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

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Message-ID: <D5086AE0-7158-49C4-8511-47F99021B2AE@nvidia.com>
Date: Mon, 12 Jan 2026 17:36:24 +0000
From: Joel Fernandes <joelagnelf@...dia.com>
To: Uladzislau Rezki <urezki@...il.com>
CC: 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>,
	"paulmck@...nel.org" <paulmck@...nel.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>, Uladzislau Rezki
	<urezki@...il.com>
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug
 operations



> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <urezki@...il.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@...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.
>> 
> 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 ;-)

Thanks.


> 
> --
> Uladzislau Rezki