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 <[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! ;-)
>
> I've merged it as a fix, so hopefully it will show up in v6.18-rc6.
Even better, thank you!!!
Thanx, Paul
