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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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@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.