[PATCH tip/core/rcu 10/12] rcu: Add sysrq rcu_node-dump capability

["Paul E. McKenney" <paulmck@linux.ibm.com>]

Life is hard if RCU manages to get stuck without triggering RCU CPU
stall warnings or triggering the rcu_check_gp_start_stall() checks
for failing to start a grace period.  This commit therefore adds a
boot-time-selectable sysrq key (commandeering "y") that allows manually
dumping Tree RCU state.  The new rcutree.sysrq_rcu kernel boot parameter
must be set for this sysrq to be available.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
 .../admin-guide/kernel-parameters.txt         |  5 ++++
 kernel/rcu/tree.c                             | 25 +++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b799bcf67d7b..b0dbb2fa401f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3721,6 +3721,11 @@
 			This wake_up() will be accompanied by a
 			WARN_ONCE() splat and an ftrace_dump().
 
+	rcutree.sysrq_rcu= [KNL]
+			Commandeer a sysrq key to dump out Tree RCU's
+			rcu_node tree with an eye towards determining
+			why a new grace period has not yet started.
+
 	rcuperf.gp_async= [KNL]
 			Measure performance of asynchronous
 			grace-period primitives such as call_rcu().
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 238150684fed..9ceb93f848cd 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -62,6 +62,7 @@
 #include <linux/suspend.h>
 #include <linux/ftrace.h>
 #include <linux/tick.h>
+#include <linux/sysrq.h>
 
 #include "tree.h"
 #include "rcu.h"
@@ -115,6 +116,9 @@ int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
 int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
 /* panic() on RCU Stall sysctl. */
 int sysctl_panic_on_rcu_stall __read_mostly;
+/* Commandeer a sysrq key to dump RCU's tree. */
+static bool sysrq_rcu;
+module_param(sysrq_rcu, bool, 0444);
 
 /*
  * The rcu_scheduler_active variable is initialized to the value
@@ -577,6 +581,27 @@ void show_rcu_gp_kthreads(void)
 }
 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
 
+/* Dump grace-period-request information due to commandeered sysrq. */
+static void sysrq_show_rcu(int key)
+{
+	show_rcu_gp_kthreads();
+}
+
+static struct sysrq_key_op sysrq_rcudump_op = {
+	.handler = sysrq_show_rcu,
+	.help_msg = "show-rcu(y)",
+	.action_msg = "Show RCU tree",
+	.enable_mask = SYSRQ_ENABLE_DUMP,
+};
+
+static int __init rcu_sysrq_init(void)
+{
+	if (sysrq_rcu)
+		return register_sysrq_key('y', &sysrq_rcudump_op);
+	return 0;
+}
+early_initcall(rcu_sysrq_init);
+
 /*
  * Send along grace-period-related data for rcutorture diagnostics.
  */
-- 
2.17.1