Re: [PATCH] rcu-tasks: Use pr_debug() for callback contend/collapse messages

[Joel Fernandes <joelagnelf@nvidia.com>]

On Tue, Nov 10, 2026 at 09:35:51PM -0800, Song Liu wrote:
> Commit 5de907d24919 ("rcu-tasks: Disable callback contend/collapse
> messages by default") added the rcupdate.rcu_task_collapse_debug module
> parameter to gate the three pr_info() messages that report RCU Tasks
> switching between CPU-0 and per-CPU callback queuing.
> 
> The kernel already has a mechanism for debug-only messages that are off
> by default: pr_debug().  Use pr_debug() for these messages, and remove
> the rcu_task_collapse_debug module parameter, its documentation, and
> its boot-time announcement.
> 
> The messages stay off by default.  On CONFIG_DYNAMIC_DEBUG=y kernels,
> they can be enabled at boot with:
> 
>   dyndbg="file kernel/rcu/tasks.h +p"
> 
> or at runtime with:
> 
>   echo 'file kernel/rcu/tasks.h +p' > /proc/dynamic_debug/control
> 
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Song Liu <song@kernel.org>

Makes sense to me,

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

Will apply for 7.5.

thanks,

Joel Fernandes


> ---
>  Documentation/admin-guide/kernel-parameters.txt |  7 -------
>  kernel/rcu/tasks.h                              | 15 +++------------
>  2 files changed, 3 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 0b3c140babd9..176648941aec 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6369,13 +6369,6 @@ Kernel parameters
>  			period to instead use normal non-expedited
>  			grace-period processing.
>  
> -	rcupdate.rcu_task_collapse_debug= [KNL]
> -			Enable debugging prints that record when RCU Tasks
> -			and RCU Tasks Trace expand to per-CPU callback
> -			queuing and collapse back to CPU-0 queuing.
> -			This is default-disabled due to the fact that
> -			some workloads can make it quite noisy.
> -
>  	rcupdate.rcu_task_collapse_lim= [KNL]
>  			Set the maximum number of callbacks present
>  			at the beginning of a grace period that allows
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index fcac7361ec51..743f777b4c18 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -178,8 +178,6 @@ static int rcu_task_contend_lim __read_mostly = 100;
>  module_param(rcu_task_contend_lim, int, 0444);
>  static int rcu_task_collapse_lim __read_mostly = 10;
>  module_param(rcu_task_collapse_lim, int, 0444);
> -static bool rcu_task_collapse_debug __read_mostly;
> -module_param(rcu_task_collapse_debug, bool, 0644);
>  static int rcu_task_lazy_lim __read_mostly = 32;
>  module_param(rcu_task_lazy_lim, int, 0444);
>  
> @@ -392,8 +390,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
>  			WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
>  			WRITE_ONCE(rtp->percpu_dequeue_lim, rcu_task_cpu_ids);
>  			smp_store_release(&rtp->percpu_enqueue_lim, rcu_task_cpu_ids);
> -			if (data_race(rcu_task_collapse_debug))
> -				pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
> +			pr_debug("Switching %s to per-CPU callback queuing.\n", rtp->name);
>  		}
>  		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
>  	}
> @@ -514,9 +511,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
>  			smp_store_release(&rtp->percpu_enqueue_lim, 1);
>  			rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
>  			gpdone = false;
> -			if (data_race(rcu_task_collapse_debug))
> -				pr_info("Starting switch %s to CPU-0 callback queuing.\n",
> -					rtp->name);
> +			pr_debug("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
>  		}
>  		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
>  	}
> @@ -524,9 +519,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
>  		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
>  		if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
>  			WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
> -			if (data_race(rcu_task_collapse_debug))
> -				pr_info("Completing switch %s to CPU-0 callback queuing.\n",
> -					rtp->name);
> +			pr_debug("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
>  		}
>  		if (rtp->percpu_dequeue_lim == 1) {
>  			for (cpu = rtp->percpu_dequeue_lim; cpu < rcu_task_cpu_ids; cpu++) {
> @@ -711,8 +704,6 @@ static void __init rcu_tasks_bootup_oddness(void)
>  		pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc);
>  		rcu_task_stall_info_mult = rtsimc;
>  	}
> -	if (rcu_task_collapse_debug)
> -		pr_info("\tTasks-RCU callback contend/collapse debug enabled.\n");
>  #endif /* #ifdef CONFIG_TASKS_RCU */
>  #ifdef CONFIG_TASKS_RCU
>  	pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
> -- 
> 2.53.0-Meta
>