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Message-ID: <1c368ffff850a00b81d4b67c15c38b826251e27d.1763492585.git.chris@chrisdown.name>
Date: Wed, 19 Nov 2025 03:07:22 +0800
From: Chris Down <chris@...isdown.name>
To: Petr Mladek <pmladek@...e.com>
Cc: linux-kernel@...r.kernel.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
John Ogness <john.ogness@...utronix.de>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Tony Lindgren <tony.lindgren@...ux.intel.com>, kernel-team@...com
Subject: [PATCH v7 06/13] printk: Support toggling per-console loglevel via
syslog() and cmdline
A new module parameter (ignore_per_console_loglevel) is added, which can
be set via the kernel command line or at runtime through
/sys/module/printk/parameters/ignore_per_console_loglevel. When set, the
per-console loglevels are ignored, and the global console loglevel
(console_loglevel) is used for all consoles.
During sysrq, we temporarily disable per-console loglevels to ensure all
requisite messages are printed to the console. This is necessary because
sysrq is often used in dire circumstances where access to
/sys/class/console may not be trivially possible.
Additionally, the syslog actions SYSLOG_ACTION_CONSOLE_ON and
SYSLOG_ACTION_CONSOLE_OFF are augmented to save and restore the state of
ignore_per_console_loglevel. This allows administrators to enable or
disable per-console loglevels dynamically using the syslog() system
call, as supported in userspace by things like dmesg.
This is useful when debugging issues with message emission, or when
needing to quickly break glass and revert to global loglevel only.
Signed-off-by: Chris Down <chris@...isdown.name>
---
Documentation/admin-guide/index.rst | 1 +
.../admin-guide/kernel-parameters.txt | 7 ++
.../admin-guide/per-console-loglevel.rst | 71 +++++++++++++++++++
MAINTAINERS | 1 +
drivers/tty/sysrq.c | 6 ++
include/linux/printk.h | 2 +
kernel/printk/printk.c | 29 +++++++-
7 files changed, 114 insertions(+), 3 deletions(-)
create mode 100644 Documentation/admin-guide/per-console-loglevel.rst
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 259d79fbeb94..5f0ae9c4017b 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -155,6 +155,7 @@ How to configure your hardware within your Linux system.
media/index
nvme-multipath
parport
+ per-console-loglevel
pnp
rapidio
rtc
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3edc5ce0e2a3..7b62a99489b8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2177,6 +2177,13 @@
could change it dynamically, usually by
/sys/module/printk/parameters/ignore_loglevel.
+ ignore_per_console_loglevel [KNL,EARLY]
+ Ignore all per-console loglevel settings
+ and use only the global console_loglevel for all
+ consoles. This can also be set at runtime via
+ /sys/module/printk/parameters/ignore_per_console_loglevel.
+ See Documentation/admin-guide/per-console-loglevel.rst.
+
ignore_rlimit_data
Ignore RLIMIT_DATA setting for data mappings,
print warning at first misuse. Can be changed via
diff --git a/Documentation/admin-guide/per-console-loglevel.rst b/Documentation/admin-guide/per-console-loglevel.rst
new file mode 100644
index 000000000000..1f8f1eabc75c
--- /dev/null
+++ b/Documentation/admin-guide/per-console-loglevel.rst
@@ -0,0 +1,71 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _per_console_loglevel:
+
+Per-console loglevel support
+============================
+
+Motivation
+----------
+
+Consoles can have vastly different latencies and throughputs. For example,
+writing a message to the serial console can take on the order of tens of
+milliseconds to get the UART to successfully write a message. While this might
+be fine for a single, one-off message, this can cause significant
+application-level stalls in situations where the kernel writes large amounts of
+information to the console.
+
+This means that while you might want to send at least INFO level messages to
+(for example) netconsole, which is relatively fast, you may only want to send at
+least WARN level messages to the serial console. This permits debugging
+using the serial console in cases that netconsole doesn't receive messages
+during particularly bad system issues, while still keeping the noise low enough
+to avoid inducing latency in userspace applications.
+
+Loglevel
+--------
+
+Kernel loglevels are defined thus:
+
++---+--------------+-----------------------------------+
+| 0 | KERN_EMERG | system is unusable |
++---+--------------+-----------------------------------+
+| 1 | KERN_ALERT | action must be taken immediately |
++---+--------------+-----------------------------------+
+| 2 | KERN_CRIT | critical conditions |
++---+--------------+-----------------------------------+
+| 3 | KERN_ERR | error conditions |
++---+--------------+-----------------------------------+
+| 4 | KERN_WARNING | warning conditions |
++---+--------------+-----------------------------------+
+| 5 | KERN_NOTICE | normal but significant condition |
++---+--------------+-----------------------------------+
+| 6 | KERN_INFO | informational |
++---+--------------+-----------------------------------+
+| 7 | KERN_DEBUG | debug-level messages |
++---+--------------+-----------------------------------+
+
+Tunables
+--------
+
+In order to allow tuning per-console loglevels, the following controls exist:
+
+Global
+~~~~~~
+
+The global loglevel is set by the ``kernel.console_loglevel`` sysctl, which can
+also be set as ``loglevel=`` on the kernel command line.
+
+The printk module also takes two parameters which modify this behaviour
+further:
+
+* ``ignore_loglevel`` on the kernel command line or set in printk parameters:
+ Emit all messages. All other controls are ignored if this is present.
+
+* ``ignore_per_console_loglevel`` on the kernel command line or set in printk
+ parameters: Ignore all per-console loglevels and use the global loglevel.
+
+The default value for ``kernel.console_loglevel`` comes from
+``CONFIG_CONSOLE_LOGLEVEL_DEFAULT``, or ``CONFIG_CONSOLE_LOGLEVEL_QUIET`` if
+``quiet`` is passed on the kernel command line.
+
diff --git a/MAINTAINERS b/MAINTAINERS
index e56494c7a956..8018a4db2d9f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20505,6 +20505,7 @@ R: John Ogness <john.ogness@...utronix.de>
R: Sergey Senozhatsky <senozhatsky@...omium.org>
S: Maintained
T: git git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux.git
+F: Documentation/admin-guide/per-console-loglevel.rst
F: Documentation/core-api/printk-basics.rst
F: include/linux/printk.h
F: kernel/printk/
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 1763eaefd581..6cd2750e962b 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -52,6 +52,7 @@
#include <linux/of.h>
#include <linux/rcupdate.h>
#include <linux/console.h>
+#include <linux/printk.h>
#include <asm/ptrace.h>
#include <asm/irq_regs.h>
@@ -599,12 +600,16 @@ static void __sysrq_put_key_op(u8 key, const struct sysrq_key_op *op_p)
void __handle_sysrq(u8 key, bool check_mask)
{
const struct sysrq_key_op *op_p;
+ bool orig_ignore_per_console_loglevel;
int orig_suppress_printk;
int i;
orig_suppress_printk = suppress_printk;
suppress_printk = 0;
+ orig_ignore_per_console_loglevel = ignore_per_console_loglevel;
+ ignore_per_console_loglevel = true;
+
rcu_sysrq_start();
rcu_read_lock();
/*
@@ -650,6 +655,7 @@ void __handle_sysrq(u8 key, bool check_mask)
rcu_read_unlock();
rcu_sysrq_end();
+ ignore_per_console_loglevel = orig_ignore_per_console_loglevel;
suppress_printk = orig_suppress_printk;
}
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 644584edf3e9..e36cae1805b2 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -73,6 +73,8 @@ extern int console_printk[];
#define minimum_console_loglevel (console_printk[2])
#define default_console_loglevel (console_printk[3])
+extern bool ignore_per_console_loglevel;
+
extern void console_verbose(void);
/* strlen("ratelimit") + 1 */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b8679b0da42f..80204cbb7bc8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -104,6 +104,9 @@ DEFINE_STATIC_SRCU(console_srcu);
*/
int __read_mostly suppress_printk;
+/* The sysrq infrastructure needs this even on !CONFIG_PRINTK. */
+bool __read_mostly ignore_per_console_loglevel;
+
#ifdef CONFIG_LOCKDEP
static struct lockdep_map console_lock_dep_map = {
.name = "console_lock"
@@ -1298,9 +1301,21 @@ module_param(ignore_loglevel, bool, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(ignore_loglevel,
"ignore loglevel setting (prints all kernel messages to the console)");
+static int __init ignore_per_console_loglevel_setup(char *str)
+{
+ ignore_per_console_loglevel = true;
+ return 0;
+}
+
+early_param("ignore_per_console_loglevel", ignore_per_console_loglevel_setup);
+module_param(ignore_per_console_loglevel, bool, 0644);
+MODULE_PARM_DESC(
+ ignore_per_console_loglevel,
+ "ignore per-console loglevel setting (only respect global console loglevel)");
+
bool has_per_console_loglevel(const struct console *con)
{
- return con && (console_srcu_read_loglevel(con) > 0);
+ return !ignore_per_console_loglevel && con && (console_srcu_read_loglevel(con) > 0);
}
/**
@@ -1340,7 +1355,7 @@ console_effective_loglevel_source(int con_level)
if (ignore_loglevel)
return LLS_IGNORE_LOGLEVEL;
- if (is_valid_per_console_loglevel(con_level))
+ if (!ignore_per_console_loglevel && is_valid_per_console_loglevel(con_level))
return LLS_LOCAL;
return LLS_GLOBAL;
@@ -1863,6 +1878,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
struct printk_info info;
bool clear = false;
static int saved_console_loglevel = LOGLEVEL_DEFAULT;
+ static bool saved_ignore_per_console_loglevel;
int error;
error = check_syslog_permissions(type, source);
@@ -1904,9 +1920,12 @@ int do_syslog(int type, char __user *buf, int len, int source)
/* Disable logging to console */
case SYSLOG_ACTION_CONSOLE_OFF:
mutex_lock(&syslog_lock);
- if (saved_console_loglevel == LOGLEVEL_DEFAULT)
+ if (saved_console_loglevel == LOGLEVEL_DEFAULT) {
saved_console_loglevel = console_loglevel;
+ saved_ignore_per_console_loglevel = ignore_per_console_loglevel;
+ }
console_loglevel = minimum_console_loglevel;
+ ignore_per_console_loglevel = true;
mutex_unlock(&syslog_lock);
break;
/* Enable logging to console */
@@ -1914,12 +1933,16 @@ int do_syslog(int type, char __user *buf, int len, int source)
mutex_lock(&syslog_lock);
if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
console_loglevel = saved_console_loglevel;
+ ignore_per_console_loglevel = saved_ignore_per_console_loglevel;
saved_console_loglevel = LOGLEVEL_DEFAULT;
}
mutex_unlock(&syslog_lock);
break;
/* Set level of messages printed to console */
case SYSLOG_ACTION_CONSOLE_LEVEL:
+ if (!ignore_per_console_loglevel)
+ pr_warn_once(
+ "SYSLOG_ACTION_CONSOLE_LEVEL is ignored by consoles with an explicitly set per-console loglevel, see Documentation/admin-guide/per-console-loglevel.rst\n");
if (len < 1 || len > 8)
return -EINVAL;
if (len < minimum_console_loglevel)
--
2.51.2
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