Re: Potential Regression in futex Performance from v6.9 to v6.10-rc1 and v6.11-rc4

[David Hildenbrand]

On 04.09.24 12:05, Anders Roxell wrote:
> On Tue, 3 Sept 2024 at 14:37, David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 03.09.24 14:21, Anders Roxell wrote:
> > > Hi,
> > >
> > > I've noticed that the futex01-thread-* tests in will-it-scale-sys-threads
> > > are running about 2% slower on v6.10-rc1 compared to v6.9, and this
> > > slowdown continues with v6.11-rc4. I am focused on identifying any
> > > performance regressions greater than 2% that occur in automated
> > > testing on arm64 HW.
> > >
> > > Using git bisect, I traced the issue to commit
> > > f002882ca369 ("mm: merge folio_is_secretmem() and
> > > folio_fast_pin_allowed() into gup_fast_folio_allowed()").
> >
> > Thanks for analyzing the (slight) regression!
> >
> > > My tests were performed on m7g.large and m7g.metal instances:
> > >
> > > * The slowdown is consistent regardless of the number of threads;
> > >      futex1-threads-128 performs similarly to futex1-threads-2, indicating
> > >      there is no scalability issue, just a minor performance overhead.
> > > * The test doesn’t involve actual futex operations, just dummy wake/wait
> > >      on a variable that isn’t accessed by other threads, so the results might
> > >      not be very significant.
> > >
> > > Given that this seems to be a minor increase in code path length rather
> > > than a scalability issue, would this be considered a genuine regression?
> >
> > Likely not, I've seen these kinds of regressions (for example in my fork
> > micro-benchmarks) simply because the compiler slightly changes the code
> > layout, or suddenly decides to not inline a functions.
> >
> > Still it is rather unexpected, so let's find out what's happening.
> >
> > My first intuition would have been that the compiler now decides to not
> > inline gup_fast_folio_allowed() anymore, adding a function call.
> >
> > LLVM seems to inline it for me. GCC not.
> >
> > Would this return the original behavior for you?
>
> David thank you for quick patch for me to try.
>
> This patch helped the original regression on v6.10-rc1, but on current mainline
> v6.11-rc6 the patch does nothing and the performance is as expeced.

Just so I understand this correctly:

It fixed itself after v6.11-rc4, but v6.11-rc4 was fixed with my patch?

If that's the case, then it's really the compiler deciding whether to 
inline or not, and on v6.11-rc6 it decides to inline again.

--
Cheers,

David / dhildenb