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

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jan 13, 2026 at 12:44:10PM +0000, Joel Fernandes wrote:
> 
> 
> > On Jan 13, 2026, at 7:19 AM, Uladzislau Rezki <urezki@gmail.com> wrote:
> > 
> > On Mon, Jan 12, 2026 at 05:36:24PM +0000, Joel Fernandes wrote:
> >> 
> >> 
> >>>> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <urezki@gmail.com> 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.
> >>>> 
> >>> IMO, we can increase that threshold. 512/1024 is not a problem at all.
> >>> But as Paul mentioned, we should consider scalability enhancement. From
> >>> the other hand it is also probably worth to get into the state when we
> >>> really see them :)
> >> 
> >> Instead of pegging to number of CPUs, perhaps the optimization should be dynamic? That is, default to it unless synchronize_rcu load is high, default to the sr_normal wake-up optimization. Of course carefully considering all corner cases, adequate testing and all that ;-)
> >> 
> > Honestly i do not see use cases when we are not up to speed to process
> > all callbacks in time keeping in mind that it is blocking context call.
> > 
> > How many of them should be in flight(blocked contexts) to make it starve... :)
> > According to my last evaluation it was ~64K.
> > 
> > Note i do not say that it should not be scaled.
> 
> But you did not test that on large system with 1000s of CPUs right? 
> 
No, no. I do not have access to such systems.

>
> So the options I see are: either default to always using the optimization,
> not just for less than 17 CPUs (what you are saying above). Or, do what I said
> above (safer for system with 1000s of CPUs and less risky).
>
You mean introduce threshold and count how many nodes are in queue?
To me it sounds not optimal and looks like a temporary solution. 

Long term wise, it is better to split it, i mean to scale.

Do you know who can test it on ~1000 CPUs system? So we have some figures.

What i have is 256 CPUs system i can test on.

--
Uladzislau Rezki