Re: [RFC][PATCH 02/10] mm: Remove likely() from mapping_unevictable()

KOSAKI Motohiro
2010-12-10T07:00:21Z

> [ Resending to Nick's real email ]
>
>
> From: Steven Rostedt <srostedt@redhat.com>
>
> The mapping_unevictable() has a likely() around the mapping parameter.
> This mapping parameter comes from page_mapping() which has an
> unlikely() that the page will be set as PAGE_MAPPING_ANON, and if
> so, it will return NULL. One would think that this unlikely() means
> that the mapping returned by page_mapping() would not be NULL, but
> where page_mapping() is used just above mapping_unevictable(), that
> unlikely() is incorrect most of the time. This means that the
> "likely(mapping)" in mapping_unevictable() is incorrect most of the
> time.
>
> Running the annotated branch profiler on my main box which runs firefox,
> evolution, xchat and is part of my distcc farm, I had this:
>
> correct incorrect % Function File Line
> ------- --------- - -------- ---- ----
> 12872836 1269443893 98 mapping_unevictable pagemap.h 51
> 35935762 1270265395 97 page_mapping mm.h 659
> 1306198001 143659 0 page_mapping mm.h 657
> 203131478 121586 0 page_mapping mm.h 657
> 5415491 1116 0 page_mapping mm.h 657
> 74899487 1116 0 page_mapping mm.h 657
> 203132845 224 0 page_mapping mm.h 659
> 5415464 27 0 page_mapping mm.h 659
> 13552 0 0 page_mapping mm.h 657
> 13552 0 0 page_mapping mm.h 659
> 242630 0 0 page_mapping mm.h 657
> 242630 0 0 page_mapping mm.h 659
> 74899487 0 0 page_mapping mm.h 659
>
> The page_mapping() is a static inline, which is why it shows up multiple
> times. The mapping_unevictable() is also a static inline but seems to
> be used only once in my setup.
>
> The unlikely in page_mapping() was correct a total of 1909540379 times and
> incorrect 1270533123 times, with a 39% being incorrect. Perhaps this is
> enough to remove the unlikely from page_mapping() as well.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Nick Piggin <nickpiggin@yahoo.com.au>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
> include/linux/pagemap.h | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 2d1ffe3..9c66e99 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -48,7 +48,7 @@ static inline void mapping_clear_unevictable(struct address_space *mapping)
>
> static inline int mapping_unevictable(struct address_space *mapping)
> {
> - if (likely(mapping))
> + if (mapping)
> return test_bit(AS_UNEVICTABLE, &mapping->flags);
> return !!mapping;
> }

I think you are right.
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>