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

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Message-ID: <20140920180800.GS2832@worktop.localdomain>
Date:	Sat, 20 Sep 2014 20:08:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Joe Perches <joe@...ches.com>, 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>
Subject: Re: [PATCH] printk: git rid of [sched_delayed] message for
 printk_deferred

On Sat, Sep 20, 2014 at 12:30:01PM -0400, Steven Rostedt wrote:
> On Sat, 20 Sep 2014 09:10:47 -0700
> Joe Perches <joe@...ches.com> wrote:
> 
> > On Sat, 2014-09-20 at 17:47 +0200, Peter Zijlstra wrote:
> > > On a whole, printk() is entirely useless for debugging these days, its
> > > far too fragile/unreliable to be taken seriously so I really don't care
> > > on that point either.
> > 
> > That's unfortunate.
> > 
> > Care to enumerate the issues that you believe make
> > printk too fragile/unreliable for debugging?
> 
> I seldom use printk these days. It's far too limited in its uses. For
> one, most things worth debugging happen thousands of times a second,
> and printk will just slow things down to a crawl if it is used.
> Another, is that it can not be used in most critical sections (NMI
> handlers and anything that deals with the scheduler). Also, as it no
> longer blocks when another CPU is doing a printk, a bug can happen
> which crashes the system and the output of that bug will never get
> printed due to the delayed output from another CPU having the console
> lock.

If you're trying to debug a dying machine the early_serial_console which
basically only does a stream of OUTBs to the right port is the only
reliable thing. For anything else, trace_printk().

Now all I need to figure out is how to keep suspend from killing the
early_serial_console setup :-)
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