iozone regression with 2.6.29-rc6

[Lin Ming <ming.m.lin@intel.com>]

bisect locates below commits,

commit 1cf6e7d83bf334cc5916137862c920a97aabc018
Author: Nick Piggin <npiggin@suse.de>
Date:   Wed Feb 18 14:48:18 2009 -0800

    mm: task dirty accounting fix

    YAMAMOTO-san noticed that task_dirty_inc doesn't seem to be called properly for
    cases where set_page_dirty is not used to dirty a page (eg. mark_buffer_dirty).

    Additionally, there is some inconsistency about when task_dirty_inc is
    called.  It is used for dirty balancing, however it even gets called for
    __set_page_dirty_no_writeback.

    So rather than increment it in a set_page_dirty wrapper, move it down to
    exactly where the dirty page accounting stats are incremented.

    Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
    Signed-off-by: Nick Piggin <npiggin@suse.de>
    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>


below data in parenthesis is the result after above commit reverted, for example,
-10% (+2%) means,
iozone has ~10% regression with 2.6.29-rc6 compared with 2.6.29-rc5.
and
iozone has ~2% improvement with 2.6.29-rc6-revert-1cf6e7d compared with 2.6.29-rc5.


			4P dual-core HT	 	2P qual-core  	2P qual-core HT
			tulsa		   	stockley	Nehalem
			--------------------------------------------------------
iozone-rewrite		-10% (+2%)		-8% (0%)	-10% (-7%)
iozone-rand-write	-50% (0%)		-20% (+10%)
iozone-read					-13% (0%)
iozone-write					-28% (-1%)
iozone-reread							-5% (-1%)
iozone-mmap-read						-7% (+2%)
iozone-mmap-reread						-7% (+2%)
iozone-mmap-rand-read						-7% (+3%)
iozone-mmap-rand-write						-5% (0%)

Lin Ming