linux-kernel - Re: [RFC][PATCH 4/8] perf/x86: Remove PERF_X86_EVENT

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Date:   Tue, 19 Mar 2019 13:48:18 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, tonyj@...e.com,
        nelson.dsouza@...el.com
Subject: Re: [RFC][PATCH 4/8] perf/x86: Remove PERF_X86_EVENT_COMMITTED

On Thu, Mar 14, 2019 at 6:11 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> The flag PERF_X86_EVENT_COMMITTED is used to find uncommitted events
> for which to call put_event_constraint() when scheduling fails.
>
> These are the newly added events to the list, and must form, per
> definition, the tail of cpuc->event_list[]. By computing the list
> index of the last successfull schedule, then iteration can start there
> and the flag is redundant.
>
> There are only 3 callers of x86_schedule_events(), notably:
>
>  - x86_pmu_add()
>  - x86_pmu_commit_txn()
>  - validate_group()
>
> For x86_pmu_add(), cpuc->n_events isn't updated until after
> schedule_events() succeeds, therefore cpuc->n_events points to the
> desired index.
>
Correct.

> For x86_pmu_commit_txn(), cpuc->n_events is updated, but we can
> trivially compute the desired value with cpuc->n_txn -- the number of
> events added in this transaction.
>
I suggest you put this explanation in the code so that it is easier to
understand.

> For validate_group(), we can make the rule for x86_pmu_add() work by
> simply setting cpuc->n_events to 0 before calling schedule_events().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Reviewed-by: Stephane Eranian <eranian@...gle.com>

> ---
>  arch/x86/events/core.c       |   21 ++++++---------------
>  arch/x86/events/perf_event.h |    1 -
>  2 files changed, 6 insertions(+), 16 deletions(-)
>
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -925,19 +925,16 @@ int x86_schedule_events(struct cpu_hw_ev
>         if (!unsched && assign) {
>                 for (i = 0; i < n; i++) {
>                         e = cpuc->event_list[i];
> -                       e->hw.flags |= PERF_X86_EVENT_COMMITTED;
>                         if (x86_pmu.commit_scheduling)
>                                 x86_pmu.commit_scheduling(cpuc, i, assign[i]);
>                 }
>         } else {
> -               for (i = 0; i < n; i++) {
> +               i = cpuc->n_events;
> +               if (cpuc->txn_flags & PERF_PMU_TXN_ADD)
> +                       i -= cpuc->n_txn;
> +
> +               for (; i < n; i++) {
>                         e = cpuc->event_list[i];
> -                       /*
> -                        * do not put_constraint() on comitted events,
> -                        * because they are good to go
> -                        */
> -                       if ((e->hw.flags & PERF_X86_EVENT_COMMITTED))
> -                               continue;
>
>                         /*
>                          * release events that failed scheduling
> @@ -1372,11 +1369,6 @@ static void x86_pmu_del(struct perf_even
>         int i;
>
>         /*
> -        * event is descheduled
> -        */
> -       event->hw.flags &= ~PERF_X86_EVENT_COMMITTED;
> -
> -       /*
>          * If we're called during a txn, we only need to undo x86_pmu.add.
>          * The events never got scheduled and ->cancel_txn will truncate
>          * the event_list.
> @@ -2079,8 +2071,7 @@ static int validate_group(struct perf_ev
>         if (n < 0)
>                 goto out;
>
> -       fake_cpuc->n_events = n;
> -
> +       fake_cpuc->n_events = 0;
>         ret = x86_pmu.schedule_events(fake_cpuc, n, NULL);
>
>  out:
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -61,7 +61,6 @@ struct event_constraint {
>  #define PERF_X86_EVENT_PEBS_LDLAT      0x0001 /* ld+ldlat data address sampling */
>  #define PERF_X86_EVENT_PEBS_ST         0x0002 /* st data address sampling */
>  #define PERF_X86_EVENT_PEBS_ST_HSW     0x0004 /* haswell style datala, store */
> -#define PERF_X86_EVENT_COMMITTED       0x0008 /* event passed commit_txn */

I would put a placeholder saying that bit 3 is available or renumbered
the other masks below

>  #define PERF_X86_EVENT_PEBS_LD_HSW     0x0010 /* haswell style datala, load */
>  #define PERF_X86_EVENT_PEBS_NA_HSW     0x0020 /* haswell style datala, unknown */
>  #define PERF_X86_EVENT_EXCL            0x0040 /* HT exclusivity on counter */
>
>