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.
-- Uladzislau Rezki
