linux-kernel - Re: Weird character in kernel message

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Date:	Mon, 15 Sep 2014 18:37:40 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Weird character in kernel message

On 2014.09.14 at 11:13 +0200, Geert Uytterhoeven wrote:
> On Sun, Sep 14, 2014 at 7:54 AM, Markus Trippelsdorf
> <markus@...ppelsdorf.de> wrote:
> > On 2014.09.14 at 07:09 +0200, Markus Trippelsdorf wrote:
> >> Just noticed this today:
> >>
> >> Sep 14 06:51:57 x4 kernel: [sched_delayed] ^a4CE: hpet increased min_delta_ns to 20115 nsec
> >>
> >> in hex:
> >> 20 01 34 43 45 3A 20
> >>
> >> Must be a recent regression.
> >
> > It looks like a combination of commit 504d58745c9ca and commit
> > 458df9fd4815b causes the issue.
> > 458df9fd4815b changed the loglevel of printk_deferred to a hardcoded
> > KERN_WARNING. And 504d58745c9ca changed the printk in
> > kernel/time/clockevents.c to printk_deferred.
> > But now the KERN_WARNING loglevel of printk_deferred in
> > kernel/time/clockevents.c is redundant and responsible for the weird
> > 01 34 character combination (KERN_SOH "4").
> 
> vprintk_emit() has:
> 
>         if (in_sched)
>                 text_len = scnprintf(text, sizeof(textbuf),
>                                      KERN_WARNING "[sched_delayed] ");
> 
>         text_len += vscnprintf(text + text_len,
>                                sizeof(textbuf) - text_len, fmt, args);
> 
> Hence if in_sched, "[sched_delayed] " is inserted before the printed
> message. As most printed messages have a KERN_* prefix, this
> prefix now ends up in the middle of the message, causing the issue.
> 
> A simple solution is to print a newline after the "[sched_delayed]", but
> this will break the line.
> 
> Better, but more complicated, would be to remove the KERN_* prefix
> (if it exists) from the next message if in_sched is set.

Or just get rid of the "[sched_delayed] " thing. Does it really convey
useful information to the user?

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1ce770687ea8..b13c8650a751 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1680,12 +1680,8 @@ asmlinkage int vprintk_emit(int facility, int level,
 	 * The printf needs to come first; we need the syslog
 	 * prefix which might be passed-in as a parameter.
 	 */
-	if (in_sched)
-		text_len = scnprintf(text, sizeof(textbuf),
-				     KERN_WARNING "[sched_delayed] ");
 
-	text_len += vscnprintf(text + text_len,
-			       sizeof(textbuf) - text_len, fmt, args);
+	text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
 
 	/* mark and strip a trailing newline */
 	if (text_len && text[text_len-1] == '\n') {

-- 
Markus
--
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