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

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Message-ID: <a746ab66-79ac-4cdc-91e8-07814e172a8b@linux.ibm.com>
Date: Tue, 13 Jan 2026 20:23:29 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Joel Fernandes <joelagnelf@...dia.com>,
        Uladzislau Rezki <urezki@...il.com>
Cc: 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>
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug
 operations

Hi.

On 1/13/26 8:02 PM, Joel Fernandes wrote:
> 
> 

>>>>>>> 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.
> 

Do you have a patch to try out? We can test it on these systems.


Note: Might take a while to test it, as those systems are bit tricky to
get.

>>
>> What i have is 256 CPUs system i can test on.
> Same boat. ;-)
> 
> thanks,
> 
>   - Joel
>