Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations

From: Shrikanth Hegde

Date: Mon Jan 12 2026 - 07:03:20 EST


Hi Vishal. Thanks for the patch.

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.camel@xxxxxxxxxxxxxxxxxx

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.