linux-kernel - Re: [PATCH] perf_events: improve x86 event scheduling (v6 incremental)

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Date:	Fri, 22 Jan 2010 21:27:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	eranian@...gle.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com
Subject: Re: [PATCH]  perf_events: improve x86 event scheduling (v6
 incremental)

On Thu, 2010-01-21 at 17:39 +0200, Stephane Eranian wrote:
> @@ -1395,40 +1430,28 @@ void hw_perf_enable(void)
>                  * apply assignment obtained either from
>                  * hw_perf_group_sched_in() or x86_pmu_enable()
>                  *
> -                * step1: save events moving to new counters
> -                * step2: reprogram moved events into new counters
> +                * We either re-enable or re-program and re-enable.
> +                * All events are disabled by the time we come here.
> +                * That means their state has been saved already.
>                  */

I'm not seeing how it is true.

Suppose a core2 with counter0 active counting a non-restricted event,
say cpu_cycles. Then we do:

perf_disable()
  hw_perf_disable()
    intel_pmu_disable_all
      wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, 0);

->enable(MEM_LOAD_RETIRED) /* constrained to counter0 */
  x86_pmu_enable()
    collect_events()
    x86_schedule_events()
    n_added = 1

    /* also slightly confused about this */
    if (hwc->idx != -1)
      x86_perf_event_set_period()

perf_enable()
  hw_perf_enable()

    /* and here we'll assign the new event to counter0
     * except we never disabled it... */

    intel_pmu_enable_all()
      wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, intel_ctrl)

Or am I missing something?

>                 for(i=0; i < cpuc->n_events; i++) {
>  
>                         event = cpuc->event_list[i];
>                         hwc = &event->hw;
>  
> -                       if (hwc->idx == -1 || hwc->idx == cpuc->assign[i])
> -                               continue;
> -
> -                       x86_pmu.disable(hwc, hwc->idx);
> -
> -                       clear_bit(hwc->idx, cpuc->active_mask);
> -                       barrier();
> -                       cpuc->events[hwc->idx] = NULL;
> -
> -                       x86_perf_event_update(event, hwc, hwc->idx);
> -
> -                       hwc->idx = -1;
> -               }
> -
> -               for(i=0; i < cpuc->n_events; i++) {
> -
> -                       event = cpuc->event_list[i];
> -                       hwc = &event->hw;
> -
> -                       if (hwc->idx == -1) {
> -                               x86_assign_hw_event(event, hwc, cpuc->assign[i]);
> +                       /*
> +                        * we can avoid reprogramming counter if:
> +                        * - assigned same counter as last time
> +                        * - running on same CPU as last time
> +                        * - no other event has used the counter since
> +                        */
> +                       if (!match_prev_assignment(hwc, cpuc, i)) {
> +                               x86_assign_hw_event(event, cpuc, cpuc->assign[i]);
>                                 x86_perf_event_set_period(event, hwc, hwc->idx);
>                         }
>                         /*
>                          * need to mark as active because x86_pmu_disable()
> -                        * clear active_mask and eventsp[] yet it preserves
> +                        * clear active_mask and events[] yet it preserves
>                          * idx
>                          */
>                         set_bit(hwc->idx, cpuc->active_mask); 

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