linux-kernel - Re: [PATCH] printk: git rid of [sched_delayed] message for printk

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Message-ID: <20140916212250.GI1205@quack.suse.cz>
Date:	Tue, 16 Sep 2014 23:22:50 +0200
From:	Jan Kara <jack@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kara <jack@...e.cz>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] printk: git rid of [sched_delayed] message for
 printk_deferred

On Tue 16-09-14 17:07:09, Steven Rostedt wrote:
> On Tue, 16 Sep 2014 22:35:10 +0200
> Jan Kara <jack@...e.cz> wrote:
> 
> > On Tue 16-09-14 11:13:31, Steven Rostedt wrote:
> > > On Tue, 16 Sep 2014 16:42:52 +0200
> > > Markus Trippelsdorf <markus@...ppelsdorf.de> wrote:
> > > 
> > > > commit 458df9fd hardcodes printk_deferred() to KERN_WARNING and inserts
> > > > the string "[sched_delayed] " before the actual message.
> > > > However it doesn't take into account the KERN_* prefix of the message,
> > > > that now ends up in the middle of the output:
> > > > 
> > > >  [sched_delayed] ^a4CE: hpet increased min_delta_ns to 20115 nsec
> > > > 
> > > > Fix this by just getting rid of the "[sched_delayed] " scnprintf().
> > > 
> > > I prefer the "[sched_delayed]" output. It lets us know that the output
> > > did not come out immediately, which is important sometimes during
> > > debugging. Otherwise it can confuse people, as printk is suppose to be
> > > a blocking write.
> >   This is a common misconception about printk. It isn't a blocking write
> > for ten years or more. If there happens to be someone else printing to
> > console, the difference between printk() and printk_deferred() is
> > marginal - it used to be bigger when scheduler had its own buffer but these
> > days message is inserted in the kernel ring buffer immediately. That's why
> > I don't think the prefix is useful anymore.
> 
> For the most part it's a blocking write. Yeah, if another CPU is
> writing, it wont be a blocking write, but it usually is, I know I
> depend on it (when I'm debugging, I usually don't have contention
> between CPUs). The important part is that they are done in order. A
> delayed print, wont be in order with other printks. That is still a
> crucial difference.
  printk_deffered() will be in order with other printks after your commit
458df9fd4815b47809875d57f42e16401674b621. Just printing to console itself
will be delayed to the next timer interrupt. Or am I missing something?

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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