(Alt+SysRq+<cmd_key>) != (echo cmd_key > /proc/sysrq-trigger)

regid · 2020-11-01 02:12:41

The following is probably only a minor bug. Never the less, I do think it is a bug. Or is it?
The way I understand https://www.kernel.org/doc/html/latest/ … sysrq.html, pressing the key combination

 Alt+SysRq+<cmd_key>

should be equivalent to

# echo cmd_key > /proc/sysrq-trigger

But that is not the case. I know that the kernel accept my

 Alt+SysRq+<cmd_key>

commands because the journal says so. But even when I set the console log level to a higher value, I do not get the output of

Alt+SysRq+h

in the VT (as per https://bbs.archlinux.org/viewtopic.php?id=200635 ). Yet I can get it in the VT, after setting the console log level to a higher value, with

# echo h > /proc/sysrq-trigger

loqs · 2020-11-01 02:31:33

What is the value of /proc/sys/kernel/sysrq ?

regid · 2020-11-01 11:20:28

The results above were with a custom kernel.
I have tried the experiment again. This time after a reboot into an arch made kernel. With the arch made kernel, I didn't get the help message in the VT at all. Not with

Alt+SysRq+h

, and not with

echo h > /proc/sysrq-trigger

. Both tried after setting the log level appropriately. Logging the help message, as well as logging the log level changes, within the journal seem to work.

regid · 2020-11-01 11:29:42

Initially,

$ cat /proc/sys/kernel/sysrq
16

, probably because of

$ grep ^kernel.sysrq /usr/lib/sysctl.d/50-default.conf 
kernel.sysrq = 16

. But I have

$ echo 1 > /proc/sys/kernel/sysrq

a priory.

Arch Linux

#1 2020-11-01 02:12:41

(Alt+SysRq+<cmd_key>) != (echo cmd_key > /proc/sysrq-trigger)

#2 2020-11-01 02:31:33

Re: (Alt+SysRq+<cmd_key>) != (echo cmd_key > /proc/sysrq-trigger)

#3 2020-11-01 11:20:28

Re: (Alt+SysRq+<cmd_key>) != (echo cmd_key > /proc/sysrq-trigger)

#4 2020-11-01 11:29:42

Re: (Alt+SysRq+<cmd_key>) != (echo cmd_key > /proc/sysrq-trigger)

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