> On Jan 12, 2026, at 9:24 AM, Peter Zijlstra <[email protected]> wrote: > > 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.
Agreed, I was just wondering if it was incredibly slow or something. Looking forward to more justification from Vishal on usecase, - Joel >
