linux-kernel - Re: printk: Add process name information to printk() output.

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Date:   Fri, 04 Sep 2020 11:53:42 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Changki Kim <changki.kim@...sung.com>, pmladek@...e.com
Cc:     sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        changbin.du@...el.com, masahiroy@...nel.org, rd.dunlap@...il.com,
        gregkh@...uxfoundation.org, krzk@...nel.org,
        linux-kernel@...r.kernel.org, Changki Kim <changki.kim@...sung.com>
Subject: Re: printk: Add process name information to printk() output.

On 2020-09-04, Changki Kim <changki.kim@...sung.com> wrote:
> Printk() meesages are the most basic and useful debug method.
> However, additional information needs in multi-processor.
> If we add messages with processor id and process name, we can find
> a problem only with messages when the problem occurs with H/W IP or CPU.
> This is very useful in narrowing down the scope of the problems.

[...]

> diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
> index e6302da041f9..fcefe9516606 100644
> --- a/kernel/printk/printk_ringbuffer.h
> +++ b/kernel/printk/printk_ringbuffer.h
> @@ -21,6 +22,12 @@ struct printk_info {
>  	u8	flags:5;	/* internal record flags */
>  	u8	level:3;	/* syslog level */
>  	u32	caller_id;	/* thread id or processor id */
> +#ifdef CONFIG_PRINTK_PROCESS
> +	int	pid;			/* process id */
> +	u8	cpu_id;			/* processor id */
> +	u8	in_interrupt;		/* interrupt conext */
> +	char process[TASK_COMM_LEN];	/* process name */
> +#endif
>  };

I can understand the desire to have more information with messages. But
IMHO adding it to the ringbuffer descriptor is the wrong place for
it. The descriptor should really be limited to data that the printk
subsystem needs for _itself_. With respect to LOG_CONT, I think we can
agree that @caller_id is not enough. But there has been discussions [0]
of having @caller_id provide a better context representation.

If we want to support adding more meta information to messages, I would
prefer that the information is either prepended directly to the message
text string or appended to the dictionary text string. We could even go
so far as providing a boot argument where a list of information could be
specified, what should be automatically added to the text/dict strings
of each message. That would not require any ringbuffer changes and would
allow new types of information to be added later.

Something like:

    printk.format=ts,cpu,comm,pid,in_atomic

John Ogness

[0] https://lkml.kernel.org/r/20200719143527.GA566@jagdpanzerIV.localdomain