[PATCH printk v5 29/30] rcu: Mark emergency sections in rcu stalls

[John Ogness <john.ogness@xxxxxxxxxxxxx>]

Mark emergency sections wherever multiple lines of
rcu stall information are generated. In an emergency
section the CPU will not perform console output for the
printk() calls. Instead, a flushing of the console
output is triggered when exiting the emergency section.
This allows the full message block to be stored as
quickly as possible in the ringbuffer.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 kernel/rcu/tree_exp.h   |  9 +++++++++
 kernel/rcu/tree_stall.h | 11 +++++++++++
 2 files changed, 20 insertions(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 6b83537480b1..bc1d8733c08f 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -7,6 +7,7 @@
  * Authors: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
  */
 
+#include <linux/console.h>
 #include <linux/lockdep.h>
 
 static void rcu_exp_handler(void *unused);
@@ -571,6 +572,9 @@ static void synchronize_rcu_expedited_wait(void)
 			return;
 		if (rcu_stall_is_suppressed())
 			continue;
+
+		nbcon_cpu_emergency_enter();
+
 		j = jiffies;
 		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
 		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
@@ -612,6 +616,7 @@ static void synchronize_rcu_expedited_wait(void)
 			}
 			pr_cont("\n");
 		}
+		nbcon_cpu_emergency_flush();
 		rcu_for_each_leaf_node(rnp) {
 			for_each_leaf_node_possible_cpu(rnp, cpu) {
 				mask = leaf_node_cpu_bit(rnp, cpu);
@@ -624,6 +629,9 @@ static void synchronize_rcu_expedited_wait(void)
 			rcu_exp_print_detail_task_stall_rnp(rnp);
 		}
 		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
+
+		nbcon_cpu_emergency_exit();
+
 		panic_on_rcu_stall();
 	}
 }
@@ -792,6 +800,7 @@ static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
 		 */
 		touch_nmi_watchdog();
 		sched_show_task(t);
+		nbcon_cpu_emergency_flush();
 	}
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5d666428546b..1ca0826545c1 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -7,6 +7,7 @@
  * Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
  */
 
+#include <linux/console.h>
 #include <linux/kvm_para.h>
 #include <linux/rcu_notifier.h>
 
@@ -260,6 +261,7 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 		 */
 		touch_nmi_watchdog();
 		sched_show_task(t);
+		nbcon_cpu_emergency_flush();
 	}
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
@@ -522,6 +524,7 @@ static void print_cpu_stall_info(int cpu)
 	       falsepositive ? " (false positive?)" : "");
 
 	print_cpu_stat_info(cpu);
+	nbcon_cpu_emergency_flush();
 }
 
 /* Complain about starvation of grace-period kthread.  */
@@ -604,6 +607,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 	if (rcu_stall_is_suppressed())
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	/*
 	 * OK, time to rat on our buddy...
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
@@ -655,6 +660,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 	rcu_check_gp_kthread_expired_fqs_timer();
 	rcu_check_gp_kthread_starvation();
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	rcu_force_quiescent_state();  /* Kick them all. */
@@ -675,6 +682,8 @@ static void print_cpu_stall(unsigned long gps)
 	if (rcu_stall_is_suppressed())
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	/*
 	 * OK, time to rat on ourselves...
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
@@ -703,6 +712,8 @@ static void print_cpu_stall(unsigned long gps)
 			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	/*
-- 
2.39.2