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
