[PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro

[Michael Ellerman]

Linus Torvalds <torvalds at linux-foundation.org> writes:
> On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak at gmail.com> wrote:
>>
>> 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.

So LGTM.

> The lockref_get() case can be quite hot under some loads, it would be
> sad if this made other architectures worse.

Do you know of a benchmark that shows it up? I tried a few things but
couldn't get lockref_get() to count for more than 1-2%.

cheers