Messages in this thread | | | Date | Mon, 28 May 2018 13:15:49 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC 2/2] perf: Sharing PMU counters across compatible events |
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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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