linux-kernel - Re: [PATCH 03/11] rcu/nocb: Invoke rcu_core() at the start of deoffloading

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Date:   Fri, 01 Oct 2021 18:50:04 +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 03/11] rcu/nocb: Invoke rcu_core() at the start of deoffloading

On 30/09/21 00:10, Frederic Weisbecker wrote:
> On PREEMPT_RT, if rcu_core() is preempted by the de-offloading process,
> some work, such as callbacks acceleration and invocation, may be left
> unattended due to the volatile checks on the offloaded state.
>
> In the worst case this work is postponed until the next rcu_pending()
> check that can take a jiffy to reach, which can be a problem in case
> of callbacks flooding.
>
> Solve that with invoking rcu_core() early in the de-offloading process.
> This way any work dismissed by an ongoing rcu_core() call fooled by
> a preempting deoffloading process will be caught up by a nearby future
> recall to rcu_core(), this time fully aware of the de-offloading state.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Valentin Schneider <valentin.schneider@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>

One comment/question below.

> @@ -990,6 +990,15 @@ static long rcu_nocb_rdp_deoffload(void *arg)
>        * will refuse to put anything into the bypass.
>        */
>       WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
> +	/*
> +	 * Start with invoking rcu_core() early. This way if the current thread
> +	 * happens to preempt an ongoing call to rcu_core() in the middle,
> +	 * leaving some work dismissed because rcu_core() still thinks the rdp is
> +	 * completely offloaded, we are guaranteed a nearby future instance of
> +	 * rcu_core() to catch up.
> +	 */
> +	rcu_segcblist_set_flags(cblist, SEGCBLIST_RCU_CORE);
> +	invoke_rcu_core();

I think your approach is a bit neater, but would there have been any issue
with keeping the setting of SEGCBLIST_RCU_CORE within
rcu_segcblist_offload() and bundling it with an invoke_rcu_core()?

>       ret = rdp_offload_toggle(rdp, false, flags);
>       swait_event_exclusive(rdp->nocb_state_wq,
>                             !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
> --
> 2.25.1