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SubjectRe: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation
On Mon, Dec 13, 2021 at 02:14:07PM +0100, Frederic Weisbecker wrote:
> On Mon, Dec 13, 2021 at 11:28:45AM +0000, David Woodhouse wrote:
> > On Mon, 2021-12-13 at 12:22 +0100, Frederic Weisbecker wrote:
> > > I was about to ack the patch but, should we really add code that isn't going to
> > > be necessary before a long while?
> >
> > Yeah, I'm torn on that. In this case it's harmless enough and it makes
> > the code reentrant in its own right instead of relying on the fact that
> > the cpuhp code won't invoke it multiple times in parallel. So I think
> > that's reasonable defensive programming.
>
> The thing is that RCU code is already quite complicated. Are we even at least
> sure that we'll ever make CPU hotplug allow concurrent CPU onlining/offlining?
>
> This will require much more thoughts and a new hotplug concurrency
> infrastructure that we'll need to base RCU on. IMHO it's a bit early to handle
> that on hotplug individual callbacks.
>
> But anyway, let's see what Paul thinks about it...

We need to at least think through parallelizing the various RCU
CPU-hotplug notifiers. For one thing, it might turn out to be necessary
to parallelize those notifiers, perhaps on some non-x86 architecture or
on some non-Amazon .config. For another thing, doing so might suggest
some simplifications, as has happened in my ongoing rcu_barrier() work.

My thought is to pull in the patches and at the very least leave a
tag recording them for later possible use.

Thanx, Paul

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