Re: [PATCH v6 04/11] printk: Support toggling per-console loglevel via syslog() and cmdline

[Petr Mladek <pmladek@suse.com>]

On Mon 2024-10-28 16:45:40, Chris Down wrote:
> 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.
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1298,6 +1313,16 @@ bool per_console_loglevel_is_set(const struct console *con)
>   * 1. con->level. The locally set, console-specific loglevel. Optional, only
>   *    valid if >0.
>   * 2. console_loglevel. The default global console loglevel, always present.

I think that I have suggested to remove the above comment because
it was obvious from the code. I am in doubts now because use
extendended it below ;-)

> + * The behaviour can be further changed by the following printk module
> + * parameters:
> + *
> + * 1. ignore_loglevel. Can be set at boot or at runtime with
> + *    /sys/module/printk/parameters/ignore_loglevel. Overrides absolutely
> + *    everything since it's used to debug.
> + * 2. ignore_per_console_loglevel. Existing per-console loglevel values are left
> + *    intact, but are ignored in favour of console_loglevel as long as this is
> + *    true. Also manipulated through syslog(SYSLOG_ACTION_CONSOLE_{ON,OFF}).

I like that it is summarized in one place. I like the comment after all ;-)

That said, it is also nicely summarized in
Documentation/admin-guide/per-console-loglevel.rst
So, it might be enough to mention it here.

>   */
>  enum loglevel_source
>  console_effective_loglevel_source(const struct console *con)

Best Regards,
Petr