linux-kernel - Re: [PATCH] rcu: Make rcu_state.n_online

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Date:   Mon, 13 Dec 2021 14:06:33 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc:     paulmck@...nel.org, dwmw@...zon.co.uk, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, joel@...lfernandes.org,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        urezki@...il.com, boqun.feng@...il.com
Subject: Re: [PATCH] rcu: Make rcu_state.n_online_cpus updates atomic

On Mon, Dec 13, 2021 at 12:30:59PM +0530, Neeraj Upadhyay wrote:
> To support onlining multiple CPUs concurrently,
> change rcu_state.n_online_cpus updates to be atomic.
> Note, it's ok for rcu_blocking_is_gp() to do a
> atomic_read(&rcu_state.n_online_cpus), as the
> value of .n_online_cpus switches from 1->2, in
> rcutree_prepare_cpu(), which runs before the new
> CPU comes online. Similarly 2->1 transition happens
> from rcutree_dead_cpu(), which executes after the
> CPU is offlined, and runs on the last online CPU.
> 
> Signed-off-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>

That's a step but I can imagine much more complications to handle while looking
at rcutree_dead_cpu() VS rcutree_dead_cpu() (or other hotplug operations)
inside the same rnp calling rcu_boost_kthread_setaffinity() concurrently
or more generally rcu_boost_kthread_setaffinity() against concurrent onlining/offlining.

This function fetches the online CPUs to decide the affinity of boosting.
This can go quite wrong if CPUs can be concurrently onlined/offlined.

And I don't know how such problems are going to be solved in the future
but some new CPU hotplug concurrency primitives will be needed...

That's one more reason why I think it is a bit early to handle this wide problem...

Thanks.