readahead: reduce unnecessary mmap_miss increases

The original INT_MAX is too large, reduce it to

- avoid unnecessarily dirtying/bouncing the cache line

- restore mmap read-around faster on changed access pattern

Background: in the mosbench exim benchmark which does multi-threaded page
faults on shared struct file, the ra->mmap_miss updates are found to cause
excessive cache line bouncing on tmpfs.  The ra state updates are needless
for tmpfs because it actually disabled readahead totally
(shmem_backing_dev_info.ra_pages == 0).

Tested-by: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index c974a28..e513139 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1566,7 +1566,8 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
                return;
        }
 
-       if (ra->mmap_miss < INT_MAX)
+       /* Avoid banging the cache line if not needed */
+       if (ra->mmap_miss < MMAP_LOTSAMISS * 10)
                ra->mmap_miss++;
 
        /*