linux-kernel - Re: [PATCH 2/3] sysrq documentation: document why the command header only is shown

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Date:	Wed, 07 Jan 2009 15:30:29 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Andy Whitcroft <apw@...onical.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Crutcher Dunnavant <crutcher+kernel@...astacks.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] sysrq documentation: document why the command header
 only is shown

Andy Whitcroft wrote:
> Document the interactions between loglevel and the sysrq output.  Also
> document how to work round it should output on the console be required.
> 
> Signed-off-by: Andy Whitcroft <apw@...onical.com>
> ---
>  Documentation/sysrq.txt |   18 ++++++++++++++++++
>  1 files changed, 18 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt
> index 265f637..a262ffb 100644
> --- a/Documentation/sysrq.txt
> +++ b/Documentation/sysrq.txt
> @@ -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 combination only the header appears on the console, why?

   *  When I hit a sysrq key combination, only the header appears on the console. Why?
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +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 cirtain that the kernel loglevel is too low.

                           certain

> +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


-- 
~Randy
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