Sent out this morning as part of this week's "locking/urgent" pull request is a performance regression fix ahead of today's Linux 6.16-rc5 release. This latest performance regression in the Linux kernel is around the new Futex code merged this cycle with a big performance hit being observed in scheduler benchmarks.

The locking/urgent pull request sent out this morning is disabling the FUTEX_PRIVATE_HASH option merged back during the start of the Linux 6.16 merge window. The FUTEX_PRIVATE_HASH functionality was introduced as part of the work on supporting task local hash maps that landed alongside [1]some nice Futex improvements in Linux 6.16 .

The FUTEX_PRIVATE_HASH code ended up causing some significant performance regressions though so is being disabled disabled for Linux 6.16 and then for Linux 6.17 the kernel developers will attempt to re-introduce it with a proper performance fix.

Meta engineer Chris Mason reported the performance regression that was able to mimic real-world performance workloads, unlike some earlier Futex benchmarks that showed some performance regressions just in micro-benchmarks. So FUTEX_PRIVATE_HASH with today's pull request is being gated by the "BROKEN" Kconfig option so it's effectively disabled for the rest of the Linux 6.16 cycle.

Chris Mason [2]reported the performance regression with a pretty sizable performance hit:

"On my big turin system, this commit slows down RPS by 36%. But even a VM on a skylake machine sees a 29% difference."

The requests per second dropping by 36% on a high-end AMD EPYC 9005 "Turin" server. Ouch.

So with [3]this pull request expected to hit Linux Git today ahead of the Linux 6.16-rc5 release, the regression will be avoided.



[1] https://www.phoronix.com/news/FUTEX2-NUMA-Linux-6.16

[2] https://lore.kernel.org/all/3ad05298-351e-4d61-9972-ca45a0a50e33@meta.com/

[3] https://lore.kernel.org/lkml/20250706081812.GAaGoxRMTfEshzatY7@fat_crate.local/