Messages in this thread | | | From | Song Liu <> | Subject | Re: [RFC 2/2] perf: Sharing PMU counters across compatible events | Date | Mon, 28 May 2018 18:24:09 +0000 |
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> On May 28, 2018, at 4:15 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > 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...
I think we can also fix the scheduling path. To achieve this, we need to limit the sharing within the ctx. In other words, events in cpuctx->ctx can only share PMU with events in cpuctx->ctx, but not with events in cpuctx->task_ctx. This will probably also solve the locking issue here. Let me try it.
Thanks, Song
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