr_events;
    > +
    > + kfree(cpuctx->dup_event_list);
    > + cpuctx->dup_event_count = 0;
    > +
    > + cpuctx->dup_event_list =
    > + kzalloc(sizeof(struct perf_event_dup) * dup_count, GFP_ATOMIC);


    __schedule()
    local_irq_disable()
    raw_spin_lock(rq->lock)
    context_switch()
    prepare_task_switch()
    perf_event_task_sched_out()
    __perf_event_task_sched_out()
    perf_event_context_sched_out()
    task_ctx_sched_out()
    ctx_sched_out()
    rebuild_event_dup_list()
    kzalloc()
    ...
    spin_lock()

    Also, as per the above, this nests a regular spin lock inside the
    (raw) rq->lock, which is a no-no.

    Not to mention that whole O(n) crud in the scheduling path...

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      Last update: 2018-05-28 14:41    [W:3.029 / U:1.468 seconds]
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