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Peter Anvin" , Thomas Huth , Sean Christopherson , Jisheng Zhang , Alexandre Ghiti , Christian Borntraeger , Sven Schnelle Subject: Re: [PATCH v5 8/8] futex: Use runtime constants for __futex_hash() hot path Message-ID: <20260701075714.GP48970@noisy.programming.kicks-ass.net> References: <20260630045531.3939-1-kprateek.nayak@amd.com> <20260630045531.3939-9-kprateek.nayak@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260630045531.3939-9-kprateek.nayak@amd.com> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jun 30, 2026 at 04:55:31AM +0000, K Prateek Nayak wrote: > From: Peter Zijlstra > > 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 # MAX_NUMNODES bloat > Not-yet-signed-off-by: Peter Zijlstra > Signed-off-by: K Prateek Nayak 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.