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

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jan 13, 2026 at 09:32:13AM -0500, Joel Fernandes wrote:
> 
> 
> On 1/13/2026 9:17 AM, Uladzislau Rezki wrote:
> > 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?
> 
> Yes.
> 
> > To me it sounds not optimal and looks like a temporary solution. 
> 
> Not more sub-optimal than the existing 16 CPU hard-coded solution I suppose.
> 
It was trial testing :) Agree we should do something with it.

> > 
> > Long term wise, it is better to split it, i mean to scale.
> 
> But the scalable solution is already there: the !synchronize_rcu_normal path,
> right? And splitting the list won't help this use case anyway.
> 
Fair point.

> > 
> > Do you know who can test it on ~1000 CPUs system? So we have some figures.
> 
> I don't have such systems either. The most I can go is ~200+ CPUs. Perhaps the
> folks on this thread have such systems as they mentioned 1900+ CPU systems. They
> should be happy to test.
> 
> > 
> > What i have is 256 CPUs system i can test on.
> Same boat. ;-)
> 
:)

--
Uladzislau Rezki