Date:	Mon, 12 Oct 2009 16:30:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Simon Kagstrom <simon.kagstrom@...insight.net>
Cc:	Artem Bityutskiy <dedekind1@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Koskinen Aaro (Nokia-D/Helsinki)" <aaro.koskinen@...ia.com>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] panic.c: export panic_on_oops


* Simon Kagstrom <simon.kagstrom@...insight.net> wrote:

> OK, I don't think we understand each other. Sorry if I'm being slow 
> here, please tell me if I'm misunderstanding something fundamental 
> below.

[ it could easily be me being confused - i dont know the mtdoops code 
  that well - i just raised an eyebrow at the export request, which 
  yelled 'layering violation' at me ;-) ]

> On Mon, 12 Oct 2009 15:15:29 +0200
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > > I'm afraid I don't really see this issue. The workqueue is used to 
> > > write the buffer to the mtd device if we are not in a panic or 
> > > interrupt context - in which case we do it directly.
> > > 
> > > So it's only used when an oops is ongoing.
> > 
> > This fixation on 'panic' is so wrong!
> > 
> > 90% of the bugs users care about dont involve any panic. And even if 
> > there is a panic down the line, most of the interesting messages are in 
> > the stream leading up to the panic - now tucked away in that async 
> > workqueue mechanism and not visible.
> 
> Well, this is what my patch [1] aims to fix. What it does is to put 
> all messages in a circular buffer, and when an oops or panic occurs it 
> writes them out. The current version only collects messages _during_ 
> an oops. I'll rework it with using kfifo as per Alans suggestion 
> though.
> 
> Neither the current code nor the new patch has them stored in the work 
> queue during a panic though. If this happens, they will call 
> panic_write (if it's available) to write it out directly.
> 
> > There's two clean solutions i think:
> > 
> > 1) add some new "ok, there's trouble!" callback to struct console and 
> >    the console driver could via that mechanism send out the _last_ 2KB 
> >    (or more) of kernel log messages. Basically we can go back in time by 
> >    looking at the dmesg buffer. The low level console driver does not 
> >    need to 'follow' the high level console state - it only wants to 
> >    print in case of trouble anyway.
> >
> > 2) or add buffered (flash-friendly) writes for all printk output - panic
> >    and non-panic alike. This would be useful to debug suspend/resume
> >    bugs for example. This would also optimize the packets of netconsole
> >    output. (last i checked we sent a packet per line.)
> 
> Well, suspend/resume hangs is one of the cases which mtdoops won't 
> catch. [...]

( Sidenote: i see no reason why that wouldnt be possible if it's 
  implemented properly. )

> [...] But at least on NAND flash, I'd be a bit weary about logging all 
> printk output for fear of wearing out the flash.

Clearly should be optional - like the s2ram debug hack to RTC registers 
is optional on x86.

> > The workqueue looks wrong in both variants. If we are panic-ing (or 
> > hanging, or ...) then we are halting the machine - the workqueue has 
> > no chance to actually execute.
> 
> but then we are using mtd->panic_write to write it out directly, not 
> via the work queue.

... i might be confused, but in which case _is_ the workqueue used?

It clearly shows up in the codepaths i've read, but maybe i've 
misinterpreted what it does.

	Ingo
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