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futex: Temporary disable FUTEX_PRIVATE_HASH

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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 <clm@meta.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250630145034.8JnINEaS@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2025-06-30 16:50:34 +02:00 committed by Peter Zijlstra
parent d0b3b7b22d
commit 9a57c37731
1 changed files with 4 additions and 0 deletions
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4
init/Kconfig
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@ -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