[RFC PATCH 2/2] NUMA balancing: avoid to migrate task to CPU-less node

[Huang Ying]

In a typical memory tiering system, there's no CPU in slow (PMEM) NUMA
nodes.  But if the number of the hint page faults on a PMEM node is
the max for a task, The current NUMA balancing policy may try to place
the task on the PMEM node instead of DRAM node.  This is unreasonable,
because there's no CPU in PMEM NUMA nodes.  To fix this, CPU-less
nodes are ignored when searching the migration target node for a task
in this patch.

To test the patch, we run a workload that accesses more memory in PMEM
node than memory in DRAM node.  Without the patch, the PMEM node will
be chosen as preferred node in task_numa_placement().  While the DRAM
node will be chosen instead with the patch.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 54e1aad1c5d7..e462ac5c1e48 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2393,6 +2393,10 @@ static void task_numa_placement(struct task_struct *p)
 			}
 		}
 
+		/* Cannot migrate task to CPU-less node */
+		if (!node_state(nid, N_CPU))
+			continue;
+
 		if (!ng) {
 			if (faults > max_faults) {
 				max_faults = faults;
-- 
2.30.2