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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