[PATCH] rcu-tasks: Dump rtpcp->lazy_timer status in show_rcu_tasks_generic_gp_kthread()

[PATCH] rcu-tasks: Dump rtpcp->lazy_timer status in show_rcu_tasks_generic_gp_kthread()

[Zqiang]

Add 'P' flag to the diagnostic line to indicate whether any per-cpu
rtpcp's lazy_timer is pending.  this helps diagnose stalls where
rcu-task callbacks are queued but the kthread stay sleep because
the lazy_timer has not yet fired and no grace period has started.

The output is as follows:

[   31.319540][   T77] call_rcu_tasks() has failed boot-time tests.
[   31.320205][   T77] rcu_tasks: RTGS_WAIT_CBS(11) since 7518 g:4 i:0 kCuUP l:150000

Signed-off-by: Zqiang <qiang.zhang@linux.dev>
---
 kernel/rcu/tasks.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 6720dc25cd39..a146d1d98c22 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -723,6 +723,7 @@ static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
 	bool havecbs = false;
 	bool haveurgent = false;
 	bool haveurgentcbs = false;
+	bool havependtimer = false;
 
 	for_each_possible_cpu(cpu) {
 		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
@@ -733,10 +734,12 @@ static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
 			haveurgent = true;
 		if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)) && data_race(rtpcp->urgent_gp))
 			haveurgentcbs = true;
-		if (havecbs && haveurgent && haveurgentcbs)
+		if (data_race(timer_pending(&rtpcp->lazy_timer)))
+			havependtimer = true;
+		if (havecbs && haveurgent && haveurgentcbs && havependtimer)
 			break;
 	}
-	pr_info("%s: %s(%d) since %lu g:%lu i:%lu %c%c%c%c l:%lu %s\n",
+	pr_info("%s: %s(%d) since %lu g:%lu i:%lu %c%c%c%c%c l:%lu %s\n",
 		rtp->kname,
 		tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
 		jiffies - data_race(rtp->gp_jiffies),
@@ -746,6 +749,7 @@ static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
 		".C"[havecbs],
 		".u"[haveurgent],
 		".U"[haveurgentcbs],
+		".P"[havependtimer],
 		rtp->lazy_jiffies,
 		s);
 }
-- 
2.17.1

Re: [PATCH] rcu-tasks: Dump rtpcp->lazy_timer status in show_rcu_tasks_generic_gp_kthread()

[Paul E. McKenney]

On Thu, Jul 16, 2026 at 05:57:54PM +0800, Zqiang wrote:
> Add 'P' flag to the diagnostic line to indicate whether any per-cpu
> rtpcp's lazy_timer is pending.  this helps diagnose stalls where
> rcu-task callbacks are queued but the kthread stay sleep because
> the lazy_timer has not yet fired and no grace period has started.
> 
> The output is as follows:
> 
> [   31.319540][   T77] call_rcu_tasks() has failed boot-time tests.
> [   31.320205][   T77] rcu_tasks: RTGS_WAIT_CBS(11) since 7518 g:4 i:0 kCuUP l:150000
> 
> Signed-off-by: Zqiang <qiang.zhang@linux.dev>

Nice!  Queued for review and testing, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/tasks.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 6720dc25cd39..a146d1d98c22 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -723,6 +723,7 @@ static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
>  	bool havecbs = false;
>  	bool haveurgent = false;
>  	bool haveurgentcbs = false;
> +	bool havependtimer = false;
>  
>  	for_each_possible_cpu(cpu) {
>  		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> @@ -733,10 +734,12 @@ static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
>  			haveurgent = true;
>  		if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)) && data_race(rtpcp->urgent_gp))
>  			haveurgentcbs = true;
> -		if (havecbs && haveurgent && haveurgentcbs)
> +		if (data_race(timer_pending(&rtpcp->lazy_timer)))
> +			havependtimer = true;
> +		if (havecbs && haveurgent && haveurgentcbs && havependtimer)
>  			break;
>  	}
> -	pr_info("%s: %s(%d) since %lu g:%lu i:%lu %c%c%c%c l:%lu %s\n",
> +	pr_info("%s: %s(%d) since %lu g:%lu i:%lu %c%c%c%c%c l:%lu %s\n",
>  		rtp->kname,
>  		tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
>  		jiffies - data_race(rtp->gp_jiffies),
> @@ -746,6 +749,7 @@ static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
>  		".C"[havecbs],
>  		".u"[haveurgent],
>  		".U"[haveurgentcbs],
> +		".P"[havependtimer],
>  		rtp->lazy_jiffies,
>  		s);
>  }
> -- 
> 2.17.1
> 
>