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? ;-)
Thanx, Paul
