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rcu-tasks: Disable callback contend/collapse messages by default

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New workloads can do large bursts of call_rcu_tasks() invocations in a
short time period, followed by a quiet time period long enough to drain
all of the callbacks, followed by another burst of call_rcu_tasks()
invocations.  This can cause RCU Tasks to switch back and forth between
queuing callbacks only on CPU 0 (during quiet periods) and on all CPUs
(during bursts).

Which is fine.  Except for the fact that each cycle from CPU-0-only to
all-CPUs queuing and back generates three console messages, one announcing
the shift to all-CPUs queuing, another announcing the start of the shift
back to CPU-0-only queuing, and the third announcing completion of this
shift after an RCU grace period.  And these console messages can overrun
console-log communications channels and obscure other console-message-based
debugging information.  And the only known use for these console messages
is debugging RCU Tasks itself.

This commit therefore adds a rcupdate.rcu_task_collapse_debug module
parameter that defaults to false (suppressing these console messages).
Those debugging or otherwise playing with RCU Tasks callback queuing
auto-adjustment can set this parameter to the value true.

[ paulmck: Apply Breno Leitao feedback. ]

Reported-by: Breno Leitao <leitao@debian.org>
Reported-by: David Dai <david.dai@linux.dev>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
This commit is contained in:
Paul E. McKenney 2026-08-11 11:44:00 -07:00
parent 852fbf7fc8
commit 5de907d249
2 changed files with 19 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/admin-guide/kernel-parameters.txt
View File

@ -6405,6 +6405,13 @@ 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

15
kernel/rcu/tasks.h
View File

@ -178,6 +178,8 @@ 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);
@ -390,7 +392,8 @@ 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);
pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
if (data_race(rcu_task_collapse_debug))
pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
@ -511,7 +514,9 @@ 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;
pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
if (data_race(rcu_task_collapse_debug))
pr_info("Starting switch %s to CPU-0 callback queuing.\n",
rtp->name);
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
}
@ -519,7 +524,9 @@ 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);
pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
if (data_race(rcu_task_collapse_debug))
pr_info("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++) {
@ -704,6 +711,8 @@ 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");