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

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Message-ID: <aRHLV8lLX0fxQICR@willie-the-truck>
Date: Mon, 10 Nov 2025 11:24:07 +0000
From: Will Deacon <will@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Catalin Marinas <catalin.marinas@....com>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org,
	frederic@...nel.org
Subject: Re: [PATCH v2 15/16] srcu: Optimize SRCU-fast-updown for arm64

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@...nel.org>
> > > Cc: Catalin Marinas <catalin.marinas@....com>
> > > Cc: Will Deacon <will@...nel.org>
> > > Cc: Mark Rutland <mark.rutland@....com>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > > Cc: <linux-arm-kernel@...ts.infradead.org>
> > > Cc: <bpf@...r.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.

Will