Message-ID: <ealgvcqabn5nwfyxk6cv6wwovrbbh6meihea7zfoomcr4gk2nr@7o4izfp2tbd5>
Date: Fri, 10 Jan 2025 11:27:53 +0100
From: Joel Granados <joel.granados@...nel.org>
To: Chris Down <chris@...isdown.name>
Cc: Petr Mladek <pmladek@...e.com>, 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: Re: [PATCH v6 06/11] printk: console: Introduce sysfs interface for
 per-console loglevels

On Mon, Oct 28, 2024 at 04:45:46PM +0000, Chris Down wrote:
> A sysfs interface under /sys/class/console/ is created that permits
> viewing and configuring per-console attributes. This is the main
> interface with which we expect users to interact with and configure
> per-console loglevels.
These are different from the knobs that you added to sysctl. right? The
ones in sysctl are general values as opposed to console specific.

Would it make sense to put everything in here (sys) instead of having
them separate?

Best
> 
> Each console device now has its own directory (for example,
> /sys/class/console/ttyS0/) containing the following attributes:
> 
> - effective_loglevel (ro): The effective loglevel for the console after
>   considering all loglevel authorities (e.g., global loglevel,
>   per-console loglevel).
> - effective_loglevel_source (ro): The source of the effective loglevel
>   (e.g., local, global, ignore_loglevel).
> - enabled (ro): Indicates whether the console is enabled (1) or disabled
>   (0).
> - loglevel (rw): The per-console loglevel. Writing a value between 0
>   (KERN_EMERG) and 8 (KERN_DEBUG + 1) sets the per-console loglevel.
>   Writing -1 disables the per-console loglevel.
> 
> In terms of technical implementation, we embed a device pointer in the
> console struct, and register each console using it so we can expose
> attributes in sysfs. We currently expose the following attributes:
> 
>     % ls -l /sys/class/console/ttyS0/
>     total 0
>     lrwxrwxrwx 1 root root    0 Oct 23 13:17 subsystem -> ../../../../class/console/
>     -r--r--r-- 1 root root 4096 Oct 23 13:18 effective_loglevel
>     -r--r--r-- 1 root root 4096 Oct 23 13:18 effective_loglevel_source
>     -r--r--r-- 1 root root 4096 Oct 23 13:18 enabled
>     -rw-r--r-- 1 root root 4096 Oct 23 13:18 loglevel
>     -rw-r--r-- 1 root root 4096 Oct 23 13:17 uevent
> 
> The lifecycle of this classdev looks like this on registration:
> 
>     register_console(con)/printk_late_init()
>       console_register_device(con)
>         device_initialize(con->classdev) # refcount++
>         device_add(con->classdev)        # refcount++
> 
> At stable state, the refcount is two.
> 
> Console unregistration looks like this:
> 
>     [con->classdev refcount drops to 0]
>       console_classdev_release(con->classdev)
>         kfree(con->classdev)
> 
>     unregister_console(con)
>       device_unregister(con->classdev)
>         device_del(con->classdev) # refcount--
>           device_remove_class_symlinks()
>             kernfs_remove_by_name_ns()
>               kernfs_drain()
>                 kernfs_drain_open_files() # wait for close()
>         put_device(con->classdev) # refcount--
> 
> Signed-off-by: Chris Down <chris@...isdown.name>
> ---
>  Documentation/ABI/testing/sysfs-class-console |  47 +++++
>  .../admin-guide/per-console-loglevel.rst      |  41 ++++
>  Documentation/core-api/printk-basics.rst      |  35 ++--
>  Documentation/networking/netconsole.rst       |  13 ++
>  MAINTAINERS                                   |   1 +
>  include/linux/console.h                       |   3 +
>  include/linux/printk.h                        |   2 +
>  kernel/printk/Makefile                        |   2 +-
>  kernel/printk/internal.h                      |   5 +
>  kernel/printk/printk.c                        |  15 ++
>  kernel/printk/sysfs.c                         | 178 ++++++++++++++++++
>  11 files changed, 324 insertions(+), 18 deletions(-)
>  create mode 100644 Documentation/ABI/testing/sysfs-class-console
>  create mode 100644 kernel/printk/sysfs.c
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-console b/Documentation/ABI/testing/sysfs-class-console
> new file mode 100644
> index 000000000000..40b90b190af3
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-class-console
> @@ -0,0 +1,47 @@
> +What:		/sys/class/console/
> +Date:		October 2024
> +Contact:	Chris Down <chris@...isdown.name>
> +Description:	Interface for viewing and setting per-console attributes, like
> +		the per-console loglevel. For a high-level document describing
> +		the motivations for this interface and related non-sysfs
> +		controls, see
> +		Documentation/admin-guide/per-console-loglevel.rst.
> +
> +What:		/sys/class/console/<C>/effective_loglevel
> +Date:		October 2024
> +Contact:	Chris Down <chris@...isdown.name>
> +Description:	Read only. The currently effective loglevel for this console.
> +		All messages emitted with a loglevel below the effective value
> +		will be emitted to the console.
> +
> +What:		/sys/class/console/<C>/effective_loglevel_source
> +Date:		October 2024
> +Contact:	Chris Down <chris@...isdown.name>
> +Description:	Read only. The currently effective loglevel source for this
> +		console -- for example, whether it was set globally, or whether
> +		it was set locally for this console.
> +
> +		Possible values are:
> +			=============== ============================================
> +			local           The loglevel comes from the per-console
> +			                loglevel.
> +			global          The loglevel comes from the global loglevel.
> +			ignore_loglevel Both the per-console loglevel and global
> +			                loglevels are ignored as ignore_loglevel is
> +			                present on the kernel command line.
> +			=============== ============================================
> +
> +What:		/sys/class/console/<C>/enabled
> +Date:		October 2024
> +Contact:	Chris Down <chris@...isdown.name>
> +Description:	Read only. "1" if the console is enabled, "0" otherwise.
> +
> +What:		/sys/class/console/<C>/loglevel
> +Date:		October 2024
> +Contact:	Chris Down <chris@...isdown.name>
> +Description:	Read write. The current per-console loglevel, which will take
> +		effect if not overridden by other non-sysfs controls (see
> +		Documentation/admin-guide/per-console-loglevel.rst). Bounds are
> +		0 (LOGLEVEL_EMERG) to 8 (LOGLEVEL_DEBUG + 1) inclusive. Also
> +		takes the special value "-1" to indicate that no per-console
> +		loglevel is set, and we should defer to the global controls.
> diff --git a/Documentation/admin-guide/per-console-loglevel.rst b/Documentation/admin-guide/per-console-loglevel.rst
> index 1ec7608f94b0..41bf3befb2f3 100644
> --- a/Documentation/admin-guide/per-console-loglevel.rst
> +++ b/Documentation/admin-guide/per-console-loglevel.rst
> @@ -68,3 +68,44 @@ further:
>  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.
> +
> +Console attributes
> +~~~~~~~~~~~~~~~~~~
> +
> +Registered consoles are exposed at ``/sys/class/console``. For example, if you
> +are using ``ttyS0``, the console backing it can be viewed at
> +``/sys/class/console/ttyS0/``. The following files are available:
> +
> +* ``effective_loglevel`` (r): The effective loglevel after considering all
> +  loglevel authorities. For example, it shows the value of the console-specific
> +  loglevel when a console-specific loglevel is defined, and shows the global
> +  console loglevel value when the console-specific one is not defined.
> +
> +* ``effective_loglevel_source`` (r): The loglevel authority which resulted in
> +  the effective loglevel being set. The following values can be present:
> +
> +    * ``local``: The console-specific loglevel is in effect.
> +
> +    * ``global``: The global loglevel (``kernel.console_loglevel``) is in
> +      effect. Set a console-specific loglevel to override it.
> +
> +    * ``ignore_loglevel``: ``ignore_loglevel`` was specified on the kernel
> +      command line or at ``/sys/module/printk/parameters/ignore_loglevel``.
> +      Disable it to use level controls.
> +
> +* ``enabled`` (r): Whether the console is enabled.
> +
> +* ``loglevel`` (rw): The local, console-specific loglevel for this console.
> +  This will be in effect if no other global control overrides it. Look at
> +  ``effective_loglevel`` and ``effective_loglevel_source`` to verify that.
> +
> +Deprecated
> +~~~~~~~~~~
> +
> +* ``kernel.printk`` sysctl: this takes four values, setting
> +  ``kernel.console_loglevel``, ``kernel.default_message_loglevel``, the minimum
> +  console loglevel, and a fourth unused value. The interface is generally
> +  considered to quite confusing, doesn't perform checks on the values given,
> +  and is unaware of per-console loglevel semantics.
> +
> +Chris Down <chris@...isdown.name>, 13-October-2024
> diff --git a/Documentation/core-api/printk-basics.rst b/Documentation/core-api/printk-basics.rst
> index 2dde24ca7d9f..bfad359505bb 100644
> --- a/Documentation/core-api/printk-basics.rst
> +++ b/Documentation/core-api/printk-basics.rst
> @@ -54,32 +54,33 @@ string, the log level is not a separate argument). The available log levels are:
>  
>  The log level specifies the importance of a message. The kernel decides whether
>  to show the message immediately (printing it to the current console) depending
> -on its log level and the current *console_loglevel* (a kernel variable). If the
> -message priority is higher (lower log level value) than the *console_loglevel*
> -the message will be printed to the console.
> +on its log level and the current global *console_loglevel* or local per-console
> +loglevel (kernel variables). If the message priority is higher (lower log level
> +value) than the effective loglevel the message will be printed to the console.
>  
>  If the log level is omitted, the message is printed with ``KERN_DEFAULT``
>  level.
>  
> -You can check the current *console_loglevel* with::
> +You can check the current console's loglevel -- for example if you want to
> +check the loglevel for serial consoles:
>  
> -  $ cat /proc/sys/kernel/printk
> -  4        4        1        7
> +  $ cat /sys/class/console/ttyS0/effective_loglevel
> +  6
> +  $ cat /sys/class/console/ttyS0/effective_loglevel_source
> +  local
>  
> -The result shows the *current*, *default*, *minimum* and *boot-time-default* log
> -levels.
> +To change the default loglevel for all consoles, simply write the desired level
> +to ``/proc/sys/kernel/console_loglevel``. For example::
>  
> -To change the current console_loglevel simply write the desired level to
> -``/proc/sys/kernel/printk``. For example, to print all messages to the console::
> +  # echo 5 > /proc/sys/kernel/console_loglevel
>  
> -  # echo 8 > /proc/sys/kernel/printk
> +This sets the console_loglevel to print KERN_WARNING (4) or more severe
> +messages to console. Consoles with a per-console loglevel set will ignore it
> +unless ``ignore_per_console_loglevel`` is set on the kernel command line or at
> +``/sys/module/printk/parameters/ignore_per_console_loglevel``.
>  
> -Another way, using ``dmesg``::
> -
> -  # dmesg -n 5
> -
> -sets the console_loglevel to print KERN_WARNING (4) or more severe messages to
> -console. See ``dmesg(1)`` for more information.
> +For more information on per-console loglevels, see
> +Documentation/admin-guide/per-console-loglevel.rst.
>  
>  As an alternative to printk() you can use the ``pr_*()`` aliases for
>  logging. This family of macros embed the log level in the macro names. For
> diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
> index d55c2a22ec7a..34419e6fe106 100644
> --- a/Documentation/networking/netconsole.rst
> +++ b/Documentation/networking/netconsole.rst
> @@ -72,6 +72,19 @@ Built-in netconsole starts immediately after the TCP stack is
>  initialized and attempts to bring up the supplied dev at the supplied
>  address.
>  
> +You can also set a loglevel at runtime::
> +
> +  $ ls -l /sys/class/console/netcon0/
> +  total 0
> +  lrwxrwxrwx 1 root root    0 May 18 13:28 subsystem -> ../../../../class/console/
> +  -r--r--r-- 1 root root 4096 May 18 13:28 effective_loglevel
> +  -r--r--r-- 1 root root 4096 May 18 13:28 effective_loglevel_source
> +  -r--r--r-- 1 root root 4096 May 18 13:28 enabled
> +  -rw-r--r-- 1 root root 4096 May 18 13:28 loglevel
> +  -rw-r--r-- 1 root root 4096 May 18 13:28 uevent
> +
> +See Documentation/admin-guide/per-console-loglevel.rst for more information.
> +
>  The remote host has several options to receive the kernel messages,
>  for example:
>  
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ea134675669e..003f999e531b 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -18449,6 +18449,7 @@ 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/ABI/testing/sysfs-class-console
>  F:	Documentation/admin-guide/per-console-loglevel.rst
>  F:	Documentation/core-api/printk-basics.rst
>  F:	include/linux/printk.h
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 3ff22bfeef2a..332eef0f95f7 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -16,6 +16,7 @@
>  
>  #include <linux/atomic.h>
>  #include <linux/bits.h>
> +#include <linux/device.h>
>  #include <linux/irq_work.h>
>  #include <linux/rculist.h>
>  #include <linux/rcuwait.h>
> @@ -322,6 +323,7 @@ struct nbcon_write_context {
>   * @data:		Driver private data
>   * @node:		hlist node for the console list
>   * @level:		Console-specific loglevel
> + * @classdev:		Console class device for /sys/class/console
>   *
>   * @nbcon_state:	State for nbcon consoles
>   * @nbcon_seq:		Sequence number of the next record for nbcon to print
> @@ -351,6 +353,7 @@ struct console {
>  	void			*data;
>  	struct hlist_node	node;
>  	int			level;
> +	struct device		*classdev;
>  
>  	/* nbcon console specific members */
>  
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 0053533dcfec..b7e8411e033d 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -77,6 +77,8 @@ extern bool ignore_per_console_loglevel;
>  
>  extern void console_verbose(void);
>  
> +int clamp_loglevel(int level);
> +
>  /* strlen("ratelimit") + 1 */
>  #define DEVKMSG_STR_MAX_SIZE 10
>  extern char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE];
> diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
> index 39a2b61c7232..3c0b6e2a8083 100644
> --- a/kernel/printk/Makefile
> +++ b/kernel/printk/Makefile
> @@ -1,5 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0-only
> -obj-y	= printk.o
> +obj-y	= printk.o sysfs.o
>  obj-$(CONFIG_PRINTK)	+= printk_safe.o nbcon.o
>  obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o
>  obj-$(CONFIG_PRINTK_INDEX)	+= index.o
> diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
> index 9303839d0551..ac607d6907a0 100644
> --- a/kernel/printk/internal.h
> +++ b/kernel/printk/internal.h
> @@ -21,6 +21,11 @@ enum loglevel_source {
>  	LLS_IGNORE_LOGLEVEL,
>  };
>  
> +void console_register_device(struct console *new);
> +void console_setup_class(void);
> +
> +int clamp_loglevel(int level);
> +
>  enum loglevel_source
>  console_effective_loglevel_source(const struct console *con);
>  int console_effective_loglevel(const struct console *con);
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 055619c5c7e8..bc456f7624d4 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -199,6 +199,12 @@ static int __init control_devkmsg(char *str)
>  }
>  __setup("printk.devkmsg=", control_devkmsg);
>  
> +int clamp_loglevel(int level)
> +{
> +	return clamp(level, minimum_console_loglevel,
> +		     CONSOLE_LOGLEVEL_MOTORMOUTH);
> +}
> +
>  char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit";
>  #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
>  int devkmsg_sysctl_set_loglvl(const struct ctl_table *table, int write,
> @@ -3894,6 +3900,8 @@ static int try_enable_preferred_console(struct console *newcon,
>  			// TODO: Will be configurable in a later patch
>  			newcon->level = -1;
>  
> +			newcon->classdev = NULL;
> +
>  			if (_braille_register_console(newcon, c))
>  				return 0;
>  
> @@ -4170,6 +4178,7 @@ void register_console(struct console *newcon)
>  	if (use_device_lock)
>  		newcon->device_unlock(newcon, flags);
>  
> +	console_register_device(newcon);
>  	console_sysfs_notify();
>  
>  	/*
> @@ -4264,6 +4273,9 @@ static int unregister_console_locked(struct console *console)
>  	if (console->flags & CON_NBCON)
>  		nbcon_free(console);
>  
> +	if (console->classdev)
> +		device_unregister(console->classdev);
> +
>  	console_sysfs_notify();
>  
>  	if (console->exit)
> @@ -4428,6 +4440,9 @@ static int __init printk_late_init(void)
>  					console_cpu_notify, NULL);
>  	WARN_ON(ret < 0);
>  	printk_sysctl_init();
> +
> +	console_setup_class();
> +
>  	return 0;
>  }
>  late_initcall(printk_late_init);
> diff --git a/kernel/printk/sysfs.c b/kernel/printk/sysfs.c
> new file mode 100644
> index 000000000000..e24590074861
> --- /dev/null
> +++ b/kernel/printk/sysfs.c
> @@ -0,0 +1,178 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/kernel.h>
> +#include <linux/console.h>
> +#include <linux/device.h>
> +#include <linux/printk.h>
> +#include <linux/slab.h>
> +#include "internal.h"
> +
> +#ifdef CONFIG_PRINTK
> +static struct class *console_class;
> +
> +static const char *
> +console_effective_loglevel_source_str(const struct console *con)
> +{
> +	enum loglevel_source source;
> +	const char *str;
> +
> +	source = console_effective_loglevel_source(con);
> +
> +	switch (source) {
> +	case LLS_IGNORE_LOGLEVEL:
> +		str = "ignore_loglevel";
> +		break;
> +	case LLS_LOCAL:
> +		str = "local";
> +		break;
> +	case LLS_GLOBAL:
> +		str = "global";
> +		break;
> +	default:
> +		pr_warn("Unhandled console loglevel source: %d", source);
> +		str = "unknown";
> +		break;
> +	}
> +
> +	return str;
> +}
> +
> +static ssize_t loglevel_show(struct device *dev, struct device_attribute *attr,
> +			     char *buf)
> +{
> +	struct console *con = dev_get_drvdata(dev);
> +
> +	return sysfs_emit(buf, "%d\n", READ_ONCE(con->level));
> +}
> +
> +static ssize_t loglevel_store(struct device *dev, struct device_attribute *attr,
> +			      const char *buf, size_t size)
> +{
> +	struct console *con = dev_get_drvdata(dev);
> +	ssize_t ret;
> +	int level;
> +
> +	ret = kstrtoint(buf, 10, &level);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (level == -1)
> +		goto out;
> +
> +	if (clamp_loglevel(level) != level)
> +		return -ERANGE;
> +
> +out:
> +	WRITE_ONCE(con->level, level);
> +
> +	return size;
> +}
> +
> +static DEVICE_ATTR_RW(loglevel);
> +
> +static ssize_t effective_loglevel_source_show(struct device *dev,
> +					      struct device_attribute *attr,
> +					      char *buf)
> +{
> +	struct console *con = dev_get_drvdata(dev);
> +
> +	return sysfs_emit(buf, "%s\n",
> +			  console_effective_loglevel_source_str(con));
> +}
> +
> +static DEVICE_ATTR_RO(effective_loglevel_source);
> +
> +static ssize_t effective_loglevel_show(struct device *dev,
> +				       struct device_attribute *attr, char *buf)
> +{
> +	struct console *con = dev_get_drvdata(dev);
> +
> +	return sysfs_emit(buf, "%d\n", console_effective_loglevel(con));
> +}
> +
> +static DEVICE_ATTR_RO(effective_loglevel);
> +
> +static ssize_t enabled_show(struct device *dev, struct device_attribute *attr,
> +			    char *buf)
> +{
> +	struct console *con = dev_get_drvdata(dev);
> +	int cookie;
> +	bool enabled;
> +
> +	cookie = console_srcu_read_lock();
> +	enabled = console_srcu_read_flags(con) & CON_ENABLED;
> +	console_srcu_read_unlock(cookie);
> +	return sysfs_emit(buf, "%d\n", enabled);
> +}
> +
> +static DEVICE_ATTR_RO(enabled);
> +
> +static struct attribute *console_sysfs_attrs[] = {
> +	&dev_attr_loglevel.attr,
> +	&dev_attr_effective_loglevel_source.attr,
> +	&dev_attr_effective_loglevel.attr,
> +	&dev_attr_enabled.attr,
> +	NULL,
> +};
> +
> +ATTRIBUTE_GROUPS(console_sysfs);
> +
> +static void console_classdev_release(struct device *dev)
> +{
> +	kfree(dev);
> +}
> +
> +void console_register_device(struct console *con)
> +{
> +	/*
> +	 * We might be called from register_console() before the class is
> +	 * registered. If that happens, we'll take care of it in
> +	 * printk_late_init.
> +	 */
> +	if (IS_ERR_OR_NULL(console_class))
> +		return;
> +
> +	if (WARN_ON(con->classdev))
> +		return;
> +
> +	con->classdev = kzalloc(sizeof(struct device), GFP_KERNEL);
> +	if (!con->classdev)
> +		return;
> +
> +	device_initialize(con->classdev);
> +	dev_set_name(con->classdev, "%s%d", con->name, con->index);
> +	dev_set_drvdata(con->classdev, con);
> +	con->classdev->release = console_classdev_release;
> +	con->classdev->class = console_class;
> +	if (device_add(con->classdev))
> +		put_device(con->classdev);
> +}
> +
> +void console_setup_class(void)
> +{
> +	struct console *con;
> +	int cookie;
> +
> +	/*
> +	 * printk exists for the lifetime of the kernel, it cannot be unloaded,
> +	 * so we should never end up back in here.
> +	 */
> +	if (WARN_ON(console_class))
> +		return;
> +
> +	console_class = class_create("console");
> +	if (!IS_ERR(console_class))
> +		console_class->dev_groups = console_sysfs_groups;
> +
> +	cookie = console_srcu_read_lock();
> +	for_each_console_srcu(con)
> +		console_register_device(con);
> +	console_srcu_read_unlock(cookie);
> +}
> +#else /* CONFIG_PRINTK */
> +void console_register_device(struct console *new)
> +{
> +}
> +void console_setup_class(void)
> +{
> +}
> +#endif
> -- 
> 2.46.0
> 

-- 

Joel Granados

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