Re: [PATCH v7 11/13] printk: docs: Add comprehensive guidance for per-console loglevels

[John Ogness <john.ogness@linutronix.de>]

On 2025-11-19, Chris Down <chris@chrisdown.name> wrote:
> diff --git a/Documentation/admin-guide/per-console-loglevel.rst b/Documentation/admin-guide/per-console-loglevel.rst
> index 4908d5d8ed4f..69eede12e20f 100644
> --- a/Documentation/admin-guide/per-console-loglevel.rst
> +++ b/Documentation/admin-guide/per-console-loglevel.rst
> @@ -97,6 +97,158 @@ are using ``ttyS0``, the console backing it can be viewed at
>    This will be in effect if no other global control overrides it. Look at
>    ``effective_loglevel`` and ``effective_loglevel_source`` to verify that.
>  
> +Examples
> +--------
> +
> +Setting per-console loglevel at runtime
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Set serial console to only show warnings and above (level 4)::
> +
> +    echo 4 > /sys/class/console/ttyS0/loglevel

Setting it to 4 would only show "above warnings" ... you will not see
warning messages.

> +
> +Set netconsole to show info and above (level 6)::
> +
> +    echo 6 > /sys/class/console/netcon0/loglevel

Setting it to 6 would only show "above info" ... you will not see info
messages.

> +Common use case - high performance with serial fallback
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +A common configuration is to set netconsole to a verbose level for normal
> +debugging, while keeping the serial console quiet to avoid performance impact,
> +but still available for emergencies::
> +
> +    # Netconsole gets INFO and above (verbose)
> +    echo 6 > /sys/class/console/netcon0/loglevel

6 only shows "above INFO" ... you will not see INFO.

> +
> +    # Serial console gets only WARN and above (quiet, for emergencies)
> +    echo 4 > /sys/class/console/ttyS0/loglevel

4 only shows "above WARN" ... you will not see WARN.

> +
> +This allows you to see informational messages on the fast netconsole without
> +the latency impact of writing them to the slow serial port.
> +
> +Performance Impact
> +------------------
> +
> +When a console has a higher (less verbose) loglevel than the global level,
> +messages that would normally be sent to that console are filtered out before
> +the console write callback is invoked. This eliminates the latency that would
> +be incurred by writing those messages to slow consoles (e.g., serial ports).
> +
> +For example, setting a serial console to WARN level (4) while keeping
> +netconsole at INFO level (6) prevents INFO and NOTICE messages from being
> +written to the slow serial port, reducing application stalls during verbose
> +logging periods.

Setting serial to 4 will also prevent WARN messages.

> +
> +Serial console writes can take tens of milliseconds per message. During
> +periods of heavy logging (e.g., during network debugging or block I/O tracing),
> +this can cause significant application-level stalls. By setting a higher
> +per-console loglevel for the serial console, you can avoid these stalls while
> +still capturing all messages on faster consoles like netconsole.
> +
> +Troubleshooting
> +---------------
> +
> +Messages not appearing on console despite setting loglevel
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +1. Check effective loglevel source::
> +
> +       cat /sys/class/console/<name>/effective_loglevel_source
> +
> +   If it shows ``ignore_loglevel``, you have the ``printk.ignore_loglevel``
> +   kernel parameter set, which overrides all level controls. Remove it from
> +   your kernel command line or set it to N in sysfs::
> +
> +       echo N > /sys/module/printk/parameters/ignore_loglevel
> +
> +2. Check if per-console loglevels are being ignored::
> +
> +       cat /sys/module/printk/parameters/ignore_per_console_loglevel
> +
> +   If it shows ``Y``, per-console settings are disabled. Set it to N::
> +
> +       echo N > /sys/module/printk/parameters/ignore_per_console_loglevel
> +
> +3. Verify the message priority is high enough::
> +
> +       cat /sys/class/console/<name>/effective_loglevel
> +
> +   Messages must have priority less than this value to appear. For example,
> +   if effective_loglevel is 4, only messages with priority 0-3 (EMERG, ALERT,
> +   CRIT, ERR) will be printed. If you want to see WARN messages (priority 4),
> +   you need to increase the effective_loglevel to at least 5.

Yay! Here it is documented correctly. ;-)

> +
> +Cannot set loglevel to 0
> +~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Per-console loglevels cannot be set to 0 (KERN_EMERG). This is by design, as
> +level 0 is reserved for the most critical system messages that should always
> +go to all consoles. To use the global loglevel, set the per-console loglevel
> +to -1::
> +
> +    echo -1 > /sys/class/console/<name>/loglevel
> +
> +Setting below minimum_console_loglevel fails
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +If you get an error when trying to set a loglevel, check the system-wide
> +minimum::
> +
> +    cat /proc/sys/kernel/console_loglevel
> +
> +Per-console loglevels cannot be set below this minimum. This is a safety
> +feature to ensure critical messages are always visible.
> +
> +Edge cases
> +~~~~~~~~~~
> +
> +**Setting all consoles to high loglevels**: If you set all consoles to
> +very high loglevels (e.g., 1 or 2), most messages won't appear anywhere.
> +This is valid but probably not what you want. Keep at least one console
> +at a reasonable level for monitoring.

Keep in mind that all messages (regardless of loglevel) are always
available to userspace via dmesg or syslogd. So saying "messages won't
appear anywhere" is a bit misleading.

John Ogness