linux-kernel - Re: [RFC][PATCH 05/10] mm: Remove likely() from grab_cache_page_write

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Date:	Mon, 06 Dec 2010 21:24:10 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC][PATCH 05/10] mm: Remove likely() from
 grab_cache_page_write_begin()

[ Resending to Nick's real email address ]

From: Steven Rostedt <srostedt@...hat.com>

Running the annotated branch profiler on a box doing average work
(firefox, evolution, xchat, distcc farm), the likely() used in
grab_cache_page_write_begin() was incorrect most of the time:

 correct incorrect  %        Function                  File              Line
 ------- ---------  -        --------                  ----              ----
 1924262 71332401  97 grab_cache_page_write_begin    filemap.c           2206

Adding a trace_printk() and running the function tracer limited to
just this function I can see:

        gconfd-2-2696  [000]  4467.268935: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=7
        gconfd-2-2696  [000]  4467.268946: grab_cache_page_write_begin <-ext3_write_begin
        gconfd-2-2696  [000]  4467.268947: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=8
        gconfd-2-2696  [000]  4467.268959: grab_cache_page_write_begin <-ext3_write_begin
        gconfd-2-2696  [000]  4467.268960: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=9
        gconfd-2-2696  [000]  4467.268972: grab_cache_page_write_begin <-ext3_write_begin
        gconfd-2-2696  [000]  4467.268973: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=10
        gconfd-2-2696  [000]  4467.268991: grab_cache_page_write_begin <-ext3_write_begin
        gconfd-2-2696  [000]  4467.268992: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=11
        gconfd-2-2696  [000]  4467.269005: grab_cache_page_write_begin <-ext3_write_begin

Which shows that a lot of calls from ext3_write_begin will result in
the page returned by "find_lock_page" will be NULL.

Cc: Nick Piggin <npiggin@...nel.dk>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 mm/filemap.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index ea89840..d557fe7 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2218,7 +2218,7 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
 		gfp_notmask = __GFP_FS;
 repeat:
 	page = find_lock_page(mapping, index);
-	if (likely(page))
+	if (page)
 		return page;
 
 	page = __page_cache_alloc(mapping_gfp_mask(mapping) & ~gfp_notmask);


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