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

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Message-ID: <20181203150603.cdqii263e4kmmibo@pathway.suse.cz>
Date:   Mon, 3 Dec 2018 16:06:03 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...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>
Subject: Re: [PATCH] printk: Add caller information to printk() output.

On Sat 2018-12-01 23:44:37, Tetsuo Handa wrote:
> On 2018/12/01 0:40, Petr Mladek wrote:
> >> Some examples for console output:
> >>
> >>   [    0.293000] [T1] smpboot: CPU0: Intel(R) Core(TM) i5-4440S CPU @ 2.80GHz (family: 0x6, model: 0x3c, stepping: 0x3)
> >>   [    0.299733] [T1] Performance Events: Haswell events, core PMU driver.
> >>   [    2.813808] [T35] clocksource: Switched to clocksource tsc
> >>   [    2.893984] [C0] random: fast init done
> >                   ^
> > 
> > Please, remove the space between the timestamp and the from field.
> 
> This space was emitted by print_time(). Do we want to modify print_time()
> not to emit this space if the from field is printed?

Exactly. This is what I thought about.
 
> If we modify print_time(), I think that the leading spaces inserted by "%5lu"
> makes little sense, for "%5lu" is too small for systems with uptime >= 1.16 days
> and parsers after all cannot assume fixed length for the timestamp field. Then,
> we could change from "%5lu.%06lu" to "%lu.%06lu" so that parsers (like /bin/awk)
> can get prefix part using white spaces as a delimiter.

My primary concern was a human readability. The different header columns
are separated by brackets and the message itself is separated by the space.

awk could easily use \[ as the separator.

But you made a good point about the column width. The text might be
hard to read when every line of text starts on a different column. And
the might be bigger differences for the task id. It might be useful to
add some reasonable default width also for the for the "from_id" column.

> If we want to reduce space, do we want to do like
> 
>   [0.293000@T1] smpboot: CPU0: Intel(R) Core(TM) i5-4440S CPU @ 2.80GHz (family: 0x6, model: 0x3c, stepping: 0x3)
>   [0.299733@T1] Performance Events: Haswell events, core PMU driver.
>   [2.813808@T35] clocksource: Switched to clocksource tsc
>   [2.893984@C0] random: fast init done

Hmm, this is pretty hard to parse by my eyes. Also it changes the format
of the timestamp column.

I think that the following might give the best human user experience:

[    0.293000][     T1] smpboot: CPU0: Intel(R) Core(TM) i5-4440S CPU @ 2.80GHz (family: 0x6, model: 0x3c, stepping: 0x3)
[    0.299733][     T1] Performance Events: Haswell events, core PMU driver.
[    2.813808][    T35] clocksource: Switched to clocksource tsc
[    2.893984][     C0] random: fast init done


> >> @@ -1037,6 +1054,9 @@ void log_buf_vmcoreinfo_setup(void)
> >>  	VMCOREINFO_OFFSET(printk_log, len);
> >>  	VMCOREINFO_OFFSET(printk_log, text_len);
> >>  	VMCOREINFO_OFFSET(printk_log, dict_len);
> >> +#ifdef CONFIG_PRINTK_FROM
> >> +	VMCOREINFO_OFFSET(printk_log, from_id);
> >> +#endif
> > 
> > The crash tool would need to be updated if anyone wanted to read
> > the log from the extended structure. Well, it might be done later
> > if people start using it more widely.
> 
> Since syzbot can utilize output from only normal consoles, I can
> keep extended records unmodified for now.

Please, add VMCOREINFO_OFFSET(printk_log, from_id) so that crashdump
can be updated when necessary.

> > 
> > I think about adding one more filed "u8 version". It would help
> > to solve the external compatibility in the long term.
> 
> /dev/kmsg format allows adding more fields, but that format did not define
> how to tell what fields are there. If fields are conditionally added by
> kernel config options, I don't think that "u8 version" field helps.
> Unless we add fields unconditionally, we will need to use $name=$value
> (where $name and $value must not contain ',' and ';') representation.

/dev/kmsg uses key=value notation. It does not need any version. The
version filed was intended for crashdump. It would make the life
easier for its maintainers.

Well, I do not resist on it. Let's put the version field aside for now.

Best Regards,
Petr