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

Paul E. McKenney posted 2 patches 1 year ago
[PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state machine
Posted by Paul E. McKenney 1 year ago 
From: Frederic Weisbecker <frederic@kernel.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@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 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
Re: [PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state machine
Posted by Frederic Weisbecker 1 year ago 
Le Thu, Dec 12, 2024 at 10:42:13AM -0800, Paul E. McKenney a écrit :
> From: Frederic Weisbecker <frederic@kernel.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@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.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
>
Re: [PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state machine
Posted by Paul E. McKenney 1 year ago 
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@kernel.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@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.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
> >
Re: [PATCH rcu 1/2] rcu/nocb: Use switch/case on NOCB timer state machine
Posted by Uladzislau Rezki 1 year ago 
On Fri, Dec 13, 2024 at 03:29:16PM -0800, Paul E. McKenney wrote:
> 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@kernel.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@kernel.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.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!
> 
Thank you. Dropped both.

--
Uladzislau Rezki

Patchew 2.1-devel-b67e4c2

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