linux-kernel - Re: [PATCH 11/11] rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread

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Message-ID: <20210930153737.lljjay2cquph7cfv@linutronix.de>
Date:   Thu, 30 Sep 2021 17:37:37 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Valentin Schneider <valentin.schneider@....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 11/11] rcu/nocb: Don't invoke local rcu core on callback
 overload from nocb kthread

On 2021-09-30 00:10:12 [+0200], Frederic Weisbecker wrote:
> rcu_core() tries to ensure that its self-invocation in case of callbacks
> overload only happen in softirq/rcuc mode. Indeed it doesn't make sense
> to trigger local RCU core from nocb_cb kthread since it can execute
> on a CPU different from the target rdp. Also in case of overload, the
> nocb_cb kthread simply iterates a new loop of callbacks processing.
> 
> However the "offloaded" check that aims at preventing that is wrong.
- that?

> First of all that state is volatile and second: softirq/rcuc can
> execute while the target rdp is offloaded. As a result rcu_core() can

can what?

> Fix that with moving the rcu_core() self-invocation to rcu_core() itself,
> irrespective of the rdp offloaded state.

Sebastian