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

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Jan 13, 2026, at 7:19 AM, Uladzislau Rezki <urezki@gmail.com> wrote:
> 
> On Mon, Jan 12, 2026 at 05:36:24PM +0000, Joel Fernandes wrote:
>> 
>> 
>>>> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <urezki@gmail.com> 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.
>>>> 
>>> IMO, we can increase that threshold. 512/1024 is not a problem at all.
>>> But as Paul mentioned, we should consider scalability enhancement. From
>>> the other hand it is also probably worth to get into the state when we
>>> really see them :)
>> 
>> Instead of pegging to number of CPUs, perhaps the optimization should be dynamic? That is, default to it unless synchronize_rcu load is high, default to the sr_normal wake-up optimization. Of course carefully considering all corner cases, adequate testing and all that ;-)
>> 
> Honestly i do not see use cases when we are not up to speed to process
> all callbacks in time keeping in mind that it is blocking context call.
> 
> How many of them should be in flight(blocked contexts) to make it starve... :)
> According to my last evaluation it was ~64K.
> 
> Note i do not say that it should not be scaled.

But you did not test that on large system with 1000s of CPUs right? 

So the options I see are: either default to always using the optimization,
not just for less than 17 CPUs (what you are saying above). Or, do what I said
above (safer for system with 1000s of CPUs and less risky).

Let me know if I missed something.  Thanks.


> 
> --
> Uladzislau Rezki