From: Sebastian Andrzej Siewior Date: Mon, 30 Jun 2025 14:50:34 +0000 (+0200) Subject: futex: Temporary disable FUTEX_PRIVATE_HASH X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=9a57c3773152a3ff2c35cc8325e088d011c9f83b;p=linux-block.git 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. Closes: https://lore.kernel.org/all/3ad05298-351e-4d61-9972-ca45a0a50e33@meta.com/ Reported-by: Chris Mason Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250630145034.8JnINEaS@linutronix.de --- diff --git a/init/Kconfig b/init/Kconfig index af4c2f085455..666783eb50ab 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