On Tue, Jan 13, 2026 at 12:44:10PM +0000, Joel Fernandes wrote: > > > > On Jan 13, 2026, at 7:19 AM, Uladzislau Rezki <[email protected]> wrote: > > > > On Mon, Jan 12, 2026 at 05:36:24PM +0000, Joel Fernandes wrote: > >> > >> > >>>> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <[email protected]> wrote: > >>> > >>> On Mon, Jan 12, 2026 at 04:09:49PM +0000, Joel Fernandes wrote: > >>>> > >>>> > >>>>>> On Jan 12, 2026, at 7:57 AM, Uladzislau Rezki <[email protected]> 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.ca...@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
