linux-kernel - Re: [PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state machine

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Message-ID: <Z1y-egJlaH9IWKsV@pavilion.home>
Date: Sat, 14 Dec 2024 00:08:42 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state
 machine

Le Thu, Dec 12, 2024 at 10:42:13AM -0800, Paul E. McKenney a écrit :
> From: Frederic Weisbecker <frederic@...nel.org>
> 
> It's more convenient to benefit from the fallthrough feature of
> switch / case to handle the timer state machine. Also a new state is
> about to be added that will take advantage of it.
> 
> No intended functional change.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

Please drop this one too. It introduced a subtle (yet desired) behaviour
change as Boqun noted. I'll resend a better version.

Thanks!

> ---
>  kernel/rcu/tree_nocb.h | 33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 2605dd234a13c..0923d60c5a338 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -271,22 +271,35 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
>  
>  	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
>  
> -	/*
> -	 * Bypass wakeup overrides previous deferments. In case of
> -	 * callback storms, no need to wake up too early.
> -	 */
> -	if (waketype == RCU_NOCB_WAKE_LAZY &&
> -	    rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
> -		mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
> -		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> -	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
> +	switch (waketype) {
> +	case RCU_NOCB_WAKE_BYPASS:
> +		/*
> +		 * Bypass wakeup overrides previous deferments. In case of
> +		 * callback storms, no need to wake up too early.
> +		 */
>  		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
>  		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> -	} else {
> +		break;
> +	case RCU_NOCB_WAKE_LAZY:
> +		if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
> +			mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
> +			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> +		}
> +		/*
> +		 * If the timer is already armed, a non-lazy enqueue may have happened
> +		 * in-between. Don't delay it and fall-through.
> +		 */
> +		break;
> +	case RCU_NOCB_WAKE:
> +		fallthrough;
> +	case RCU_NOCB_WAKE_FORCE:
>  		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
>  			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
>  		if (rdp_gp->nocb_defer_wakeup < waketype)
>  			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
>  	}
>  
>  	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
> -- 
> 2.40.1
>