linux-kernel - Re: [RFC PATCH 1/2] printk: Add context information to the header of /dev/kmsg

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Date:	Mon, 19 May 2014 20:35:06 +0900
From:	Kay Sievers <kay@...y.org>
To:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Petr Mladek <pmladek@...e.cz>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	yrl.pp-manager.tt@...achi.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Joe Perches <joe@...ches.com>, Arun KS <arunks.linux@...il.com>
Subject: Re: [RFC PATCH 1/2] printk: Add context information to the header of /dev/kmsg

On Mon, May 19, 2014 at 7:28 PM, Yoshihiro YUNOMAE
<yoshihiro.yunomae.ez@...achi.com> wrote:
> Add context information to the header of /dev/kmsg.
>
> Two printk messages connected with KERN_CONT can be divided in multiple lines
> by a different process context message. If the different context message seems
> like the 1st divided message, it is difficult to understand which the 2nd
> divided message belongs to. This problem can also occur for the situation where
> multiple message lines without KERN_CONT are broken into by similar messages.
> For example, SCSI disk error messages can be show as follows:
>
>         [110781.736171] sd 2:0:0:0: [sdb]
>         [110781.736170] sd 3:0:0:0: [sdc] Unhandled sense code
>         [110781.736172] sd 3:0:0:0: [sdc]
>         [110781.736175] Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
>         [110781.736177] sd 3:0:0:0: [sdc]
>         [110781.736178] Sense Key : Medium Error [current]
>         [110781.736187] Sense Key : Recovered Error
>         [110781.736189] [current]

[...]

> This patch adds PID and interrupt context flag to the header of /dev/kmsg as
> the context information in order to understand relation of output messages. If
> PID values of two messages and the interrupt context flags are same, it means
> that those messages are same context, so those message have some relation. If
> not so, it means that those messages are different context, so users do not
> need to take care about the relation of the messages.

[...]

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -197,6 +197,8 @@ enum log_flags {
>
>  struct printk_log {
>         u64 ts_nsec;            /* timestamp in nanoseconds */
> +       pid_t pid;              /* identify PID */
> +       u32 irq_count;          /* identify irq_count */

I don't think it is worth to blow-up this heavily used struct up even
more, just because SCSI cannot log in simple single calls. How about
fixing SCSI to log to a local buffer if it cannot safely print one
line at once.

I'm not convinced, that turning the /dev/kmsg format into an
"annotation language', and requiring a rather complex state machine to
re-construct the "broken" logging makes too much sense here. If tools
rely on properly formatted logging messages, the logging should be
fixed at the source while it is logged, not be reconstructed later.

It is not not my call, but I don't this makes much sense. Continuation
lines are "best effort" not a facility that is or ever was reliable in
the past. I think the proper fix is how the log is created, not how it
is exported.

Thanks,
Kay
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