Re: [patch 1/3] move WARN_ON() out of line

[Olof Johansson]

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@xxxxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>
> 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.


-Olof

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