linux-kernel - Re: [patch 1/3] move WARN

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Date:	Thu, 03 Jan 2008 22:03:13 +0100
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Olof Johansson <olof@...om.net>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 1/3] move WARN_ON() out of line

Olof Johansson wrote:
> On Thu, Jan 03, 2008 at 01:56:58AM +0100, Arjan van de Ven wrote:
>> Subject: move WARN_ON() out of line
>> From: Arjan van de Ven <arjan@...ux.intel.com>
>> CC: Ingo Molnar <mingo@...e.hu>
>> CC: Andrew Morton <akpm@...ux-foundation.org>
>>
>> A quick grep shows that there are currently 1145 instances of WARN_ON
>> in the kernel. Currently, WARN_ON is pretty much entirely inlined,
>> which makes it hard to enhance it without growing the size of the kernel
>> (and getting Andrew unhappy).
>>
>> This patch moves WARN_ON() out of line entirely. I've considered keeping
>> the test inline and moving only the slowpath out of line, but I decided
>> against that: an out of line test reduces the pressure on the CPUs
>> branch predictor logic and gives smaller code, while a function call
>> to a fixed location is quite fast. Likewise I've considered doing something
>> similar to BUG() (eg use a trapping instruction) but that's not really
>> better (it needs the test inline again and recovering from an invalid
>> instruction isn't quite fun).
> 
> Hi Arjan,
> 
> I've got a couple of patches in -mm at the moment that introduces __WARN()
> and uses that (and lets architectures override __WARN, since for example
> powerpc does use trapping instructions similarly to BUG()).
> 
> The two patches in question are:
> 
> bugh-remove-have_arch_bug--have_arch_warn.patch
> powerpc-switch-to-generic-warn_on-bug_on.patch
> 
> Care to do this incrementally on top of that instead? I.e. call
> do_warn_on() from the asm-generic/bug.h __WARN() instead.
> 
> 
ok just did that; will post shortly
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