Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [RFC][PATCH 02/10] mm: Remove likely() from mapping_unevictable() | Date | Fri, 10 Dec 2010 16:00:16 +0900 (JST) |
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> [ 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>
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