On Sun, 26 Nov 2023 at 08:39, Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> Here is my optimization advice:
>
> #define CMPXCHG_LOOP(CODE, SUCCESS) do { \
> int retry = 100; \
> struct lockref old; \
> BUILD_BUG_ON(sizeof(old) != 8); \
> + prefetchw(lockref); \\
No.
We're not adding software prefetches to generic code. Been there, done
that. They *never* improve performance on good hardware. They end up
helping on some random (usually particularly bad) microarchitecture,
and then they hurt everybody else.
And the real optimization advice is: "don't run on crap hardware".
It really is that simple. Good hardware does OoO and sees the future write.
> Micro-arch could give prefetchw more guarantee:
Well, in practice, they never do, and in fact they are often buggy and
cause problems because they weren't actually tested very much.
Linus
