Re: [PATCH v2 15/16] srcu: Optimize SRCU-fast-updown for arm64

[Frederic Weisbecker <frederic@kernel.org>]

Le Mon, Nov 24, 2025 at 09:20:25AM -0800, Paul E. McKenney a écrit :
> On Mon, Nov 24, 2025 at 01:04:20PM +0000, Will Deacon wrote:
> > On Mon, Nov 10, 2025 at 09:29:43AM -0800, Paul E. McKenney wrote:
> > > On Mon, Nov 10, 2025 at 11:24:07AM +0000, Will Deacon wrote:
> > > > On Sat, Nov 08, 2025 at 10:38:32AM -0800, Paul E. McKenney wrote:
> > > > > On Sat, Nov 08, 2025 at 01:07:45PM +0000, Will Deacon wrote:
> > > > > > On Wed, Nov 05, 2025 at 12:32:15PM -0800, Paul E. McKenney wrote:
> > > > > > > Some arm64 platforms have slow per-CPU atomic operations, for example,
> > > > > > > the Neoverse V2.  This commit therefore moves SRCU-fast from per-CPU
> > > > > > > atomic operations to interrupt-disabled non-read-modify-write-atomic
> > > > > > > atomic_read()/atomic_set() operations.  This works because
> > > > > > > SRCU-fast-updown is not invoked from read-side primitives, which
> > > > > > > means that if srcu_read_unlock_fast() NMI handlers.  This means that
> > > > > > > srcu_read_lock_fast_updown() and srcu_read_unlock_fast_updown() can
> > > > > > > exclude themselves and each other
> > > > > > > 
> > > > > > > This reduces the overhead of calls to srcu_read_lock_fast_updown() and
> > > > > > > srcu_read_unlock_fast_updown() from about 100ns to about 12ns on an ARM
> > > > > > > Neoverse V2.  Although this is not excellent compared to about 2ns on x86,
> > > > > > > it sure beats 100ns.
> > > > > > > 
> > > > > > > This command was used to measure the overhead:
> > > > > > > 
> > > > > > > tools/testing/selftests/rcutorture/bin/kvm.sh --torture refscale --allcpus --duration 5 --configs NOPREEMPT --kconfig "CONFIG_NR_CPUS=64 CONFIG_TASKS_TRACE_RCU=y" --bootargs "refscale.loops=100000 refscale.guest_os_delay=5 refscale.nreaders=64 refscale.holdoff=30 torture.disable_onoff_at_boot refscale.scale_type=srcu-fast-updown refscale.verbose_batched=8 torture.verbose_sleep_frequency=8 torture.verbose_sleep_duration=8 refscale.nruns=100" --trust-make
> > > > > > > 
> > > > > > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > > > > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > > > > > Cc: Will Deacon <will@kernel.org>
> > > > > > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > > > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > > > > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > > > > > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > > > > > > Cc: <linux-arm-kernel@lists.infradead.org>
> > > > > > > Cc: <bpf@vger.kernel.org>
> > > > > > > ---
> > > > > > >  include/linux/srcutree.h | 51 +++++++++++++++++++++++++++++++++++++---
> > > > > > >  1 file changed, 48 insertions(+), 3 deletions(-)
> > > > > > 
> > > > > > I've queued the per-cpu tweak from Catalin in the arm64 fixes tree [1]
> > > > > > for 6.18, so please can you drop this SRCU commit from your tree?
> > > > > 
> > > > > Very good!  Adding Frederic on CC since he is doing the pull request
> > > > > for the upcoming merge window.
> > > > > 
> > > > > But if this doesn't show up in -rc1, we reserve the right to put it
> > > > > back in.
> > > > > 
> > > > > Sorry, couldn't resist!   ;-)
> > > > 
> > > > I've merged it as a fix, so hopefully it will show up in v6.18-rc6.
> > > 
> > > Even better, thank you!!!
> > 
> > It landed in Linus' tree here:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/arm64?id=535fdfc5a228524552ee8810c9175e877e127c27
> 
> Again, thank you, and Breno has started backporting it for use in
> our fleet.
> 
> > Please can you drop the SRCU change from -next? It still shows up in
> > 20251121.
> 
> This one?
> 
> 11f748499236 ("srcu: Optimize SRCU-fast-updown for arm64")
> 
> if so, Frederic, could you please drop this commit?

Dropped, thanks!

(And I'm glad to do so given how error-prone it can be).

-- 
Frederic Weisbecker
SUSE Labs