linux-kernel - [PATCH] printk: Add CONFIG_CONSOLE_LOGLEVEL

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Date:   Tue, 22 Jun 2021 15:33:50 +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] printk: Add CONFIG_CONSOLE_LOGLEVEL_PANIC

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 CONFIG_CONSOLE_LOGLEVEL_PANIC, which allows to choose how
verbose the kernel must be on oops/panic.

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>
---
 include/linux/printk.h |  4 ++--
 lib/Kconfig.debug      | 13 +++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index fe7eb2351610..5a65a719f917 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -76,8 +76,8 @@ static inline void console_silent(void)
 
 static inline void console_verbose(void)
 {
-	if (console_loglevel)
-		console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
+	if (console_loglevel && (CONFIG_CONSOLE_LOGLEVEL_PANIC > 0))
+		console_loglevel = CONFIG_CONSOLE_LOGLEVEL_PANIC;
 }
 
 /* strlen("ratelimit") + 1 */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 678c13967580..0c12cafd9d8b 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -61,6 +61,19 @@ config CONSOLE_LOGLEVEL_QUIET
 	  will be used as the loglevel. IOW passing "quiet" will be the
 	  equivalent of passing "loglevel=<CONSOLE_LOGLEVEL_QUIET>"
 
+config CONSOLE_LOGLEVEL_PANIC
+	int "panic console loglevel (1-15)"
+	range 0 15
+	default "15"
+	help
+	  loglevel to use in kernel panic or oopses.
+
+	  Usually in order to provide more debug information on console upon
+	  panic, one wants to see everything being printed (loglevel = 15).
+	  With an exception to setups with low baudrate on serial console,
+	  keeping this value high is a good choice.
+	  0 value is to keep the loglevel during panic/oops unchanged.
+
 config MESSAGE_LOGLEVEL_DEFAULT
 	int "Default message log level (1-7)"
 	range 1 7
-- 
2.31.1