On Tue, Mar 02, 2021 at 12:57:37PM -0800, Josh Don wrote:
> From: Clement Courbet <cour...@google.com>
>
> A significant portion of __calc_delta time is spent in the loop
> shifting a u64 by 32 bits. Use `fls` instead of iterating.
>
> This is ~7x faster on benchmarks.
>
> The generic `fls` implementation (`generic_fls`) is still ~4x faster
> than the loop.
> Architectures that have a better implementation will make use of it. For
> example, on X86 we get an additional factor 2 in speed without dedicated
> implementation.
>
> On gcc, the asm versions of `fls` are about the same speed as the
> builtin. On clang, the versions that use fls (fls,fls64) are more than
> twice as slow as the builtin. This is because the way the `fls` function
> is written, clang puts the value in memory:
> https://godbolt.org/z/EfMbYe. This can be fixed in a separate patch.
>
> ```
> name cpu/op
> BM_Calc<__calc_delta_loop> 9.57ms ±12%
> BM_Calc<__calc_delta_generic_fls> 2.36ms ±13%
> BM_Calc<__calc_delta_asm_fls> 2.45ms ±13%
> BM_Calc<__calc_delta_asm_fls_nomem> 1.66ms ±12%
> BM_Calc<__calc_delta_asm_fls64> 2.46ms ±13%
> BM_Calc<__calc_delta_asm_fls64_nomem> 1.34ms ±15%
> BM_Calc<__calc_delta_builtin> 1.32ms ±11%
> ```
>
> Signed-off-by: Clement Courbet <cour...@google.com>
> Signed-off-by: Josh Don <josh...@google.com>
> ---
> kernel/sched/fair.c | 30 ++++++++++++++++++++++--------
> kernel/sched/sched.h | 1 +
> 2 files changed, 23 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a8bd7b13634..67e5a1d536ad 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -214,6 +214,16 @@ static void __update_inv_weight(struct load_weight *lw)
> lw->inv_weight = WMULT_CONST / w;
> }
>
> +/*
> + * An fls that handles an u32 value on architectures
> + * where `sizeof(unsigned int) < 32`.
> + */
> +#if (__SIZEOF_INT__ >= 32)
This should never happen, we use ILP32 or LP64 for Linux.
> +# define FLS_AT_LEAST_32(v) fls(v)
> +#else
> +# define FLS_AT_LEAST_32(v) fls64(v)
> +#endif
> +
> /*
> * delta_exec * weight / lw.weight
> * OR
> @@ -229,27 +239,31 @@ static void __update_inv_weight(struct load_weight *lw)
> static u64 __calc_delta(u64 delta_exec, unsigned long weight, struct
> load_weight *lw)
> {
> u64 fact = scale_load_down(weight);
> + u32 fact_hi = (u32)(fact >> 32);
> int shift = WMULT_SHIFT;
> + int fs;
>
> __update_inv_weight(lw);
>
> - if (unlikely(fact >> 32)) {
> - while (fact >> 32) {
> - fact >>= 1;
> - shift--;
> - }
> + if (unlikely(fact_hi)) {
> + fs = FLS_AT_LEAST_32(fact_hi);
you made fact_hi u32, why can't we unconditionally use fls() ?
> + shift -= fs;
> + fact >>= fs;
> }
>
> fact = mul_u32_u32(fact, lw->inv_weight);
>
> - while (fact >> 32) {
> - fact >>= 1;
> - shift--;
> + fact_hi = (u32)(fact >> 32);
> + if (fact_hi) {
> + fs = FLS_AT_LEAST_32(fact_hi);
> + shift -= fs;
> + fact >>= fs;
> }
>
> return mul_u64_u32_shr(delta_exec, fact, shift);
> }
Horrific whitespace damage..
