cpusets: new round-robin rotor for SLAB allocations

We have observed several workloads running on multi-node systems where
memory is assigned unevenly across the nodes in the system.  There are
numerous reasons for this but one is the round-robin rotor in
cpuset_mem_spread_node().

For example, a simple test that writes a multi-page file will allocate
pages on nodes 0 2 4 6 ...  Odd nodes are skipped.  (Sometimes it
allocates on odd nodes & skips even nodes).

An example is shown below.  The program "lfile" writes a file consisting
of 10 pages.  The program then mmaps the file & uses get_mempolicy(...,
MPOL_F_NODE) to determine the nodes where the file pages were allocated.
The output is shown below:

	# ./lfile
	 allocated on nodes: 2 4 6 0 1 2 6 0 2

There is a single rotor that is used for allocating both file pages & slab
pages.  Writing the file allocates both a data page & a slab page
(buffer_head).  This advances the RR rotor 2 nodes for each page
allocated.

A quick confirmation seems to confirm this is the cause of the uneven
allocation:

	# echo 0 >/dev/cpuset/memory_spread_slab
	# ./lfile
	 allocated on nodes: 6 7 8 9 0 1 2 3 4 5

This patch introduces a second rotor that is used for slab allocations.

Signed-off-by: Jack Steiner <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: Jack Steiner <[email protected]>
Cc: Robin Holt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/slab.c

@@ -3219,7 +3219,7 @@ static void *alternate_node_alloc(struct kmem_cache *cachep, gfp_t flags)
 	nid_alloc = nid_here = numa_node_id();
 	get_mems_allowed();
 	if (cpuset_do_slab_mem_spread() && (cachep->flags & SLAB_MEM_SPREAD))
-		nid_alloc = cpuset_mem_spread_node();
+		nid_alloc = cpuset_slab_spread_node();
 	else if (current->mempolicy)
 		nid_alloc = slab_node(current->mempolicy);
 	put_mems_allowed();