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

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Message-ID: <20140920154733.GM2832@worktop.localdomain>
Date:	Sat, 20 Sep 2014 17:47:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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 07:12:24AM +0200, Jan Kara wrote:
> On Thu 18-09-14 19:34:14, Peter Zijlstra wrote:
> > On Wed, Sep 17, 2014 at 08:31:35PM -0400, Steven Rostedt wrote:
> > > I totally didn't get what you wrote.
> > 
> > :-)
> > 
> > > We don't want to know if it got delayed, then the patch to remove that
> > > print seems correct.
> > 
> > Why would you not want to know that? Also was that the actual argument?
> > Lemme go check the earlier emails -- I cannot find that argument in the
> > first few emails.
>   Well, so what gets delayed is printing from kernel buffer to console.
> So this is the same as when you do printk() when console lock is taken by
> someone else. So it seems a bit strange to prepend [delayed] in some cases
> and not in others.

The difference is that when someone else has the console lock, he
guarantees it gets out. Whereas with the delayed thing it can take a
virtual forever to get out.

> Another question is what the [delayed] prefix would be useful for? If the
> message eventually gets printed to console I don't see why you would care
> it was printed few ms after it entered the kernel buffer (after all the
> time stamp before the line will be the time when it entered the kernel
> buffer). And if the kernel crashes in such a way that the message doesn't
> get printed, then bad luck but prefix in the kernel log buffer isn't going
> to make that any better :)
> 
> This all feels like bikeshedding, I don't deeply care what gets done but I
> wanted to point out I don't really see a use for [delayed]...

Sure, I was just pointing out that those arguments had not been made. I
think you're right, if you see the msg it obviously made it out. If you
don't see it, you don't know either way.

But a patch removing it _must_ make those arguments, it did not.

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.
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