mm: do_migrate_pages() calls migrate_to_node() even if task is already on a correct node

While running an application that moves tasks from one cpuset to another
I noticed that it takes much longer and moves many more pages than
expected.

The reason for this is do_migrate_pages() does its best to preserve the
relative node differential from the first node of the cpuset because the
application may have been written with that in mind.  If memory was
interleaved on the nodes of the source cpuset by an application
do_migrate_pages() will try its best to maintain that interleaving on
the nodes of the destination cpuset.  This means copying the memory from
all source nodes to the destination nodes even if the source and
destination nodes overlap.

This is a problem for userspace NUMA placement tools.  The amount of
time spent doing extra memory moves cancels out some of the NUMA
performance improvements.  Furthermore, if the number of source and
destination nodes are to maintain the previous interleaving layout
anyway.

This patch changes do_migrate_pages() to only preserve the relative
layout inside the program if the number of NUMA nodes in the source and
destination mask are the same.  If the number is different, we do a much
more efficient migration by not touching memory that is in an allowed
node.

This preserves the old behaviour for programs that want it, while
allowing a userspace NUMA placement tool to use the new, faster
migration.  This improves performance in our tests by up to a factor of
7.

Without this change migrating tasks from a cpuset containing nodes 0-7
to a cpuset containing nodes 3-4, we migrate from ALL the nodes even if
they are in the both the source and destination nodesets:

   Migrating 7 to 4
   Migrating 6 to 3
   Migrating 5 to 4
   Migrating 4 to 3
   Migrating 1 to 4
   Migrating 3 to 4
   Migrating 0 to 3
   Migrating 2 to 3

With this change we only migrate from nodes that are not in the
destination nodesets:

   Migrating 7 to 4
   Migrating 6 to 3
   Migrating 5 to 4
   Migrating 2 to 3
   Migrating 1 to 4
   Migrating 0 to 3

Yet if we move from a cpuset containing nodes 2,3,4 to a cpuset
containing 3,4,5 we still do move everything so that we preserve the
desired NUMA offsets:

   Migrating 4 to 5
   Migrating 3 to 4
   Migrating 2 to 3

As far as performance is concerned this simple patch improves the time
it takes to move 14, 20 and 26 large tasks from a cpuset containing
nodes 0-7 to a cpuset containing nodes 1 & 3 by up to a factor of 7.
Here are the timings with and without the patch:

BEFORE PATCH -- Move times: 59, 140, 651 seconds
============

  Moving 14 tasks from nodes (0-7) to nodes (1,3)
  numad(8780) do_migrate_pages (mm=0xffff88081d414400
  from_nodes=0xffff880818c81d28 to_nodes=0xffff880818c81ce8 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x7 dest=0x3 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x6 dest=0x1 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x5 dest=0x3 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x4 dest=0x1 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x2 dest=0x1 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x1 dest=0x3 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d414400 source=0x0 dest=0x1 flags=0x4)
  (Above moves repeated for each of the 14 tasks...)
  PID 8890 moved to node(s) 1,3 in 59.2 seconds

  Moving 20 tasks from nodes (0-7) to nodes (1,4-5)
  numad(8780) do_migrate_pages (mm=0xffff88081d88c700
  from_nodes=0xffff880818c81d28 to_nodes=0xffff880818c81ce8 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d88c700 source=0x7 dest=0x4 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d88c700 source=0x6 dest=0x1 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d88c700 source=0x3 dest=0x1 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d88c700 source=0x2 dest=0x5 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d88c700 source=0x1 dest=0x4 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d88c700 source=0x0 dest=0x1 flags=0x4)
  (Above moves repeated for each of the 20 tasks...)
  PID 8962 moved to node(s) 1,4-5 in 139.88 seconds

  Moving 26 tasks from nodes (0-7) to nodes (1-3,5)
  numad(8780) do_migrate_pages (mm=0xffff88081d5bc740
  from_nodes=0xffff880818c81d28 to_nodes=0xffff880818c81ce8 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x7 dest=0x5 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x6 dest=0x3 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x5 dest=0x2 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x3 dest=0x5 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x2 dest=0x3 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x1 dest=0x2 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x0 dest=0x1 flags=0x4)
  numad(8780) migrate_to_node (mm=0xffff88081d5bc740 source=0x4 dest=0x1 flags=0x4)
  (Above moves repeated for each of the 26 tasks...)
  PID 9058 moved to node(s) 1-3,5 in 651.45 seconds

AFTER PATCH -- Move times: 42, 56, 93 seconds
===========

  Moving 14 tasks from nodes (0-7) to nodes (5,7)
  numad(33209) do_migrate_pages (mm=0xffff88101d5ff140
  from_nodes=0xffff88101e7b5d28 to_nodes=0xffff88101e7b5ce8 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d5ff140 source=0x6 dest=0x5 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d5ff140 source=0x4 dest=0x5 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d5ff140 source=0x3 dest=0x7 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d5ff140 source=0x2 dest=0x5 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d5ff140 source=0x1 dest=0x7 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d5ff140 source=0x0 dest=0x5 flags=0x4)
  (Above moves repeated for each of the 14 tasks...)
  PID 33221 moved to node(s) 5,7 in 41.67 seconds

  Moving 20 tasks from nodes (0-7) to nodes (1,3,5)
  numad(33209) do_migrate_pages (mm=0xffff88101d6c37c0
  from_nodes=0xffff88101e7b5d28 to_nodes=0xffff88101e7b5ce8 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d6c37c0 source=0x7 dest=0x3 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d6c37c0 source=0x6 dest=0x1 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d6c37c0 source=0x4 dest=0x3 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d6c37c0 source=0x2 dest=0x5 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d6c37c0 source=0x0 dest=0x1 flags=0x4)
  (Above moves repeated for each of the 20 tasks...)
  PID 33289 moved to node(s) 1,3,5 in 56.3 seconds

  Moving 26 tasks from nodes (0-7) to nodes (1,3,5,7)
  numad(33209) do_migrate_pages (mm=0xffff88101d924400
  from_nodes=0xffff88101e7b5d28 to_nodes=0xffff88101e7b5ce8 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d924400 source=0x6 dest=0x5 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d924400 source=0x4 dest=0x1 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d924400 source=0x2 dest=0x5 flags=0x4)
  numad(33209) migrate_to_node (mm=0xffff88101d924400 source=0x0 dest=0x1 flags=0x4)
  (Above moves repeated for each of the 26 tasks...)
  PID 33372 moved to node(s) 1,3,5,7 in 92.67 seconds

[akpm@linux-foundation.org: clean up comment layout]
Signed-off-by: NLarry Woodman <lwoodman@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: NRik van Riel <riel@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1005,6 +1005,26 @@ int do_migrate_pages(struct mm_struct *mm,
 		int dest = 0;
 
 		for_each_node_mask(s, tmp) {
+
+			/*
+			 * do_migrate_pages() tries to maintain the relative
+			 * node relationship of the pages established between
+			 * threads and memory areas.
+                         *
+			 * However if the number of source nodes is not equal to
+			 * the number of destination nodes we can not preserve
+			 * this node relative relationship.  In that case, skip
+			 * copying memory from a node that is in the destination
+			 * mask.
+			 *
+			 * Example: [2,3,4] -> [3,4,5] moves everything.
+			 *          [0-7] - > [3,4,5] moves only 0,1,2,6,7.
+			 */
+
+			if ((nodes_weight(*from_nodes) != nodes_weight(*to_nodes)) &&
+						(node_isset(s, *to_nodes)))
+				continue;
+
 			d = node_remap(s, *from_nodes, *to_nodes);
 			if (s == d)
 				continue;