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

[Peter Zijlstra]

On Tue, Jun 30, 2026 at 04:55:31AM +0000, K Prateek Nayak wrote:
> From: Peter Zijlstra <peterz at infradead.org>
> 
> Runtime constify the read-only after init data  __futex_shift(shift_32),
> __futex_mask(mask_32), and __futex_queues(ptr) used in __futex_hash()
> hot path to avoid referencing global variable.
> 
> This also allows __futex_queues to be allocated dynamically to
> "nr_node_ids" slots instead of reserving config dependent MAX_NUMNODES
> (1 << CONFIG_NODES_SHIFT) worth of slots upfront.
> 
> Runtime constants are initialized before their first access and
> runtime_const_init() provides necessary barrier to ensure subsequent
> accesses are not reordered against their initialization.
> 
> No functional changes intended.
> 
>   [ prateek: Dynamically allocate __futex_queues, mark the global data
>     __ro_after_init since they are constified after futex_init(). ]
> 
> Link: https://patch.msgid.link/20260227161841.GH606826@noisy.programming.kicks-ass.net
> Reported-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de> # MAX_NUMNODES bloat
> Not-yet-signed-off-by: Peter Zijlstra <peterz at infradead.org>
> Signed-off-by: K Prateek Nayak <kprateek.nayak at amd.com>


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.