tree ecdbfa22cb2278c5dc97ab6217bcb468f642d527
parent d8846374a85f4290a473a4e2a64c1ba046c4a0e1
author Mel Gorman <mgorman@suse.de> 1401923434 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> 1401926050 -0700

mm: avoid unnecessary atomic operations during end_page_writeback()

If a page is marked for immediate reclaim then it is moved to the tail of
the LRU list.  This occurs when the system is under enough memory pressure
for pages under writeback to reach the end of the LRU but we test for this
using atomic operations on every writeback.  This patch uses an optimistic
non-atomic test first.  It'll miss some pages in rare cases but the
consequences are not severe enough to warrant such a penalty.

While the function does not dominate profiles during a simple dd test the
cost of it is reduced.

73048     0.7428  vmlinux-3.15.0-rc5-mmotm-20140513 end_page_writeback
23740     0.2409  vmlinux-3.15.0-rc5-lessatomic     end_page_writeback

Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>