linux-kernel - Re: [PATCH] printk: robustify printk

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Date:	Mon, 11 Aug 2008 13:22:06 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	marcin.slusarz@...il.com, linux-kernel@...r.kernel.org,
	davem@...emloft.net, rostedt@...dmis.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] printk: robustify printk

On Mon, 2008-08-11 at 13:03 +0200, Andi Kleen wrote:
> Ingo Molnar <mingo@...e.hu> writes:
> 
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> >> On Fri, 2008-08-08 at 21:21 +0200, Peter Zijlstra wrote:
> >> 
> >> > The initial printk_tick() based implementation didn't suffer this
> >> > problem, should we revert to that scheme?
> >> 
> >> Just in case people care..
> >> 
> >> ---
> >> Subject: printk: robustify printk
> >> 
> >> Avoid deadlocks against rq->lock and xtime_lock by deferring the klogd 
> >> wakeup by polling from the timer tick.
> >
> > i missed most of the discussion, but this seems like the simplest (and 
> > hence ultimately the best) approach to me.
> 
> The problem is that it means any printk data output that is more
> than DMESG-BUFFER-SIZE bytes during one clock tick is going to lose data.
> It loses the natural adaption to higher printk rates that you
> got previously.
> 
> Now we could say that for debugging etc. people should switch
> to other mechanisms like relayfs, but I would still worry about
> some corner cases where losing printk data that wasn't lost before
> could be a severe regression (e.g. consider firewall log messages
> or similar)

You only loose the msgs with klogd, console still gets everything. If
firewalls are generating that much data, perhaps its time to think about
alternative ways to channel that.

> Essentially it makes printk (much?) less reliable than it was before
> in the general case. Not sure that's a good thing. So the patch
> title is definitely misleading.

Depends, I don't give a rats arse about klogd - I get everything through
serial onto another machine. 

> As Linus pointed out earlier we've survived with this restriction
> (not doing printk in the scheduler) for a long time, so is there
> really a that pressing need to change that?

Why not fix it if its acceptable - the deadlock is just ugly.

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