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

diff --git a/init/Kconfig b/init/Kconfig
index af4c2f0854554bbcdf193852cf5c1d2c2accc64f..666783eb50abd7de2edaf5e367b0750a693fefbb 100644 (file)
--- 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