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SubjectRe: [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@intel.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@intel.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
>

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