[PATCH rcu 0/2] No-CB changes for v6.14

[PATCH rcu 0/2] No-CB changes for v6.14

[Paul E. McKenney]

Hello!

This series contains NOCB updates:

1.	Use switch/case on NOCB timer state machine, courtesy of Frederic
	Weisbecker.

2.	Fix rcuog wake-up from offline softirq, courtesy of Frederic
	Weisbecker.

						Thanx, Paul

------------------------------------------------------------------------

 b/kernel/rcu/tree.h      |    1 +
 b/kernel/rcu/tree_nocb.h |   33 +++++++++++++++++++++++----------
 kernel/rcu/tree_nocb.h   |   14 ++++++++++++--
 3 files changed, 36 insertions(+), 12 deletions(-)

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

[Paul E. McKenney]

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

[Frederic Weisbecker]

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

[Paul E. McKenney]

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

[Uladzislau Rezki]

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

[PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

After a CPU has set itself offline and before it eventually calls
rcutree_report_cpu_dead(), there are still opportunities for callbacks
to be enqueued, for example from an IRQ. When that happens on NOCB, the
rcuog wake-up is deferred through an IPI to an online CPU in order not
to call into the scheduler and risk arming the RT-bandwidth after
hrtimers have been migrated out and disabled.

But performing a synchronized IPI from an IRQ is buggy as reported in
the following scenario:

	WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single
	Modules linked in: rcutorture torture
	CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1
	Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120
	RIP: 0010:smp_call_function_single
	<IRQ>
	swake_up_one_online
	__call_rcu_nocb_wake
	__call_rcu_common
	? rcu_torture_one_read
	call_timer_fn
	__run_timers
	run_timer_softirq
	handle_softirqs
	irq_exit_rcu
	? tick_handle_periodic
	sysvec_apic_timer_interrupt
	</IRQ>

The periodic tick must be shutdown when the CPU is offline, just like is
done for oneshot tick. This must be fixed but this is not enough:
softirqs can happen on any hardirq tail and reproduce the above scenario.

Fix this with introducing a special deferred rcuog wake up mode when the
CPU is offline. This deferred wake up doesn't arm any timer and simply
wait for rcu_report_cpu_dead() to be called in order to flush any
pending rcuog wake up.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@intel.com
Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.h      |  1 +
 kernel/rcu/tree_nocb.h | 14 ++++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a9a811d9d7a37..7ed060edd12b1 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -290,6 +290,7 @@ struct rcu_data {
 #define RCU_NOCB_WAKE_LAZY	2
 #define RCU_NOCB_WAKE		3
 #define RCU_NOCB_WAKE_FORCE	4
+#define RCU_NOCB_WAKE_OFFLINE   5
 
 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
 					/* For jiffies_till_first_fqs and */
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0923d60c5a338..78841346e1c13 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -295,6 +295,8 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 	case RCU_NOCB_WAKE_FORCE:
 		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
 			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
+		fallthrough;
+	case RCU_NOCB_WAKE_OFFLINE:
 		if (rdp_gp->nocb_defer_wakeup < waketype)
 			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
 		break;
@@ -562,8 +564,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	lazy_len = READ_ONCE(rdp->lazy_len);
 	if (was_alldone) {
 		rdp->qlen_last_fqs_check = len;
-		// Only lazy CBs in bypass list
-		if (lazy_len && bypass_len == lazy_len) {
+		if (cpu_is_offline(rdp->cpu)) {
+			/*
+			 * Offline CPUs can't call swake_up_one_online() from IRQs. Rely
+			 * on the final deferred wake-up rcutree_report_cpu_dead()
+			 */
+			rcu_nocb_unlock(rdp);
+			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_OFFLINE,
+					   TPS("WakeEmptyIsDeferredOffline"));
+		} else if (lazy_len && bypass_len == lazy_len) {
+			// Only lazy CBs in bypass list
 			rcu_nocb_unlock(rdp);
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
 					   TPS("WakeLazy"));
-- 
2.40.1

Re: [PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq

[Frederic Weisbecker]

Le Thu, Dec 12, 2024 at 10:42:14AM -0800, Paul E. McKenney a écrit :
> From: Frederic Weisbecker <frederic@kernel.org>
> 
> After a CPU has set itself offline and before it eventually calls
> rcutree_report_cpu_dead(), there are still opportunities for callbacks
> to be enqueued, for example from an IRQ. When that happens on NOCB, the
> rcuog wake-up is deferred through an IPI to an online CPU in order not
> to call into the scheduler and risk arming the RT-bandwidth after
> hrtimers have been migrated out and disabled.
> 
> But performing a synchronized IPI from an IRQ is buggy as reported in
> the following scenario:
> 
> 	WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single
> 	Modules linked in: rcutorture torture
> 	CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1
> 	Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120
> 	RIP: 0010:smp_call_function_single
> 	<IRQ>
> 	swake_up_one_online
> 	__call_rcu_nocb_wake
> 	__call_rcu_common
> 	? rcu_torture_one_read
> 	call_timer_fn
> 	__run_timers
> 	run_timer_softirq
> 	handle_softirqs
> 	irq_exit_rcu
> 	? tick_handle_periodic
> 	sysvec_apic_timer_interrupt
> 	</IRQ>
> 
> The periodic tick must be shutdown when the CPU is offline, just like is
> done for oneshot tick. This must be fixed but this is not enough:
> softirqs can happen on any hardirq tail and reproduce the above scenario.
> 
> Fix this with introducing a special deferred rcuog wake up mode when the
> CPU is offline. This deferred wake up doesn't arm any timer and simply
> wait for rcu_report_cpu_dead() to be called in order to flush any
> pending rcuog wake up.
> 
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@intel.com
> Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

You can drop this patch, it has been replaced with another version upstream.

Thanks!

Re: [PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq

[Uladzislau Rezki]

On Sat, Dec 14, 2024 at 12:07:25AM +0100, Frederic Weisbecker wrote:
> Le Thu, Dec 12, 2024 at 10:42:14AM -0800, Paul E. McKenney a écrit :
> > From: Frederic Weisbecker <frederic@kernel.org>
> > 
> > After a CPU has set itself offline and before it eventually calls
> > rcutree_report_cpu_dead(), there are still opportunities for callbacks
> > to be enqueued, for example from an IRQ. When that happens on NOCB, the
> > rcuog wake-up is deferred through an IPI to an online CPU in order not
> > to call into the scheduler and risk arming the RT-bandwidth after
> > hrtimers have been migrated out and disabled.
> > 
> > But performing a synchronized IPI from an IRQ is buggy as reported in
> > the following scenario:
> > 
> > 	WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single
> > 	Modules linked in: rcutorture torture
> > 	CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1
> > 	Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120
> > 	RIP: 0010:smp_call_function_single
> > 	<IRQ>
> > 	swake_up_one_online
> > 	__call_rcu_nocb_wake
> > 	__call_rcu_common
> > 	? rcu_torture_one_read
> > 	call_timer_fn
> > 	__run_timers
> > 	run_timer_softirq
> > 	handle_softirqs
> > 	irq_exit_rcu
> > 	? tick_handle_periodic
> > 	sysvec_apic_timer_interrupt
> > 	</IRQ>
> > 
> > The periodic tick must be shutdown when the CPU is offline, just like is
> > done for oneshot tick. This must be fixed but this is not enough:
> > softirqs can happen on any hardirq tail and reproduce the above scenario.
> > 
> > Fix this with introducing a special deferred rcuog wake up mode when the
> > CPU is offline. This deferred wake up doesn't arm any timer and simply
> > wait for rcu_report_cpu_dead() to be called in order to flush any
> > pending rcuog wake up.
> > 
> > Reported-by: kernel test robot <oliver.sang@intel.com>
> > Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@intel.com
> > Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> You can drop this patch, it has been replaced with another version upstream.
> 
Dropped. Thank you!

--
Uladzislau Rezki