Commit 9a57c377 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Peter Zijlstra
Browse files

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: default avatarChris Mason <clm@meta.com>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250630145034.8JnINEaS@linutronix.de
parent d0b3b7b2

init/Kconfig

+4 −0
Original line number Diff line number Diff line
@@ -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