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SubjectRe: [RFC PATCH] Allow for stack traces to be printed on serial console
On Mon, Oct 3, 2011 at 5:30 AM, Srivatsa S. Bhat
<srivatsa.bhat@linux.vnet.ibm.com> wrote:
> On 09/30/2011 10:19 AM, Pavel Ivanov wrote:
>> When I tried to chase some mysterious lockup of my PC the only means to
>> see some error reports for me was serial console. But it looks like only
>> printk with KERN_ERR is sent to serial console. So when hung task
>> detector tried to show me stack traces for hung tasks I didn't see them
>> on serial console and I didn't see them after reboot as they couldn't be
>> saved to dmesg. Thus the following patch allowed me to actually see
>> those stack traces.
>
> Which kernel messages gets displayed on your serial console is controlled by
> your system configuration which can be altered by modifying /etc/rsyslog.conf
> or /etc/sysconfig/syslog.conf
> There you can specify that you want to print every kernel message by specifying
> kern.* for example.

I've tried to add "kern.* /dev/ttyS0" and "*.* /dev/ttyS0" to
/etc/rsyslog.conf but it didn't help at all.

> There are also some kernel command line options you can use to specify which
> level of kernel messages you want to get displayed.

I've tried to add loglevel=7 as bootup parameter but it didn't help.
Then I've tried to
"echo $'7\t7\t1\t7' >/proc/sys/kernel/printk" and it helped a little
(message with level KERN_INFO was printed from sched_show_task() to
serial console). But still I wasn't able to see stack traces. Do I
miss something?

> Since you clearly have a system configuration problem here, you don't need to alter the
> kernel source to print with KERN_ERR everywhere. Doing that would also defeat the very
> purpose of having that nice distinction between informational messages, error messages
> and so on, isn't it?

Sure. But what configuration parameter should I tune?

I proposed this patch because I didn't believe that there can be cases
when kernel wants to print stack trace and somebody looking at serial
console doesn't want to see it. But of course I can live with some
configuration parameter as long as I find it.


>> I believe kernel prints stack traces only in some serious cases and
>> expects that they would be actually seen, so KERN_ERR looks justified
>> for me in all changed places. I understand that patch is incomplete and
>> would need some other places to be changed for consistency (at least
>> other arch code). But first I want to hear comments on whether this
>> approach is good enough to be included in mainline or it should be left
>> for me as a hack.


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