Re: [PATCH v6 8/8] futex: Use runtime constants for __futex_hash() hot path

From: Peter Zijlstra <peterz@infradead.org>
To: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: "Arnd Bergmann" <arnd@arndb.de>,
	"Thomas Gleixner" <tglx@kernel.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Sebastian Andrzej Siewior" <bigeasy@linutronix.de>,
	"Charlie Jenkins" <thecharlesjenkins@gmail.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will@kernel.org>, "Paul Walmsley" <pjw@kernel.org>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Heiko Carstens" <hca@linux.ibm.com>,
	"Vasily Gorbik" <gor@linux.ibm.com>,
	"Alexander Gordeev" <agordeev@linux.ibm.com>,
	"Darren Hart" <dvhart@infradead.org>,
	"Davidlohr Bueso" <dave@stgolabs.net>,
	"André Almeida" <andrealmeid@igalia.com>,
	linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Sean Christopherson" <seanjc@google.com>,
	"Thomas Huth" <thuth@redhat.com>,
	"Jisheng Zhang" <jszhang@kernel.org>,
	"Ben Dooks" <ben.dooks@codethink.co.uk>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Radim Krčmář" <rkrcmar@ventanamicro.com>,
	"Guo Ren" <guoren@kernel.org>,
	"Christian Borntraeger" <borntraeger@linux.ibm.com>,
	"Sven Schnelle" <svens@linux.ibm.com>
Subject: Re: [PATCH v6 8/8] futex: Use runtime constants for __futex_hash() hot path
Date: Tue, 28 Jul 2026 13:08:06 +0200	[thread overview]
Message-ID: <20260728110806.GJ49951@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20260728104653.GH49951@noisy.programming.kicks-ass.net>

On Tue, Jul 28, 2026 at 12:46:53PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 28, 2026 at 05:25:40AM +0000, K Prateek Nayak wrote:
> > From: Peter Zijlstra <peterz@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@linutronix.de> # MAX_NUMNODES bloat
> > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> > Reviewed-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
> > Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
> > Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> > ---
> > changelog v5..v6:
> > 
> > o Picked up tags from Charlie. (Thanks a ton!)
> > 
> > o Picked up Peter's S-o-b from his tree.
> 
> Right, so I also munged the Changelog in my tree to include those
> benchmark results. I failed to actually push out those patches before
> disappearing on holidays, but I recon I should refresh them patches and
> re-munge the changelog?

With a bit of luck I didn't wreck things and this new version should now
be available in queue/locking/core.

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv