linux-kernel - Re: [RFC PATCH 11/12] rcu: Locally accelerate callbacks as long as offloading isn't complete

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Date:   Mon, 21 Sep 2020 17:24:52 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC PATCH 11/12] rcu: Locally accelerate callbacks as long as
 offloading isn't complete

On Mon, Sep 21, 2020 at 02:43:50PM +0200, Frederic Weisbecker wrote:
> I have no idea what I'm doing but doing that looks necessary to me.

Now -there- is a commit log that inspires confidence!!!  ;-)

> Inspired-by: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 928907e9ba94..8cef5ea8d1f0 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2619,7 +2619,6 @@ static __latent_entropy void rcu_core(void)
>  	unsigned long flags;
>  	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
>  	struct rcu_node *rnp = rdp->mynode;
> -	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
>  	const bool do_batch = !rcu_segcblist_completely_offloaded(&rdp->cblist);
>  
>  	if (cpu_is_offline(smp_processor_id()))
> @@ -2640,11 +2639,11 @@ static __latent_entropy void rcu_core(void)
>  
>  	/* No grace period and unregistered callbacks? */
>  	if (!rcu_gp_in_progress() &&
> -	    rcu_segcblist_is_enabled(&rdp->cblist) && !offloaded) {
> -		local_irq_save(flags);
> +	    rcu_segcblist_is_enabled(&rdp->cblist) && do_batch) {
> +		rcu_nocb_lock_irqsave(rdp, flags);
>  		if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
>  			rcu_accelerate_cbs_unlocked(rnp, rdp);

It might also be that rcu_advance_cbs() is needed somewhere.  I will
need to look carefully to work out whether either of them need to be
invoked at this particular point in the code.

							Thanx, Paul

> -		local_irq_restore(flags);
> +		rcu_nocb_unlock_irqrestore(rdp, flags);
>  	}
>  
>  	rcu_check_gp_start_stall(rnp, rdp, rcu_jiffies_till_stall_check());
> -- 
> 2.28.0
>