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

[Steven Rostedt]

[ Resending to Nick's real email ]


From: Steven Rostedt <srostedt@xxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxx>
Cc: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 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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