[PATCH v5 8/8] futex: Use runtime constants for __futex_hash() hot path

[K Prateek Nayak]

Hello Peter, Sebastian,

On 7/1/2026 2:11 PM, Sebastian Andrzej Siewior wrote:
> On 2026-07-01 09:57:14 [+0200], Peter Zijlstra wrote:
>> The big $1M question: does it actually make it go faster? The whole
>> point here was performance, right? But I'm not seeing numbers showing
>> how awesome these patches are.
> 
> I did complain about the about the size of __futex_data which is blown
> on distro kernels due to CONFIG_NODES_SHIFT=10 on Debian for instance.
> This makes it go away at no extra price but yeah let me boot a big box
> and see.
> If the performance remains unchanged it is still worth considering due
> to size savings on the average box with 1 node. The biggest box I have
> access to has four nodes. If I remember correctly, Prateek was saying
> that AMD has "normal" boxes which would require =9 for normal operation
> and they do run distro kernels so lowering that value is not an option.

Rationale there was with CCX as NUMA, we have 32 NUMA nodes on chip and
with CXL, there is a possibility of 2x that so I suggested NODE_SHIFT
of 7 or 8 should probably cover almost all real hardware without any
added NUMA emulation weirdness.

To answer the million dollar question, I see the following on running
perf bench futex on a 3rd Gen EPYC (2 x 64C/128T)

  +----------------+-----------+-----------+-----------+--------------+
  | Benchmark      | Kernel 1  | Kernel 2  |   Unit    | % Improvement|
  |                |  (avg/5)  |  (avg/5)  |           | (K2 vs K1)   |
  +----------------+-----------+-----------+-----------+--------------+
  | Wake-parallel  |  0.01614  |  0.00456  |    ms     |   +71.75%    |
  | Requeue        |  0.26394  |  0.24644  |    ms     |    +6.63%    |
  | Lock-pi        |     34.0  |     57.2  |  ops/sec  |   +68.24%    |
  +----------------+-----------+-----------+-----------+--------------+

Kernel1 is tip at base commit and Kernel 2 is tip + this series.
perf bench futex hash some insane bimodal behavior on my system with
both tip and tip + series so I've left that variant out for now.

This is only from 5 runs from a single boot. I'll try to grab a
bigger system and check is it makes a difference there.

-- 
Thanks and Regards,
Prateek