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

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Jan 12, 2026 at 05:24:40PM -0500, Joel Fernandes wrote:
> 
> 
> On 1/12/2026 11:48 AM, 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?  ;-)
> > 
> Yes I think you are considering the correct one, the concern you have is
> regarding large number of wake ups initiated from the GP thread, correct?
> 
> I was suggesting on the thread, a more dynamic approach where using
> synchronize_rcu_normal() until it gets overloaded with requests. One approach
> might be to measure the length of the rcu_state.srs_next to detect an overload
> condition, similar to qhimark? Or perhaps qhimark itself can be used. And under
> lightly loaded conditions, default to synchronize_rcu_normal() without checking
> for the 16 CPU count.
> 
> Thoughts?

Or maintain multiple lists.  Systems with 1000+ CPUs can be a bit
unforgiving of pretty much any form of contention.

							Thanx, Paul