linux-kernel - Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting

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Message-ID: <e0f58203-22ef-44c0-9f7b-b15c6007249b@linux.ibm.com>
Date: Tue, 15 Jul 2025 21:29:34 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Cc: André Almeida <andrealmeid@...lia.com>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Valentin Schneider <vschneid@...hat.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting



On 7/10/25 16:30, Sebastian Andrzej Siewior wrote:
> I picked up PeterZ futex patch from
>      https://lore.kernel.org/all/20250624190118.GB1490279@noisy.programming.kicks-ass.net/
> 
> and I am posting it here it now so it can be staged for v6.17.
> 
> This survived a few days on my machine and compile robot reported that
> is passes its tests.
> 
> v1…v2 https://lore.kernel.org/all/20250707143623.70325-1-bigeasy@linutronix.de
>   - Removed the IMMUTABLE bits
>   - There was a race if the application exits while the RCU callback is
>     pending. Stuffed with mmget()/ mmput_async().
> 
> Changes since its initial posting:
> - A patch description has been added
> - The testuite is "fixed" slightly different and has been split out
> - futex_mm_init() is fixed up.
> - The guard(preempt) has been replaced with guard(rcu) since there is
>    no reason to disable preemption.
> 
> Since it was not yet released, should we rip out the IMMUTABLE bits and
> just stick with GET/SET slots?
> 
> Peter Zijlstra (1):
>    futex: Use RCU-based per-CPU reference counting instead of rcuref_t
> 
> Sebastian Andrzej Siewior (5):
>    selftests/futex: Adapt the private hash test to RCU related changes
>    futex: Make futex_private_hash_get() static
>    futex: Remove support for IMMUTABLE
>    selftests/futex: Remove support for IMMUTABLE
>    perf bench futex: Remove support for IMMUTABLE
> 
>   include/linux/futex.h                         |  16 +-
>   include/linux/mm_types.h                      |   5 +
>   include/linux/sched/mm.h                      |   2 +-
>   include/uapi/linux/prctl.h                    |   2 -
>   init/Kconfig                                  |   4 -
>   kernel/fork.c                                 |   8 +-
>   kernel/futex/core.c                           | 281 ++++++++++++++----
>   kernel/futex/futex.h                          |   2 -
>   tools/include/uapi/linux/prctl.h              |   2 -
>   tools/perf/bench/futex-hash.c                 |   1 -
>   tools/perf/bench/futex-lock-pi.c              |   1 -
>   tools/perf/bench/futex-requeue.c              |   1 -
>   tools/perf/bench/futex-wake-parallel.c        |   1 -
>   tools/perf/bench/futex-wake.c                 |   1 -
>   tools/perf/bench/futex.c                      |  21 +-
>   tools/perf/bench/futex.h                      |   1 -
>   .../trace/beauty/include/uapi/linux/prctl.h   |   2 -
>   .../futex/functional/futex_priv_hash.c        | 113 +++----
>   18 files changed, 315 insertions(+), 149 deletions(-)
> 

Hi. Sorry for not stumble upon this earlier. Saw these now.

Since perf bench had shown a significant regression last time around, and
for which immutable option was added, gave perf futex a try again.

Below are the results: Ran on 5 core LPAR(VM) on power. perf was compiled from tools/perf.

===========
baseline:
===========
tip/master at
commit 8784fb5fa2e0042fe3b1632d4876e1037b695f56 (HEAD)
Author: Borislav Petkov (AMD) <bp@...en8.de>

./perf bench futex hash
Averaged 1559643 operations/sec (+- 0.09%), total secs = 10
Futex hashing: global hash

schbench -t 64 -r 5 -i 5
current rps: 2629.85

schbench -L -m 4 -M auto -t 64 -n 0 -r 5 -i 5
current rps: 1538674.22

=================
baseline + series
=================

./perf bench futex hash
Averaged 306403 operations/sec (+- 0.29%), total secs = 10    <<<  around 1/5th of baseline.
Futex hashing: auto resized to 256 buckets                    <<<  maybe resize doesn't happen fast?


./perf bench futex hash -b 512                                <<< Gave 512 buckets,
Averaged 1412543 operations/sec (+- 0.14%), total secs = 10   <<< much better numbers, still off by 8-10%.
Futex hashing: 512 hash buckets

(512 is the number of buckets that baseline would have used, increased the buckets to 8192 for trial)

./perf bench futex hash -b 8192
Averaged 1441627 operations/sec (+- 0.14%), total secs = 10
Futex hashing: 8192 hash buckets


schbench -t 64 -r 5 -i 5
current rps: 2656.85                                          <<< schbench seems good.

schbench -L -m 4 -M auto -t 64 -n 0 -r 5 -i 5
current rps: 1539273.79