Re: [RFC][PATCH 5/8] perf/x86/intel: Optimize intel_get_excl_constraints()

[Stephane Eranian]

On Thu, Mar 14, 2019 at 6:11 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Avoid the POPCNT by noting we can decrement the weight for each
> cleared bit.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Looks good to me.
Reviewed-by: Stephane Eranian <eranian@xxxxxxxxxx>

> ---
>  arch/x86/events/intel/core.c |   22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
>
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -2838,7 +2838,7 @@ intel_get_excl_constraints(struct cpu_hw
>         struct intel_excl_cntrs *excl_cntrs = cpuc->excl_cntrs;
>         struct intel_excl_states *xlo;
>         int tid = cpuc->excl_thread_id;
> -       int is_excl, i;
> +       int is_excl, i, w;
>
>         /*
>          * validating a group does not require
> @@ -2894,36 +2894,40 @@ intel_get_excl_constraints(struct cpu_hw
>          * SHARED   : sibling counter measuring non-exclusive event
>          * UNUSED   : sibling counter unused
>          */
> +       w = c->weight;
>         for_each_set_bit(i, c->idxmsk, X86_PMC_IDX_MAX) {
>                 /*
>                  * exclusive event in sibling counter
>                  * our corresponding counter cannot be used
>                  * regardless of our event
>                  */
> -               if (xlo->state[i] == INTEL_EXCL_EXCLUSIVE)
> +               if (xlo->state[i] == INTEL_EXCL_EXCLUSIVE) {
>                         __clear_bit(i, c->idxmsk);
> +                       w--;
> +                       continue;
> +               }
>                 /*
>                  * if measuring an exclusive event, sibling
>                  * measuring non-exclusive, then counter cannot
>                  * be used
>                  */
> -               if (is_excl && xlo->state[i] == INTEL_EXCL_SHARED)
> +               if (is_excl && xlo->state[i] == INTEL_EXCL_SHARED) {
>                         __clear_bit(i, c->idxmsk);
> +                       w--;
> +                       continue;
> +               }
>         }
>
>         /*
> -        * recompute actual bit weight for scheduling algorithm
> -        */
> -       c->weight = hweight64(c->idxmsk64);
> -
> -       /*
>          * if we return an empty mask, then switch
>          * back to static empty constraint to avoid
>          * the cost of freeing later on
>          */
> -       if (c->weight == 0)
> +       if (!w)
>                 c = &emptyconstraint;
>
> +       c->weight = w;
> +
>         return c;
>  }
>
>
>