Re: [PATCH v2] sched: Optimize __calc_delta.

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 03, 2021 at 02:46:53PM -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 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 bug is filed at
> https://bugs.llvm.org/show_bug.cgi?id=49406.
> 
> ```
> 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>

Thanks!