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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 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? thanks, - Joel