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

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jan 13, 2026 at 08:23:29PM +0530, Shrikanth Hegde wrote:
> 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.
> 
Let me prepare something. I will come back.

--
Uladzislau Rezki