On 1/13/2026 9:17 AM, Uladzislau Rezki wrote:
> On Tue, Jan 13, 2026 at 12:44:10PM +0000, Joel Fernandes wrote:
>>
>>
>>> On Jan 13, 2026, at 7:19 AM, Uladzislau Rezki <[email protected]> wrote:
>>>
>>> On Mon, Jan 12, 2026 at 05:36:24PM +0000, Joel Fernandes wrote:
>>>>
>>>>
>>>>>> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <[email protected]> wrote:
>>>>>
>>>>> 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.
>>>>>>
>>>>> 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?
>>
> No, no. I do not have access to such systems.
>
>>
>> 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).
>>
> You mean introduce threshold and count how many nodes are in queue?
Yes.
> To me it sounds not optimal and looks like a temporary solution.
Not more sub-optimal than the existing 16 CPU hard-coded solution I suppose.
>
> Long term wise, it is better to split it, i mean to scale.
But the scalable solution is already there: the !synchronize_rcu_normal path,
right? And splitting the list won't help this use case anyway.
>
> Do you know who can test it on ~1000 CPUs system? So we have some figures.
I don't have such systems either. The most I can go is ~200+ CPUs. Perhaps the
folks on this thread have such systems as they mentioned 1900+ CPU systems. They
should be happy to test.
>
> What i have is 256 CPUs system i can test on.
Same boat. ;-)
thanks,
- Joel
