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Message-ID: <20250630145034.8JnINEaS@linutronix.de> Date: Mon, 30 Jun 2025 16:50:34 +0200 From: Sebastian Andrzej Siewior <bigeasy@...utronix.de> To: Peter Zijlstra <peterz@...radead.org> Cc: Chris Mason <clm@...a.com>, linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de> Subject: [PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH Chris Mason reported a performance regression on big iron. Reports of this kind were usually reported as part of a micro benchmark but Chris' test did mimic his real workload. This makes it a real regression. The root cause is rcuref_get() which is invoked during each futex operation. If all threads of an application do this simultaneously then it leads to cache line bouncing and the performance drops. Disable FUTEX_PRIVATE_HASH entirely for this cycle. The performance regression will be addressed in the following cycle enabling the option again. Reported-by: Chris Mason <clm@...a.com> Closes: https://lore.kernel.org/all/3ad05298-351e-4d61-9972-ca45a0a50e33@meta.com/ Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de> --- init/Kconfig | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/init/Kconfig b/init/Kconfig index af4c2f0854554..666783eb50abd 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1716,9 +1716,13 @@ config FUTEX_PI depends on FUTEX && RT_MUTEXES default y +# +# marked broken for performance reasons; gives us one more cycle to sort things out. +# config FUTEX_PRIVATE_HASH bool depends on FUTEX && !BASE_SMALL && MMU + depends on BROKEN default y config FUTEX_MPOL -- 2.50.0
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