Messages in this thread | | | From | Michael Ellerman <> | Subject | Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro | Date | Thu, 26 May 2022 22:14:59 +1000 |
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Linus Torvalds <torvalds@linux-foundation.org> writes: > On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak@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
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