Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Jan 12, 2026 at 08:48:42AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 12, 2026 at 04:09:49PM +0000, Joel Fernandes wrote:
> > 
> > 
> > > On Jan 12, 2026, at 7:57 AM, Uladzislau Rezki <urezki@gmail.com> wrote:
> > > 
> > > Hello, Shrikanth!
> > > 
> > >> 
> > >>> On 1/12/26 3:38 PM, Uladzislau Rezki wrote:
> > >>> On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote:
> > >>>> Bulk CPU hotplug operations—such as switching SMT modes across all
> > >>>> cores—require hotplugging multiple CPUs in rapid succession. On large
> > >>>> systems, this process takes significant time, increasing as the number
> > >>>> of CPUs grows, leading to substantial delays on high-core-count
> > >>>> machines. Analysis [1] reveals that the majority of this time is spent
> > >>>> waiting for synchronize_rcu().
> > >>>> 
> > >>>> Expedite synchronize_rcu() during the hotplug path to accelerate the
> > >>>> operation. Since CPU hotplug is a user-initiated administrative task,
> > >>>> it should complete as quickly as possible.
> > >>>> 
> > >>>> Performance data on a PPC64 system with 400 CPUs:
> > >>>> 
> > >>>> + ppc64_cpu --smt=1 (SMT8 to SMT1)
> > >>>> Before: real 1m14.792s
> > >>>> After:  real 0m03.205s  # ~23x improvement
> > >>>> 
> > >>>> + ppc64_cpu --smt=8 (SMT1 to SMT8)
> > >>>> Before: real 2m27.695s
> > >>>> After:  real 0m02.510s  # ~58x improvement
> > >>>> 
> > >>>> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b
> > >>>> 
> > >>>> [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@linux.vnet.ibm.com
> > >>>> 
> > >>> Also you can try: echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> > >>> to speedup regular synchronize_rcu() call. But i am not saying that it would beat
> > >>> your "expedited switch" improvement.
> > >>> 
> > >> 
> > >> Hi Uladzislau.
> > >> 
> > >> Had a discussion on this at LPC, having in kernel solution is likely
> > >> better than having it in userspace.
> > >> 
> > >> - Having it in kernel would make it work across all archs. Why should
> > >>  any user wait when one initiates the hotplug.
> > >> 
> > >> - userspace tools are spread across such as chcpu, ppc64_cpu etc.
> > >>  though internally most do "0/1 > /sys/devices/system/cpu/cpuN/online".
> > >>  We will have to repeat the same in each tool.
> > >> 
> > >> - There is already /sys/kernel/rcu_expedited which is better if at all
> > >>  we need to fallback to userspace.
> > >> 
> > > Sounds good to me. I agree it is better to bypass parameters.
> > 
> > Another way to make it in-kernel would be to make the RCU normal wake from GP optimization enabled for > 16 CPUs by default.
> > 
> > I was considering this, but I did not bring it up because I did not know that there are large systems that might benefit from it until now.
> 
> This would require increasing the scalability of this optimization,
> right?  Or am I thinking of the wrong optimization?  ;-)
> 
I tested this before. I noticed that after 64K of simultaneous
synchronize_rcu() calls the scalability is required. Everything
less was faster with a new approach.

I can retest. Should i? :)

--
Uladzsislau Rezki