mm: page_alloc: take the ALLOC_NO_WATERMARK check out of the fast path

ALLOC_NO_WATERMARK is set in a few cases.  Always by kswapd, always for
__GFP_MEMALLOC, sometimes for swap-over-nfs, tasks etc.  Each of these
cases are relatively rare events but the ALLOC_NO_WATERMARK check is an
unlikely branch in the fast path.  This patch moves the check out of the
fast path and after it has been determined that the watermarks have not
been met.  This helps the common fast path at the cost of making the slow
path slower and hitting kswapd with a performance cost.  It's a reasonable
tradeoff.

Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 30f327a720fd746f1841a36e7db26bb440406716..485932c577e7630aaf96fc1fbf4c6ea4e0ca027b 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1937,9 +1937,6 @@ zonelist_scan:
                        (alloc_flags & ALLOC_CPUSET) &&
                        !cpuset_zone_allowed_softwall(zone, gfp_mask))
                                continue;
-               BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
-               if (unlikely(alloc_flags & ALLOC_NO_WATERMARKS))
-                       goto try_this_zone;
                /*
                 * Distribute pages in proportion to the individual
                 * zone size to ensure fair page aging.  The zone a
@@ -1986,6 +1983,11 @@ zonelist_scan:
                                       classzone_idx, alloc_flags)) {
                        int ret;
 
+                       /* Checked here to keep the fast path fast */
+                       BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
+                       if (alloc_flags & ALLOC_NO_WATERMARKS)
+                               goto try_this_zone;
+
                        if (IS_ENABLED(CONFIG_NUMA) &&
                                        !did_zlc_setup && nr_online_nodes > 1) {
                                /*