Subject: [PATCH 31/48] rcu: Mark emergency sections in rcu stalls
From: John Ogness <[email-masked]>
Date: Mon, 11 Sep 2023 15:53:39 +0000

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 <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/rcu/tree_exp.h   |    7 +++++++
 kernel/rcu/tree_stall.h |    9 +++++++++
 2 files changed, 16 insertions(+)

Index: linux-6.8.2-rt10/kernel/rcu/tree_exp.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/rcu/tree_exp.h
+++ linux-6.8.2-rt10/kernel/rcu/tree_exp.h
@@ -7,6 +7,7 @@
  * Authors: Paul E. McKenney <[email-masked]>
  */
 
+#include <linux/console.h>
 #include <linux/lockdep.h>
 
 static void rcu_exp_handler(void *unused);
@@ -636,6 +637,9 @@ static void synchronize_rcu_expedited_wa
 			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"));
@@ -689,6 +693,9 @@ static void synchronize_rcu_expedited_wa
 			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();
 	}
 }
Index: linux-6.8.2-rt10/kernel/rcu/tree_stall.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/rcu/tree_stall.h
+++ linux-6.8.2-rt10/kernel/rcu/tree_stall.h
@@ -7,6 +7,7 @@
  * Author: Paul E. McKenney <[email-masked]>
  */
 
+#include <linux/console.h>
 #include <linux/kvm_para.h>
 #include <linux/rcu_notifier.h>
 
@@ -604,6 +605,8 @@ static void print_other_cpu_stall(unsign
 	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 +658,8 @@ static void print_other_cpu_stall(unsign
 	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 +680,8 @@ static void print_cpu_stall(unsigned lon
 	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 +710,8 @@ static void print_cpu_stall(unsigned lon
 			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	/*