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

[Will Deacon <will@kernel.org>]

Hi Paul,

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?

Cheers,

Will

[1] https://git.kernel.org/arm64/c/535fdfc5a228