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

[Song Liu <song@kernel.org>]

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>
---
 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