linux-kernel - Re: sysrq loglevel

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Message-Id: <20090107112539.972e60e2.akpm@linux-foundation.org>
Date:	Wed, 7 Jan 2009 11:25:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andy Whitcroft <apw@...onical.com>
Cc:	Crutcher Dunnavant <crutcher+kernel@...astacks.com>,
	linux-kernel@...r.kernel.org
Subject: Re: sysrq loglevel

On Wed, 7 Jan 2009 12:37:58 +0000 Andy Whitcroft <apw@...onical.com> wrote:

> It seems that we deliberatly manage the console_loglevel while handling a
> sysrq request.  Raising it to 7 to emit the sysrq command header, and then
> lower it before processing the command itself.  When booting the kernel
> 'quiet' this means that we only see the header of the command and not its
> output on the console, the whole thing is in dmesg and thereby in syslog
> (if it is working).

I always thought it was fairly stupid.  Wouldn't we get the same effect
by tossing that code and switching those printks to KERN_EMERG?

> void __handle_sysrq(int key, struct tty_struct *tty, int check_mask)
> [...]
>         console_loglevel = 7;
>         printk(KERN_INFO "SysRq : ");
> [...]
>                         printk("%s\n", op_p->action_msg);
>                         console_loglevel = orig_log_level;
>                         op_p->handler(key, tty);
> [...]
> 
> Is this intentional?  I can see arguments both ways.  One way to look at
> it would be that I asked for the output so I should get it regardless.
> The other side might be that consoles can be really slow (serial or
> something) and so only outputting it there if logging is enabled
> generally is sane.
> 
> Obviously we can work round this at the moment using sysrq-7 to up the
> loglevel before the command and sysrq-4 after to restore quiet.
> 
> What do people think.  If we are happy with the status quo then I will
> spin a documentation patch to point out this behaviour and the work
> around.  Else I will happily spin a patch to fix it.
> 

There is a legitimate use case, I think: to emit the sysrq command's
output into the log bufffer and not to the console[s].  So you can do

	echo t > /proc/sysrq-trigger
	dmesg -s 1000000 > foo


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