On Sat, Nov 08, 2025 at 01:07:45PM +0000, Will Deacon wrote:
> 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 <[email protected]>
> > Cc: Catalin Marinas <[email protected]>
> > Cc: Will Deacon <[email protected]>
> > Cc: Mark Rutland <[email protected]>
> > Cc: Mathieu Desnoyers <[email protected]>
> > Cc: Steven Rostedt <[email protected]>
> > Cc: Sebastian Andrzej Siewior <[email protected]>
> > Cc: <[email protected]>
> > Cc: <[email protected]>
> > ---
> > 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! ;-)
Thanx, Paul
> Cheers,
>
> Will
>
> [1] https://git.kernel.org/arm64/c/535fdfc5a228
