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

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Jan 12, 2026, at 7:01 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Mon, Jan 12, 2026 at 05:24:40PM -0500, Joel Fernandes wrote:
>> 
>> 
>>> On 1/12/2026 11:48 AM, 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 <urezki@gmail.com> 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@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?  ;-)
>>> 
>> Yes I think you are considering the correct one, the concern you have is
>> regarding large number of wake ups initiated from the GP thread, correct?
>> 
>> I was suggesting on the thread, a more dynamic approach where using
>> synchronize_rcu_normal() until it gets overloaded with requests. One approach
>> might be to measure the length of the rcu_state.srs_next to detect an overload
>> condition, similar to qhimark? Or perhaps qhimark itself can be used. And under
>> lightly loaded conditions, default to synchronize_rcu_normal() without checking
>> for the 16 CPU count.
>> 
>> Thoughts?
> 
> Or maintain multiple lists.  Systems with 1000+ CPUs can be a bit
> unforgiving of pretty much any form of contention.

Makes sense. We could also just have a single list but a much smaller threshold for switching synchronize_rcu_normal off.

That would address the conveyor belt pattern Vishal expressed.

thanks,

 - Joel 



> 
>                            Thanx, Paul