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

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Date:   Mon, 13 Dec 2021 11:32:56 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>
Cc:     "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "urezki@...il.com" <urezki@...il.com>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>
Subject: Re: [PATCH] rcu: Make rcu_state.n_online_cpus updates atomic

On Mon, Dec 13, 2021 at 08:09:22AM +0000, Woodhouse, David wrote:
> On Mon, 2021-12-13 at 12:30 +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>
> 
> In my parallel-bringup series, the prepare stages are still being
> executed in series on the BSP, so I don't think this patch is needed
> yet. I'm not sure we'd ever end up with the prepare stages being done
> in parallel — the most I see us doing is onlining a single *batch* of
> CPUs at a time, much like bringup_nonboot_cpus() does.
> 
> But this patch certainly doesn't *hurt*.
> 
> Acked-by: David Woodhouse <dwmw@...zon.co.uk>

Queued for further review and testing.

To Frederic's point, this won't go to mainline unless it is actually
needed, but it will at least be pulled into a branch in -rcu marked with
a tag for future reference.

							Thanx, Paul