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sysrq documentation: document why the command header only is shown

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Document the interactions between loglevel and the sysrq output.  Also
document how to work round it should output be required on the console.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Cc: Martin Mares <mj@ucw.cz>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andy Whitcroft 2009-01-15 13:50:51 -08:00 committed by Linus Torvalds
parent 89365e2641
commit 47c33d9c19
1 changed files with 18 additions and 0 deletions
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18
Documentation/sysrq.txt
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@ -210,6 +210,24 @@ within a function called by handle_sysrq, you must be aware that you are in
a lock (you are also in an interrupt handler, which means don't sleep!), so
you must call __handle_sysrq_nolock instead.
* When I hit a SysRq key combination only the header appears on the console?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Sysrq output is subject to the same console loglevel control as all
other console output. This means that if the kernel was booted 'quiet'
as is common on distro kernels the output may not appear on the actual
console, even though it will appear in the dmesg buffer, and be accessible
via the dmesg command and to the consumers of /proc/kmsg. As a specific
exception the header line from the sysrq command is passed to all console
consumers as if the current loglevel was maximum. If only the header
is emitted it is almost certain that the kernel loglevel is too low.
Should you require the output on the console channel then you will need
to temporarily up the console loglevel using alt-sysrq-8 or:
echo 8 > /proc/sysrq-trigger
Remember to return the loglevel to normal after triggering the sysrq
command you are interested in.
* I have more questions, who can I ask?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
And I'll answer any questions about the registration system you got, also