Date:   Mon, 13 Dec 2021 11:00:05 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     David Woodhouse <dwmw2@infradead.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, urezki@gmail.com,
        boqun.feng@gmail.com
Subject: Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation
Message-ID: <20211213190005.GP641268@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20211211170139.27711-1-quic_neeraju@quicinc.com>
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 <601ecb12-ae2e-9608-7127-c2cddc8038a6@quicinc.com>
 <20211213112246.GA782195@lothringen>
 <984a63d4c11d04e2ee8a83fc9c61006413bf209e.camel@infradead.org>
 <20211213131407.GD782195@lothringen>
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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