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

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Message-ID: <0d069012-ab5f-4126-89b4-5f2c186b18ae@paulmck-laptop>
Date: Fri, 13 Dec 2024 15:29:16 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, urezki@...il.com
Subject: Re: [PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state
 machine

On Sat, Dec 14, 2024 at 12:08:42AM +0100, Frederic Weisbecker wrote:
> 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.

I will drop both on my next rebase.  Uladzislau, over to you!

							Thanx, Paul

> 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
> >