Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

Andrew Morton
2006-10-04T03:35:14Z

On Tue, 3 Oct 2006 23:24:27 -0400
Andrew James Wade <andrew.j.wade@gmail.com> wrote:

> On Tuesday 03 October 2006 21:14, Andrew Morton wrote:
> > There are changes here: in the old code we'll avoid reading the static
> > variable. In the new code we'll read the static variable, but we'll avoid
> > evaluating the condition.
>
> Tim Chen's patch goes back to the old behaviour. I suspect the cache
> misses on __warn_once is what he is measuring. If so, the (untested)
> patch below should reduce the cache misses back to those of the old
> code.
>
> signed-off-by: Andrew Wade <andrew.j.wade@gmail.com>
> diff -rupN a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> --- a/include/asm-generic/bug.h 2006-10-03 13:58:40.000000000 -0400
> +++ b/include/asm-generic/bug.h 2006-10-03 23:17:37.000000000 -0400
> @@ -45,9 +45,10 @@
> static int __warn_once = 1; \
> typeof(condition) __ret_warn_once = (condition);\
> \
> - if (likely(__warn_once)) \
> - if (WARN_ON(__ret_warn_once)) \
> + if (unlikely(__ret_warn_once) && __warn_once) { \
> __warn_once = 0; \
> + WARN_ON(1); \
> + }; \
> unlikely(__ret_warn_once); \
> })

It might help, but we still don't know what's going on (I think).

I mean, if cache misses against __warn_once were sufficiently high for it
to affect performance, then __warn_once would be, err, in cache?