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

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Message-ID: <471d6615f245168d4c6c96c7ac1ccabf56b75945.camel@amazon.co.uk>
Date:   Mon, 13 Dec 2021 08:09:22 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>
CC:     "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, 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>





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