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

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Message-ID: <f3fe6be4-723e-45b8-baa6-5c285cc5c150@redhat.com>
Date: Tue, 3 Sep 2024 14:37:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Anders Roxell <anders.roxell@...aro.org>
Cc: Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 dvhart@...radead.org, dave@...olabs.net, andrealmeid@...lia.com,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Linux-MM <linux-mm@...ck.org>
Subject: Re: Potential Regression in futex Performance from v6.9 to v6.10-rc1
 and v6.11-rc4

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?

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