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.
-- Uladzislau Rezki
