Hello Peter,
On Mon, Jan 12, 2026 at 03:24:40PM +0100, Peter Zijlstra 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 > > > > But who cares? Its not like you'd *ever* do this, right? Users dynamically adjust SMT modes to optimize performance of the workload being run. And, yes it doesn't happen too often, but when it does, on machines with (>= 1920 CPUs) it takes more than 20 mins to finish. - vishal
