linux-kernel - Re: + printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch added to -mm tree

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Date:	Tue, 6 May 2014 15:05:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Jan Kara <jack@...e.cz>,
	"mm-commits@...r.kernel.org" <mm-commits@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"kay@...y.org" <kay@...y.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: +
 printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch
 added to -mm tree

On Tue, 6 May 2014 14:12:34 +0100 Will Deacon <will.deacon@....com> wrote:

> > > > My opinion is that when you are printing from each and every interrupt
> > > > which happens so often, then you have a problem and disabling IRQs in
> > > > printk so that your interrupt doesn't happen that often seems like a poor
> > > > solution to me. You could as well just ratelimit your debug messages,
> > > > couldn't you?
> > > 
> > > My use-case was basically using printk as a debug trace during early boot
> > > when bringing up Linux on a new CPU core. I don't think ratelimiting would
> > > be the right thing there, since I really did want as many messages to
> > > reach the console as possible (which is also why I wrote this patch, not
> > > just the other one in the series).
> >   OK, I understand. It just seems wrong to me to throttle all interrupts on
> > the cpu while doing printing just because someone might be doing debug
> > printing from the interrupt. Sure it's fine as a debug hack but on a
> > production machine that seems rather counterproductive.
> 
> Perhaps, but the one time I *really* want printk to be reliable is when I'm
> using it to debug a problem.

If you're debugging a problem, you're able to alter printk!  So perhaps
one way out of this is some developer-only ifdef to robustify printk
for particular usage patterns.

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