> On Jan 12, 2026, at 7:01 PM, Paul E. McKenney <[email protected]> wrote: > > 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 <[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.> >>> 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.
Makes sense. We could also just have a single list but a much smaller threshold for switching synchronize_rcu_normal off. That would address the conveyor belt pattern Vishal expressed. thanks, - Joel > > Thanx, Paul
