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

[Anders Roxell]

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.


Cheers,
Anders

>
> diff --git a/mm/gup.c b/mm/gup.c
> index 69c483e2cc32d..6642f09c95881 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2726,7 +2726,8 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
>    * in the fast path, so instead we whitelist known good cases and if in doubt,
>    * fall back to the slow path.
>    */
> -static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
> +static __always_inline bool gup_fast_folio_allowed(struct folio *folio,
> +               unsigned int flags)
>   {
>          bool reject_file_backed = false;
>          struct address_space *mapping;
>
>
> --
> Cheers,
>
> David / dhildenb
>