linux-kernel - Re: [PATCH v2 3/3] rcu/tree: Make FQS complaining about offline CPU more aggressive

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Date:   Thu, 30 Jul 2020 18:36:07 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 3/3] rcu/tree: Make FQS complaining about offline CPU
 more aggressive

On Thu, Jul 30, 2020 at 08:40:12PM -0400, Joel Fernandes (Google) wrote:
> Make FQS loop consider it an immediate failure if the case of an offline CPU
> reporting QS is detected, instead of a full second.
> 
> This is because rcu_report_dead() already reports quiescent states and
> updates ->qsmaskinitnext under node lock.
> 
> Light testing with TREE03 and hotplug shows no warnings.
> 
> Convert the warning as well to WARN_ON_ONCE() to reduce log spam.

I will give you a chance to upgrade the above on your V3.
And the comment below.

I do very much like the change to WARN_ON_ONCE(), by the way!

							Thanx, Paul

> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a621932cc385..39bdd744ba97 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1208,13 +1208,15 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>  		return 1;
>  	}
>  
> -	/* If waiting too long on an offline CPU, complain. */
> -	if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp)) &&
> -	    time_after(jiffies, rcu_state.gp_start + HZ)) {
> +	/*
> +	 * Complain if an offline CPU by RCU's books has not reported QS. Node
> +	 * lock is held ensuring offlining does not race here.
> +	 */
> +	if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
>  		bool onl;
>  		struct rcu_node *rnp1;
>  
> -		WARN_ON(1);  /* Offline CPUs are supposed to report QS! */
> +		WARN_ON_ONCE(1);  /* Offline CPUs are supposed to report QS! */
>  		pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
>  			__func__, rnp->grplo, rnp->grphi, rnp->level,
>  			(long)rnp->gp_seq, (long)rnp->completedqs);
> -- 
> 2.28.0.163.g6104cc2f0b6-goog
>