linux-kernel - Re: [PATCH] printk: Add caller information to printk() output.

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Date:   Fri, 22 Mar 2019 11:48:04 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH] printk: Add caller information to printk() output.

Petr Mladek <pmladek@...e.com> writes:
> On Thu 2019-03-21 13:59:53, Michael Ellerman wrote:
>> Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> writes:
>> ...
>> > From 91f85d2bd494df2f73c605d8b4747e8cc0a61ae2 Mon Sep 17 00:00:00 2001
>> > From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> > Date: Tue, 18 Dec 2018 05:53:04 +0900
>> > Subject: [PATCH] printk: Add caller information to printk() output.
>> >
>> > Sometimes we want to print a series of printk() messages to consoles
>> > without being disturbed by concurrent printk() from interrupts and/or
>> > other threads. But we can't enforce printk() callers to use their local
>> > buffers because we need to ask them to make too much changes. Also, even
>> > buffering up to one line inside printk() might cause failing to emit
>> > an important clue under critical situation.
>> >
>> > Therefore, instead of trying to help buffering, let's try to help
>> > reconstructing messages by saving caller information as of calling
>> > log_store() and adding it as "[T$thread_id]" or "[C$processor_id]"
>> > upon printing to consoles.
>> >
>> > Some examples for console output:
>> >
>> >   [    1.222773][    T1] x86: Booting SMP configuration:
>> >   [    2.779635][    T1] pci 0000:00:01.0: PCI bridge to [bus 01]
>> >   [    5.069193][  T268] Fusion MPT base driver 3.04.20
>> >   [    9.316504][    C2] random: fast init done
>> >   [   13.413336][ T3355] Initialized host personality
>> >
>> > Some examples for /dev/kmsg output:
>> >
>> >   6,496,1222773,-,caller=T1;x86: Booting SMP configuration:
>> >   6,968,2779635,-,caller=T1;pci 0000:00:01.0: PCI bridge to [bus 01]
>> >    SUBSYSTEM=pci
>> >    DEVICE=+pci:0000:00:01.0
>> >   6,1353,5069193,-,caller=T268;Fusion MPT base driver 3.04.20
>> >   5,1526,9316504,-,caller=C2;random: fast init done
>> >   6,1575,13413336,-,caller=T3355;Initialized host personality
>> >
>> > Note that this patch changes max length of messages which can be printed
>> > by printk() or written to /dev/kmsg interface from 992 bytes to 976 bytes,
>> > based on an assumption that userspace won't try to write messages hitting
>> > that border line to /dev/kmsg interface.
>> 
>> Do you have any plans to update dmesg or other userspace tools to show
>> the caller information?
>
> dmesg already works via the syslog interface, try dmesg -S.

Oh nice, thanks.

> The current implementation does not pass the information into
> the /dev/kmsg interface. It has the following format:
>
>     "<level>,<sequnum>,<timestamp>,<contflag>[,additional_values, ... ];<message text>\n"
>
> It would be possible to add it as an additional value. To be hones
> I am not sure how they are handled by userspace tools.

OK, yeah I guess there's the potential for a backward compatibility can
of worms there.

I'm happy with dmesg -S :)

cheers