Re: [PATCH] sched: Optimize __calc_delta.

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 02, 2021 at 12:57:37PM -0800, Josh Don wrote:
> From: Clement Courbet <courbet@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 <courbet@google.com>
> Signed-off-by: Josh Don <joshdon@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..