linux-kernel - Re: printk() vs tty

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Date:	Thu, 15 Dec 2011 19:07:25 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ted Ts'o <tytso@....edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <greg@...ah.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: printk() vs tty_io

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Dec 15, 2011 at 1:56 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > We probably could, I can have a closer look, but the main question is,
> > are we going to commit to no wakeups from console implementations? That
> > would mean removing the USB serial console support and other such stuff.
> 
> I guess we can't.
> 
> So new approach:
> 
>  - screw it. the rq lock is just too central, you cannot call 
> printk from under it. And if you have an oops under it, you're 
> screwed.
> 
>    Peter, why do you want to try to work from under the rq 
>    lock?

Well, no real strong reason: we had that nasty xtime_lock 
related lockup in printk() (which was arguably much worse than 
the rq lock dependency - and it got fixed), and everyone assumed 
that somehow we could just remove the rq lock dependency from 
printk() as well.

It appears we can't - and that's OK.

Was an intellectual excercise which didnt work out.

>  - Make a special "debug printk" that is not synchronous. Just 
> make it buffer things, and have it actually print out from a 
> worker thread or whatever. This one *only* takes the lock for 
> that buffer itself, and works everywhere. We could probably 
> even do tricks to make it NMI-safe.

We kind of already have such a thing: trace_printk() - which 
works from the weirdest of atomic contexts as well.

IIRC Peter uses trace_printk() to develop the scheduler all the 
time.

What we could perhaps do is a sort of 'shut up regular printk 
and feed all printks into the trace buffers' kind of debug 
switch.

>    Make code that isn't an oops or other very synchronous 
> "have to print out *now*" aim to use this "softer" printk.

For early and nast oopses we have a very primitive printk: 
earlyprintk=keep which can just act as a full printk 
replacement. I use earlyprintk=keep while regular printk is 
disabled (there's no console).

[ earlyprintk can be used to debug everything except printk() 
  lockups, obviously. ]

Thanks,

	Ingo
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