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

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>

0 files changed, 0 insertions, 0 deletions