linux-kernel - Re: [PATCH] Fix WARN_ON / WARN_ON

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Date:	Tue, 3 Oct 2006 20:32:44 -0700
From:	Andrew Morton <akpm@...l.org>
To:	andrew.j.wade@...il.com
Cc:	tim.c.chen@...ux.intel.com, herbert@...dor.apana.org.au,
	linux-kernel@...r.kernel.org, leonid.i.ananiev@...el.com
Subject: Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

On Tue, 3 Oct 2006 23:24:27 -0400
Andrew James Wade <andrew.j.wade@...il.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@...il.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?
-
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