mm: sched: numa: Delay PTE scanning until a task is scheduled on a new node

Due to the fact that migrations are driven by the CPU a task is running
on there is no point tracking NUMA faults until one task runs on a new
node. This patch tracks the first node used by an address space. Until
it changes, PTE scanning is disabled and no NUMA hinting faults are
trapped. This should help workloads that are short-lived, do not care
about NUMA placement or have bound themselves to a single node.

This takes advantage of the logic in "mm: sched: numa: Implement slow
start for working set sampling" to delay when the checks are made. This
will take advantage of processes that set their CPU and node bindings
early in their lifetime. It will also potentially allow any initial load
balancing to take place.

Signed-off-by: Mel Gorman <mgorman@suse.de>

1 files changed, 18 insertions, 0 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a02a2082e95..3e18f611a5aa 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -861,6 +861,24 @@ void task_numa_work(struct callback_head *work)
                 return;
 
         /*
+         * We do not care about task placement until a task runs on a node
+         * other than the first one used by the address space. This is
+         * largely because migrations are driven by what CPU the task
+         * is running on. If it's never scheduled on another node, it'll
+         * not migrate so why bother trapping the fault.
+         */
+        if (mm->first_nid == NUMA_PTE_SCAN_INIT)
+                mm->first_nid = numa_node_id();
+        if (mm->first_nid != NUMA_PTE_SCAN_ACTIVE) {
+                /* Are we running on a new node yet? */
+                if (numa_node_id() == mm->first_nid &&
+                    !sched_feat_numa(NUMA_FORCE))
+                        return;
+
+                mm->first_nid = NUMA_PTE_SCAN_ACTIVE;
+        }
+
+        /*
          * Reset the scan period if enough time has gone by. Objective is that
          * scanning will be reduced if pages are properly placed. As tasks
          * can enter different phases this needs to be re-examined. Lacking