Subject: [PATCH 47/48] printk: Provide threadprintk boot argument
From: John Ogness <[email-masked]>
Date: Tue, 6 Feb 2024 14:19:34 +0000

For PREEMPT_RT, legacy console printing is performed in a dedicated
kthread. However, this behavior can also be interesting for other
preemption models as it minimizes the duration of printk() calls by
deferring all printing.

Provide a new boot argument "threadprintk" that will create the
dedicated kthread for legacy console printing for !PREEMPT_RT
systems.

The implementation is the same as "threadirqs" boot argument.

Users should be aware that if this option is enabled, the shutdown,
reboot, and panic messages probably will not be visible on the
legacy consoles.

Non-legacy consoles (NBCON) already have their own dedicated kernel
threads for printing and reliable shutdown, reboot, and panic
printing. This option really only applies to legacy consoles.

Users can view /proc/consoles to see if their console driver is
legacy or not. NBCON console drivers are shown with 'N'.

Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 Documentation/admin-guide/kernel-parameters.txt |   12 ++++++++++++
 kernel/printk/internal.h                        |    4 +++-
 kernel/printk/printk.c                          |   11 +++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)

Index: linux-6.8.2-rt10/Documentation/admin-guide/kernel-parameters.txt
===================================================================
--- linux-6.8.2-rt10.orig/Documentation/admin-guide/kernel-parameters.txt
+++ linux-6.8.2-rt10/Documentation/admin-guide/kernel-parameters.txt
@@ -6508,6 +6508,18 @@
 			Force threading of all interrupt handlers except those
 			marked explicitly IRQF_NO_THREAD.
 
+	threadprintk	[KNL]
+			Force threaded printing of all legacy consoles. Be
+			aware that with this option, the shutdown, reboot, and
+			panic messages may not be printed on the legacy
+			consoles. Also, earlycon/earlyprintk printing will be
+			delayed until a regular console or the kthread is
+			available.
+
+			Users can view /proc/consoles to see if their console
+			driver is legacy or not. Non-legacy (NBCON) console
+			drivers are already threaded and are shown with 'N'.
+
 	topology=	[S390]
 			Format: {off | on}
 			Specify if the kernel should make use of the cpu
Index: linux-6.8.2-rt10/kernel/printk/internal.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/internal.h
+++ linux-6.8.2-rt10/kernel/printk/internal.h
@@ -3,6 +3,7 @@
  * internal.h - printk internal definitions
  */
 #include <linux/console.h>
+#include <linux/jump_label.h>
 #include <linux/percpu.h>
 #include <linux/types.h>
 
@@ -24,7 +25,8 @@ int devkmsg_sysctl_set_loglvl(struct ctl
 #ifdef CONFIG_PREEMPT_RT
 # define force_printkthreads()		(true)
 #else
-# define force_printkthreads()		(false)
+DECLARE_STATIC_KEY_FALSE(force_printkthreads_key);
+# define force_printkthreads()		(static_branch_unlikely(&force_printkthreads_key))
 #endif
 
 #ifdef CONFIG_PRINTK
Index: linux-6.8.2-rt10/kernel/printk/printk.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/printk.c
+++ linux-6.8.2-rt10/kernel/printk/printk.c
@@ -195,6 +195,17 @@ static int __init control_devkmsg(char *
 }
 __setup("printk.devkmsg=", control_devkmsg);
 
+#if !defined(CONFIG_PREEMPT_RT)
+DEFINE_STATIC_KEY_FALSE(force_printkthreads_key);
+
+static int __init setup_forced_printkthreads(char *arg)
+{
+	static_branch_enable(&force_printkthreads_key);
+	return 0;
+}
+early_param("threadprintk", setup_forced_printkthreads);
+#endif
+
 char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit";
 #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
 int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,