Commit 30534755 authored May 19, 2016 by Mel Gorman Committed by Linus Torvalds May 19, 2016

mm, page_alloc: reduce cost of fair zone allocation policy retry

The fair zone allocation policy is not without cost but it can be
reduced slightly. This patch removes an unnecessary local variable,
checks the likely conditions of the fair zone policy first, uses a bool
instead of a flags check and falls through when a remote node is
encountered instead of doing a full restart. The benefit is marginal
but it's there

4.6.0-rc2 4.6.0-rc2
decstat-v1r20 optfair-v1r20
Min alloc-odr0-1 377.00 ( 0.00%) 380.00 ( -0.80%)
Min alloc-odr0-2 273.00 ( 0.00%) 273.00 ( 0.00%)
Min alloc-odr0-4 226.00 ( 0.00%) 227.00 ( -0.44%)
Min alloc-odr0-8 196.00 ( 0.00%) 196.00 ( 0.00%)
Min alloc-odr0-16 183.00 ( 0.00%) 183.00 ( 0.00%)
Min alloc-odr0-32 175.00 ( 0.00%) 173.00 ( 1.14%)
Min alloc-odr0-64 172.00 ( 0.00%) 169.00 ( 1.74%)
Min alloc-odr0-128 170.00 ( 0.00%) 169.00 ( 0.59%)
Min alloc-odr0-256 183.00 ( 0.00%) 180.00 ( 1.64%)
Min alloc-odr0-512 191.00 ( 0.00%) 190.00 ( 0.52%)
Min alloc-odr0-1024 199.00 ( 0.00%) 198.00 ( 0.50%)
Min alloc-odr0-2048 204.00 ( 0.00%) 204.00 ( 0.00%)
Min alloc-odr0-4096 210.00 ( 0.00%) 209.00 ( 0.48%)
Min alloc-odr0-8192 213.00 ( 0.00%) 213.00 ( 0.00%)
Min alloc-odr0-16384 214.00 ( 0.00%) 214.00 ( 0.00%)

The benefit is marginal at best but one of the most important benefits,
avoiding a second search when falling back to another node is not
triggered by this particular test so the benefit for some corner cases
is understated.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 4fcb0971

mm/page_alloc.c

+14 −18

Original line number	Diff line number	Diff line
		@@ -2680,12 +2680,10 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
		{
		struct zoneref *z;
		struct zone *zone;
		bool fair_skipped;
		bool zonelist_rescan;
		bool fair_skipped = false;
		bool apply_fair = (alloc_flags & ALLOC_FAIR);

		zonelist_scan:
		zonelist_rescan = false;

		/*
		* Scan zonelist, looking for a zone with enough free.
		* See also __cpuset_node_allowed() comment in kernel/cpuset.c.
		@@ -2705,13 +2703,16 @@ zonelist_scan:
		* page was allocated in should have no effect on the
		* time the page has in memory before being reclaimed.
		*/
		if (alloc_flags & ALLOC_FAIR) {
		if (!zone_local(ac->preferred_zone, zone))
		break;
		if (apply_fair) {
		if (test_bit(ZONE_FAIR_DEPLETED, &zone->flags)) {
		fair_skipped = true;
		continue;
		}
		if (!zone_local(ac->preferred_zone, zone)) {
		if (fair_skipped)
		goto reset_fair;
		apply_fair = false;
		}
		}
		/*
		* When allocating a page cache page for writing, we
		@@ -2800,18 +2801,13 @@ try_this_zone:
		* include remote zones now, before entering the slowpath and waking
		* kswapd: prefer spilling to a remote zone over swapping locally.
		*/
		if (alloc_flags & ALLOC_FAIR) {
		alloc_flags &= ~ALLOC_FAIR;
		if (fair_skipped) {
		zonelist_rescan = true;
		reset_fair:
		apply_fair = false;
		fair_skipped = false;
		reset_alloc_batches(ac->preferred_zone);
		}
		if (nr_online_nodes > 1)
		zonelist_rescan = true;
		}

		if (zonelist_rescan)
		goto zonelist_scan;
		}

		return NULL;
		}

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