rcu-tasks: Print pre-stall-warning informational messages

RCU-tasks stall-warning messages are printed after the grace period is ten
minutes old.  Unfortunately, most of us will have rebooted the system in
response to an apparently-hung command long before the ten minutes is up,
and will thus see what looks to be a silent hang.

This commit therefore adds pr_info() messages that are printed earlier.
These should avoid being classified as errors, but should give impatient
users a hint.  These are controlled by new rcupdate.rcu_task_stall_info
and rcupdate.rcu_task_stall_info_mult kernel-boot parameters.  The former
defines the initial delay in jiffies (defaulting to 10 seconds) and the
latter defines the multiplier (defaulting to 3).  Thus, by default, the
first message will appear 10 seconds into the RCU-tasks grace period,
the second 40 seconds in, and the third 160 seconds in.  There would be
a fourth at 640 seconds in, but the stall warning message appears 600
seconds in, and once a stall warning is printed for a given grace period,
no further informational messages are printed.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 27 insertions, 3 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3f1cc5e317ed..babc701d4864 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4955,10 +4955,34 @@
 			number avoids disturbing real-time workloads,
 			but lengthens grace periods.
 
+	rcupdate.rcu_task_stall_info= [KNL]
+			Set initial timeout in jiffies for RCU task stall
+			informational messages, which give some indication
+			of the problem for those not patient enough to
+			wait for ten minutes.  Informational messages are
+			only printed prior to the stall-warning message
+			for a given grace period. Disable with a value
+			less than or equal to zero.  Defaults to ten
+			seconds.  A change in value does not take effect
+			until the beginning of the next grace period.
+
+	rcupdate.rcu_task_stall_info_mult= [KNL]
+			Multiplier for time interval between successive
+			RCU task stall informational messages for a given
+			RCU tasks grace period.  This value is clamped
+			to one through ten, inclusive.	It defaults to
+			the value three, so that the first informational
+			message is printed 10 seconds into the grace
+			period, the second at 40 seconds, the third at
+			160 seconds, and then the stall warning at 600
+			seconds would prevent a fourth at 640 seconds.
+
 	rcupdate.rcu_task_stall_timeout= [KNL]
-			Set timeout in jiffies for RCU task stall warning
-			messages.  Disable with a value less than or equal
-			to zero.
+			Set timeout in jiffies for RCU task stall
+			warning messages.  Disable with a value less
+			than or equal to zero.	Defaults to ten minutes.
+			A change in value does not take effect until
+			the beginning of the next grace period.
 
 	rcupdate.rcu_self_test= [KNL]
 			Run the RCU early boot self tests