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

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Message-ID: <20250715163134.pM1J2XO9@linutronix.de>
Date: Tue, 15 Jul 2025 18:31:34 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	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 2025-07-15 21:29:34 [+0530], Shrikanth Hegde wrote:
> 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.
Thank you.

If you use perf-bench with -b then the buckets are applied
"immediately". It mostly works also with auto scaling. The problem is
that perf creates the threads and immediately after it starts the test.
While the RCU kicks in shortly after there is no transition happening
until after all the test completes/ the threads terminate. The reason is
that several private-hash references are in use because a some threads
are always in the futex() syscall.

It would require something like commit
    a255b78d14324 ("selftests/futex: Adapt the private hash test to RCU related changes")

to have this transition before the test starts.
Your schbench seems not affected?

If you use -b, is it better than or equal compared to the immutable
option? This isn't quite clear.

Sebastian