linux-kernel - Re: [PATCH 05/11] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe

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Date:   Fri, 01 Oct 2021 18:50:39 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 05/11] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe

On 30/09/21 00:10, Frederic Weisbecker wrote:
> When callbacks are offloaded, the NOCB kthreads handle the callbacks
> progression on behalf of rcu_core().
>
> However during the (de-)offloading process, the kthread may not be
> entirely up to the task. As a result some callbacks grace period
> sequence number may remain stale for a while because rcu_core() won't
> take care of them either.
>

But that should be taken care of at the tail end of the (de)offloading
process, either by rcu_core() or by the NOCB kthreads, no?

Or is it e.g. in the case of offloading, we want to make sure an rcu_core()
invocation runs callback acceleration because even if the NOCB GP/CB
kthreads are being set up, we're not guaranteed is going to do that
straight away?

IIUC it would be a similar case for deoffload when we stash the NOCB GP/CB
kthreads and get rcu_core() running concurrently.

> Fix this with forcing callbacks acceleration from rcu_core() as long
> as the offloading process isn't complete.
>