Re: [RFC 2/2] perf/core: Invoke pmu::sched_task callback for per-cpu events

[Stephane Eranian <eranian@google.com>]

On Thu, Nov 5, 2020 at 11:40 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>
>
>
> On 11/5/2020 10:54 AM, Namhyung Kim wrote:
> >> -void perf_sched_cb_inc(struct pmu *pmu)
> >> +void perf_sched_cb_inc(struct pmu *pmu, bool systemwide)
> >>    {
> >>          struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
> >>
> >> -       if (!cpuctx->sched_cb_usage++)
> >> -               list_add(&cpuctx->sched_cb_entry, this_cpu_ptr(&sched_cb_list));
> >> +       cpuctx->sched_cb_usage++;
> >>
> >> -       this_cpu_inc(perf_sched_cb_usages);
> >> +       if (systemwide) {
> >> +               this_cpu_inc(perf_sched_cb_usages);
> >> +               list_add(&cpuctx->sched_cb_entry, this_cpu_ptr(&sched_cb_list));
> > You need to check the value and make sure it's added only once.
>
> Right, maybe we have to add a new variable for that.
>
Sure, I tend to agree here that we need a narrower scope trigger, only
when needed, i.e., an event
or PMU feature that requires ctxsw work. In fact, I am also interested
in splitting ctxswin and ctswout
callbacks. The reason is that you have overhead in the way the
callback is invoked. You may end up
calling the sched_task on ctxswout when only ctxwin is needed. In
doing that you pay the cost of
stopping/starting the PMU for possibly nothing. Stopping the PMU can
be expensive, like on AMD
where you need multiple wrmsr.

So splitting or adding a flag to convey that either CTXSW_IN or
CTXSW_OUT is needed would help.
I am suggesting this now given you are adding a flag.

>
> diff --git a/arch/powerpc/perf/core-book3s.c
> b/arch/powerpc/perf/core-book3s.c
> index 6586f7e71cfb..63c9b87cab5e 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -380,7 +380,7 @@ static void power_pmu_bhrb_enable(struct perf_event
> *event)
>                 cpuhw->bhrb_context = event->ctx;
>         }
>         cpuhw->bhrb_users++;
> -       perf_sched_cb_inc(event->ctx->pmu);
> +       perf_sched_cb_inc(event->ctx->pmu, !(event->attach_state &
> PERF_ATTACH_TASK));
>   }
>
>   static void power_pmu_bhrb_disable(struct perf_event *event)
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index 444e5f061d04..a34b90c7fa6d 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -1022,9 +1022,9 @@ pebs_update_state(bool needed_cb, struct
> cpu_hw_events *cpuc,
>
>         if (needed_cb != pebs_needs_sched_cb(cpuc)) {
>                 if (!needed_cb)
> -                       perf_sched_cb_inc(pmu);
> +                       perf_sched_cb_inc(pmu, !(event->attach_state & PERF_ATTACH_TASK));
>                 else
> -                       perf_sched_cb_dec(pmu);
> +                       perf_sched_cb_dec(pmu, !(event->attach_state & PERF_ATTACH_TASK));
>
>                 update = true;
>         }
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 8961653c5dd2..8d4d02cde3d4 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -693,7 +693,7 @@ void intel_pmu_lbr_add(struct perf_event *event)
>          */
>         if (x86_pmu.intel_cap.pebs_baseline && event->attr.precise_ip > 0)
>                 cpuc->lbr_pebs_users++;
> -       perf_sched_cb_inc(event->ctx->pmu);
> +       perf_sched_cb_inc(event->ctx->pmu, !(event->attach_state &
> PERF_ATTACH_TASK));
>         if (!cpuc->lbr_users++ && !event->total_time_running)
>                 intel_pmu_lbr_reset();
>
> @@ -740,7 +740,7 @@ void intel_pmu_lbr_del(struct perf_event *event)
>         cpuc->lbr_users--;
>         WARN_ON_ONCE(cpuc->lbr_users < 0);
>         WARN_ON_ONCE(cpuc->lbr_pebs_users < 0);
> -       perf_sched_cb_dec(event->ctx->pmu);
> +       perf_sched_cb_dec(event->ctx->pmu, !(event->attach_state &
> PERF_ATTACH_TASK));
>   }
>
>   static inline bool vlbr_exclude_host(void)
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index a1b91f2de264..14f936385cc8 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -875,6 +875,7 @@ struct perf_cpu_context {
>
>         struct list_head                sched_cb_entry;
>         int                             sched_cb_usage;
> +       int                             sched_cb_sw_usage;
>
>         int                             online;
>         /*
> @@ -967,8 +968,8 @@ extern const struct perf_event_attr
> *perf_event_attrs(struct perf_event *event);
>   extern void perf_event_print_debug(void);
>   extern void perf_pmu_disable(struct pmu *pmu);
>   extern void perf_pmu_enable(struct pmu *pmu);
> -extern void perf_sched_cb_dec(struct pmu *pmu);
> -extern void perf_sched_cb_inc(struct pmu *pmu);
> +extern void perf_sched_cb_dec(struct pmu *pmu, bool systemwide);
> +extern void perf_sched_cb_inc(struct pmu *pmu, bool systemwide);
>   extern int perf_event_task_disable(void);
>   extern int perf_event_task_enable(void);
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 66a9bd71f3da..af75859c9138 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3484,22 +3484,32 @@ static void perf_event_context_sched_out(struct
> task_struct *task, int ctxn,
>
>   static DEFINE_PER_CPU(struct list_head, sched_cb_list);
>
> -void perf_sched_cb_dec(struct pmu *pmu)
> +void perf_sched_cb_dec(struct pmu *pmu, bool systemwide)
>   {
>         struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
>
> +       --cpuctx->sched_cb_usage;
> +
> +       if (!systemwide)
> +               return;
> +
>         this_cpu_dec(perf_sched_cb_usages);
>
> -       if (!--cpuctx->sched_cb_usage)
> +       if (!--cpuctx->sched_cb_sw_usage)
>                 list_del(&cpuctx->sched_cb_entry);
>   }
>
>
> -void perf_sched_cb_inc(struct pmu *pmu)
> +void perf_sched_cb_inc(struct pmu *pmu, bool systemwide)
>   {
>         struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
>
> -       if (!cpuctx->sched_cb_usage++)
> +       cpuctx->sched_cb_usage++;
> +
> +       if (!systemwide)
> +               return;
> +
> +       if (!cpuctx->sched_cb_sw_usage++)
>                 list_add(&cpuctx->sched_cb_entry, this_cpu_ptr(&sched_cb_list));
>
>         this_cpu_inc(perf_sched_cb_usages);
>
>
>
> Thanks,
> Kan