linux-kernel - Re: [PATCH] cpuhp: Expedite synchronize

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Message-ID: <3525b8af-7e14-44e4-a909-ad46a0764d94@nvidia.com>
Date: Mon, 12 Jan 2026 17:24:40 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: paulmck@...nel.org
Cc: Uladzislau Rezki <urezki@...il.com>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>,
 Vishal Chourasia <vishalc@...ux.ibm.com>,
 "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "frederic@...nel.org" <frederic@...nel.org>,
 "neeraj.upadhyay@...nel.org" <neeraj.upadhyay@...nel.org>,
 "josh@...htriplett.org" <josh@...htriplett.org>,
 "boqun.feng@...il.com" <boqun.feng@...il.com>,
 "rostedt@...dmis.org" <rostedt@...dmis.org>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "peterz@...radead.org" <peterz@...radead.org>,
 "srikar@...ux.ibm.com" <srikar@...ux.ibm.com>
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug
 operations



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@...il.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?

thanks,

 - Joel