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    SubjectRe: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression
    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?
    -
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