> 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. thanks, - Joel > > -- > Uladzislau Rezki
