Re: [PATCH v6 2/2] tty/sysrq: Replay kernel log messages on consoles via sysrq

[Sreenath Vijayan]

On Wed, Mar 13, 2024 at 03:52:52PM +0530, Sreenath Vijayan wrote:
> When terminal is unresponsive, one cannot use dmesg to view
> the printk ring buffer messages. Also, syslog services may be
> disabled, especially on embedded systems, to check the messages
> after a reboot. In this scenario, replay the messages in printk
> ring buffer on consoles via sysrq by pressing sysrq+R.
> 
> The console loglevel will determine which all kernel log messages
> are displayed. The messages will be displayed only when
> console_trylock() succeeds. Users could repeat the sysrq key when
> it fails. If the owner of console subsystem lock is stuck,
> repeating the key won't work.
> 
> Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
> Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@xxxxxxxx>
> Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> Signed-off-by: Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>
> ---
>  Documentation/admin-guide/sysrq.rst |  9 +++++++++
>  drivers/tty/sysrq.c                 | 13 ++++++++++++-
>  2 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
> index 2f2e5bd440f9..a85b3384d1e7 100644
> --- a/Documentation/admin-guide/sysrq.rst
> +++ b/Documentation/admin-guide/sysrq.rst
> @@ -161,6 +161,8 @@ Command	    Function
>              will be printed to your console. (``0``, for example would make
>              it so that only emergency messages like PANICs or OOPSes would
>              make it to your console.)
> +
> +``R``	    Replay the kernel log messages on consoles.
>  =========== ===================================================================
>  
>  Okay, so what can I use them for?
> @@ -211,6 +213,13 @@ processes.
>  "just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
>  frozen (probably root) filesystem via the FIFREEZE ioctl.
>  
> +``Replay logs(R)`` is useful to view the kernel log messages when system is hung
> +or you are not able to use dmesg command to view the messages in printk buffer.
> +User may have to press the key combination multiple times if console system is
> +busy. If it is completely locked up, then messages won't be printed. Output
> +messages depend on current console loglevel, which can be modified using
> +sysrq[0-9] (see above).
> +
>  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 02217e3c916b..e5974b8239c9 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -450,6 +450,17 @@ static const struct sysrq_key_op sysrq_unrt_op = {
>  	.enable_mask	= SYSRQ_ENABLE_RTNICE,
>  };
>  
> +static void sysrq_handle_replay_logs(u8 key)
> +{
> +	console_replay_all();
> +}
> +static struct sysrq_key_op sysrq_replay_logs_op = {
> +	.handler        = sysrq_handle_replay_logs,
> +	.help_msg       = "replay-kernel-logs(R)",
> +	.action_msg     = "Replay kernel logs on consoles",
> +	.enable_mask    = SYSRQ_ENABLE_DUMP,
> +};
> +
>  /* Key Operations table and lock */
>  static DEFINE_SPINLOCK(sysrq_key_table_lock);
>  
> @@ -519,7 +530,7 @@ static const struct sysrq_key_op *sysrq_key_table[62] = {
>  	NULL,				/* O */
>  	NULL,				/* P */
>  	NULL,				/* Q */
> -	NULL,				/* R */
> +	&sysrq_replay_logs_op,		/* R */
>  	NULL,				/* S */
>  	NULL,				/* T */
>  	NULL,				/* U */
> -- 
> 2.25.1
> 

Hi,

Kindly let me know if there are any review comments for the above changes.

Regards,
Sreenath