linux-kernel - Re: [PATCH] panic.c: export panic_on

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Date:	Mon, 12 Oct 2009 14:20:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Simon Kagstrom <simon.kagstrom@...insight.net>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Koskinen Aaro (Nokia-D/Helsinki)" <aaro.koskinen@...ia.com>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] panic.c: export panic_on_oops

* David Woodhouse <dwmw2@...radead.org> wrote:

> On Mon, 2009-10-12 at 14:09 +0200, Ingo Molnar wrote:
> > Also, would it be possible to just simplify the thing and not do any 
> > buffering at all? Extra buffering complexity in a console driver is only 
> > asking for trouble. Or is flash storage write cycles optimization that 
> > important in this case?
> 
> That and the fact that on NAND flash you have to write full pages at a 
> time -- that's 512 bytes, 2KiB or 4KiB depending on the type of chip. 
> So we really do want to buffer it where we can.
> 
> We don't want to write a 2KiB page for every line of printk output.

Then i think the buffering is at the wrong place: we should instead 
buffer in the generic layer and pass it to lowlevel if we know that we 
have gone past a 2K boundary.

The size of the generic log buffer is always a power of two so detecting 
2K boundaries is very easy. On any emergency the generic console layer 
will do faster flushes - this is nothing the console driver itself 
should bother with.

And that would avoid the whole workqueue logic - which is fragile to be 
done in a printk to begin with.

So what we need is an extension to struct console that sets a buffering 
limit. Zero (the default) means unbuffered.

(Btw., things like netconsole might make use of such buffering too.)

Agreed?

	Ingo
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