On Mon, Jan 12, 2026 at 02:20:44PM +0000, Joel Fernandes wrote: > > > > On Jan 12, 2026, at 9:03 AM, Joel Fernandes <[email protected]> wrote: > > > > > > > >> On Jan 12, 2026, at 4:44 AM, Vishal Chourasia <[email protected]> > >> 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. > > > > When does the user initiate this in your system? > > > > Hotplug should not be happening that often to begin with, it is a slow path > > that > > depends on the disruptive stop-machine mechanism. > > > >> > >> 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 > > > > This does look compelling but, Could you provide more information about how > > this was tested - what does the ppc binary do (how many hot plugs , how > > does the performance change with cycle count etc)? > > > > Can you also run rcutorture testing? Some of the scenarios like TREE03 > > stress hotplug. > > Also, why not just use the expedite api at the callsite that is slow > than blanket expediting everything between hotplug lock and unlock. > That is more specific fix than this fix which applies more broadly to > all operations. It appears the report you provided does provide the > culprit callsite.
Because hotplug is not a fast path; there is no expectation of performance here.
