Messages in this thread Patch in this message | | | Subject | Re: [RFC][PATCH 02/10] mm: Remove likely() from mapping_unevictable() | From | Steven Rostedt <> | Date | Mon, 06 Dec 2010 21:22:13 -0500 |
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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; }
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