[PATCH v2] mm: Reset numa stats for boot pagesets

[Sandipan Das <sandipan@linux.ibm.com>]

Initially, the per-cpu pagesets of each zone are set to the
boot pagesets. The real pagesets are allocated later but
before that happens, page allocations do occur and the numa
stats for the boot pagesets get incremented since they are
common to all zones at that point.

The real pagesets, however, are allocated for the populated
zones only. Unpopulated zones, like those associated with
memory-less nodes, continue using the boot pageset and end
up skewing the numa stats of the corresponding node.

E.g.

  $ numactl -H
  available: 2 nodes (0-1)
  node 0 cpus: 0 1 2 3
  node 0 size: 0 MB
  node 0 free: 0 MB
  node 1 cpus: 4 5 6 7
  node 1 size: 8131 MB
  node 1 free: 6980 MB
  node distances:
  node   0   1
    0:  10  40
    1:  40  10

  $ numastat
                             node0           node1
  numa_hit                     108           56495
  numa_miss                      0               0
  numa_foreign                   0               0
  interleave_hit                 0            4537
  local_node                   108           31547
  other_node                     0           24948

Hence, the boot pageset stats need to be cleared after
the real pagesets are allocated.

From this point onwards, the stats of the boot pagesets do
not change as page allocations requested for a memory-less
node will either fail (if __GFP_THISNODE is used) or get
fulfilled by a preferred zone of a different node based on
the fallback zonelist.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
---

The previous version and discussion around it can be found at
https://lore.kernel.org/linux-mm/20200504070304.127361-1-sandipan@linux.ibm.com/

Changes in v2:

- Reset the stats of the boot pagesets instead of explicitly
  returning zero as suggested by Vlastimil.

- Changed the subject to reflect the above.

---
 mm/page_alloc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 69827d4fa052..1543e32f7e4e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6256,6 +6256,25 @@ void __init setup_per_cpu_pageset(void)
 	for_each_populated_zone(zone)
 		setup_zone_pageset(zone);
 
+#ifdef CONFIG_NUMA
+	if (static_branch_likely(&vm_numa_stat_key)) {
+		struct per_cpu_pageset *pcp;
+		int cpu;
+
+		/*
+		 * Unpopulated zones continue using the boot pagesets.
+		 * The numa stats for these pagesets need to be reset.
+		 * Otherwise, they will end up skewing the stats of
+		 * the nodes these zones are associated with.
+		 */
+		for_each_possible_cpu(cpu) {
+			pcp = &per_cpu(boot_pageset, cpu);
+			memset(pcp->vm_numa_stat_diff, 0,
+			       sizeof(pcp->vm_numa_stat_diff));
+		}
+	}
+#endif
+
 	for_each_online_pgdat(pgdat)
 		pgdat->per_cpu_nodestats =
 			alloc_percpu(struct per_cpu_nodestat);
-- 
2.17.1