linux-kernel - [PATCH v3 2/2] printk: Add printk.console_no_auto

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Message-Id: <20210727130635.675184-3-dima@arista.com>
Date:   Tue, 27 Jul 2021 14:06:35 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH v3 2/2] printk: Add printk.console_no_auto_verbose boot parameter

console_verbose() increases console loglevel to CONSOLE_LOGLEVEL_MOTORMOUTH,
which provides more information to debug a panic/oops.

Unfortunately, in Arista we maintain some DUTs (Device Under Test) that
are configured to have 9600 baud rate. While verbose console messages
have their value to post-analyze crashes, on such setup they:
- may prevent panic/oops messages being printed
- take too long to flush on console resulting in watchdog reboot

In all our setups we use kdump which saves dmesg buffer after panic,
so in reality those extra messages on console provide no additional value,
but rather add risk of not getting to __crash_kexec().

Provide printk.console_no_auto_verbose boot parameter, which allows
to switch off printk being verbose on oops/panic/lockdep.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: John Ogness <john.ogness@...utronix.de>
Cc: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Dmitry Safonov <dima@...sta.com>
Suggested-by: Petr Mladek <pmladek@...e.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
 include/linux/printk.h                          |  6 +-----
 kernel/printk/printk.c                          | 12 ++++++++++++
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bdb22006f713..a4dd5814a83a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4167,6 +4167,15 @@
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
 			default: disabled
 
+	printk.console_no_auto_verbose=
+			Disable console loglevel raise on oops, panic
+			or lockdep-detected issues (only if lock debug is on).
+			With an exception to setups with low baudrate on
+			serial console, keeping this 0 is a good choice
+			in order to provide more debug information.
+			Format: <bool>
+			default: 0 (auto_verbose is enabled)
+
 	printk.devkmsg={on,off,ratelimit}
 			Control writing to /dev/kmsg.
 			on - unlimited logging to /dev/kmsg from userspace
diff --git a/include/linux/printk.h b/include/linux/printk.h
index a63f468a8239..0484274158d2 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -70,11 +70,7 @@ extern int console_printk[];
 #define minimum_console_loglevel (console_printk[2])
 #define default_console_loglevel (console_printk[3])
 
-static inline void console_verbose(void)
-{
-	if (console_loglevel)
-		console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
-}
+extern void console_verbose(void);
 
 /* strlen("ratelimit") + 1 */
 #define DEVKMSG_STR_MAX_SIZE 10
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 142a58d124d9..a6b94c3c5ac5 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2404,6 +2404,18 @@ module_param_named(console_suspend, console_suspend_enabled,
 MODULE_PARM_DESC(console_suspend, "suspend console during suspend"
 	" and hibernate operations");
 
+static bool printk_console_no_auto_verbose;
+
+void console_verbose(void)
+{
+	if (console_loglevel && !printk_console_no_auto_verbose)
+		console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
+}
+EXPORT_SYMBOL_GPL(console_verbose);
+
+module_param_named(console_no_auto_verbose, printk_console_no_auto_verbose, bool, 0644);
+MODULE_PARM_DESC(console_no_auto_verbose, "Disable console loglevel raise to highest on oops/panic/etc");
+
 /**
  * suspend_console - suspend the console subsystem
  *
-- 
2.32.0