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

[Uladzislau Rezki]

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.

--
Uladzislau Rezki