On Thu, May 26, 2022 at 10:14:59PM +1000, Michael Ellerman wrote:

Linus Torvalds [off-list ref] writes:

quoted

On Wed, May 25, 2022 at 7:40 AM Uros Bizjak [off-list ref] wrote:

quoted

Use try_cmpxchg64 instead of cmpxchg64 in CMPXCHG_LOOP macro.
x86 CMPXCHG instruction returns success in ZF flag, so this
change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg). The main loop of lockref_get improves from:

Ack on this one regardless of the 32-bit x86 question.

HOWEVER.

I'd like other architectures to pipe up too, because I think right now
x86 is the only one that implements that "arch_try_cmpxchg()" family
of operations natively, and I think the generic fallback for when it
is missing might be kind of nasty.

Maybe it ends up generating ok code, but it's also possible that it
just didn't matter when it was only used in one place in the
scheduler.

This patch seems to generate slightly *better* code on powerpc.

I see one register-to-register move that gets shifted slightly later, so
that it's skipped on the path that returns directly via the SUCCESS
case.

FWIW, I see the same on arm64; a register-to-register move gets moved out of
the success path. That changes the register allocation, and resulting in one
fewer move, but otherwise the code generation is the same.

Thanks,
Mark.