[tip: locking/urgent] futex: Temporary disable FUTEX_PRIVATE_HASH

["tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     9a57c3773152a3ff2c35cc8325e088d011c9f83b
Gitweb:        https://git.kernel.org/tip/9a57c3773152a3ff2c35cc8325e088d011c9f83b
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Mon, 30 Jun 2025 16:50:34 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 01 Jul 2025 15:02:05 +02:00

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 <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
---
 init/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index af4c2f0..666783e 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