linux-kernel - Re: [PATCH v1 07/10] sched/fair: Remove useless 64-bit to 32-bit variable conversion

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Date:   Thu, 25 Aug 2016 09:11:56 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Yuyang Du <yuyang.du@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH v1 07/10] sched/fair: Remove useless 64-bit to 32-bit
 variable conversion

On 10 August 2016 at 02:14, Yuyang Du <yuyang.du@...el.com> wrote:
> In __decay_sum(), the 64-bit to 32-bit variable conversion makes no
> performance nor correctness use.

Are you sure that there is no impact on 32bits system ?
Even this will be mainly shift manipulation


>
> Minor cleanup and no functionality change.
>
> Signed-off-by: Yuyang Du <yuyang.du@...el.com>
> ---
>  kernel/sched/fair.c |   13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 283e2c2..eea3349 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2616,16 +2616,11 @@ static const u32 __accumulated_sum_N32[] = {
>   */
>  static __always_inline u64 __decay_sum(u64 val, u64 n)
>  {
> -       unsigned int local_n;
> -
>         if (!n)
>                 return val;
>         else if (unlikely(n > SCHED_AVG_HALFLIFE * 63))
>                 return 0;
>
> -       /* after bounds checking we can collapse to 32-bit */
> -       local_n = n;
> -
>         /*
>          * As y^HALFLIFE = 1/2, we can combine
>          *    y^n = 1/2^(n/HALFLIFE) * y^(n%HALFLIFE)
> @@ -2633,12 +2628,12 @@ static __always_inline u64 __decay_sum(u64 val, u64 n)
>          *
>          * To achieve constant time __decay_load.
>          */
> -       if (unlikely(local_n >= SCHED_AVG_HALFLIFE)) {
> -               val >>= local_n / SCHED_AVG_HALFLIFE;
> -               local_n %= SCHED_AVG_HALFLIFE;
> +       if (unlikely(n >= SCHED_AVG_HALFLIFE)) {
> +               val >>= n / SCHED_AVG_HALFLIFE;
> +               n %= SCHED_AVG_HALFLIFE;
>         }
>
> -       val = mul_u64_u32_shr(val, __decay_inv_multiply_N[local_n], 32);
> +       val = mul_u64_u32_shr(val, __decay_inv_multiply_N[n], 32);
>         return val;
>  }
>
> --
> 1.7.9.5
>