Re: [RFC][PATCH 7/8] perf/x86: Optimize x86_schedule_events()

[Stephane Eranian <eranian@google.com>]

On Thu, Mar 14, 2019 at 6:11 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Now that cpuc->event_constraint[] is retained, we can avoid calling
> get_event_constraints() over and over again.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/events/core.c       |   25 +++++++++++++++++++++----
>  arch/x86/events/intel/core.c |    3 ++-
>  2 files changed, 23 insertions(+), 5 deletions(-)
>
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -844,6 +844,12 @@ int perf_assign_events(struct event_cons
>  }
>  EXPORT_SYMBOL_GPL(perf_assign_events);
>
> +static inline bool is_ht_workaround_active(struct cpu_hw_events *cpuc)
> +{
> +       return is_ht_workaround_enabled() && !cpuc->is_fake &&
> +              READ_ONCE(cpuc->excl_cntrs->exclusive_present);
> +}
> +
>  int x86_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign)
>  {
>         struct event_constraint *c;
> @@ -858,8 +864,20 @@ int x86_schedule_events(struct cpu_hw_ev
>                 x86_pmu.start_scheduling(cpuc);
>
>         for (i = 0, wmin = X86_PMC_IDX_MAX, wmax = 0; i < n; i++) {
> -               c = x86_pmu.get_event_constraints(cpuc, i, cpuc->event_list[i]);
> -               cpuc->event_constraint[i] = c;
> +               c = cpuc->event_constraint[i];
> +
> +               /*
> +                * Request constraints for new events; or for those events that
> +                * have a dynamic constraint due to the HT workaround -- for
> +                * those the constraint can change due to scheduling activity
> +                * on the other sibling.
> +                */
> +               if (!c || ((c->flags & PERF_X86_EVENT_DYNAMIC) &&
> +                          is_ht_workaround_active(cpuc))) {
> +
> +                       c = x86_pmu.get_event_constraints(cpuc, i, cpuc->event_list[i]);
> +                       cpuc->event_constraint[i] = c;
> +               }
On this one, I think there may be a problem with events with
shared_regs constraints.
Constraint is dynamic as it depends on other events which share the
same MSR, yet it
is not marked as DYNAMIC. But this may be okay because these other
events are all
on the same CPU and thus scheduled during the same ctx_sched_in(). Yet with the
swapping in intel_alt_er(), we need to double-check that we cannot
reuse a constraint
which could be stale. I believe this is okay, just double-check.

>
>                 wmin = min(wmin, c->weight);
>                 wmax = max(wmax, c->weight);
> @@ -903,8 +921,7 @@ int x86_schedule_events(struct cpu_hw_ev
>                  * N/2 counters can be used. This helps with events with
>                  * specific counter constraints.
>                  */
> -               if (is_ht_workaround_enabled() && !cpuc->is_fake &&
> -                   READ_ONCE(cpuc->excl_cntrs->exclusive_present))
> +               if (is_ht_workaround_active(cpuc))
>                         gpmax /= 2;
>
>                 unsched = perf_assign_events(cpuc->event_constraint, n, wmin,
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -2945,7 +2945,8 @@ intel_get_event_constraints(struct cpu_h
>          * - dynamic constraint: handled by intel_get_excl_constraints()
>          */
>         c2 = __intel_get_event_constraints(cpuc, idx, event);
> -       if (c1 && (c1->flags & PERF_X86_EVENT_DYNAMIC)) {
> +       if (c1) {
> +               WARN_ON_ONCE(!(c1->flags & PERF_X86_EVENT_DYNAMIC));
>                 bitmap_copy(c1->idxmsk, c2->idxmsk, X86_PMC_IDX_MAX);
>                 c1->weight = c2->weight;
>                 c2 = c1;
>
>