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

[Joel Fernandes]

> On Jan 12, 2026, at 7:57 AM, Uladzislau Rezki <urezki@xxxxxxxxx> 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.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.
>> 
> 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.

thanks,

 - Joel


> 
> --
> Uladzislau Rezki