On Mon, Jan 12, 2026 at 08:48:42AM -0800, Paul E. McKenney wrote: > On Mon, Jan 12, 2026 at 04:09:49PM +0000, Joel Fernandes wrote: > > > > > > > On Jan 12, 2026, at 7:57 AM, Uladzislau Rezki <[email protected]> wrote: > > > > > > 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. > > > > Another way to make it in-kernel would be to make the RCU normal wake from > > GP optimization enabled for > 16 CPUs by default. > > > > I was considering this, but I did not bring it up because I did not know > > that there are large systems that might benefit from it until now. > > This would require increasing the scalability of this optimization, > right? Or am I thinking of the wrong optimization? ;-) > I tested this before. I noticed that after 64K of simultaneous synchronize_rcu() calls the scalability is required. Everything less was faster with a new approach.
I can retest. Should i? :) -- Uladzsislau Rezki
