[PATCH] sched/fair: Skip NUMA balancing scan on memoryless nodes

[Phineas Su <pohaosu@google.com>]

On systems with memoryless NUMA nodes (e.g. CPU-only nodes created via
memory hiding, socket topologies with unpopulated memory, or CPU-only
NUMA nodes), tasks running on these CPUs cause automatic NUMA balancing
(kernel.numa_balancing=1) to repeatedly schedule task_numa_work() from
task_tick_numa().

When task_numa_work() executes, it unmaps VMAs (PROT_NONE) to induce
NUMA hinting faults (do_numa_page()). Fault handling then attempts page
migration (migrate_misplaced_folio()) to the task's current CPU NUMA
node. However, because the node has no managed memory (N_MEMORY is
false), page allocations continuously fail (TNF_MIGRATE_FAIL), while
task_tick_numa() repeatedly reschedules VMA scanning every scan period.
This results in heavy kernel system overhead (%sys CPU usage spiking up
to ~78%) and continuous page fault storms without any possible NUMA
placement benefit.

Fix this by:
1. Checking node_state(task_node(curr), N_MEMORY) in task_tick_numa() so
   tasks executing on CPUs of memoryless nodes do not schedule
   numa_work callbacks via task_work_add().
2. Checking node_state(task_node(p), N_MEMORY) in task_numa_work() as a
   safeguard to immediately abort VMA scanning if a task migrated to a
   memoryless node while numa_work was already enqueued.

Signed-off-by: Phineas Su <pohaosu@google.com>
---
 kernel/sched/fair.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee1..214cf0f2c692 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4101,6 +4101,9 @@ static void task_numa_work(struct callback_head *work)
 	if (p->flags & PF_EXITING)
 		return;
 
+	if (!node_state(task_node(p), N_MEMORY))
+		return;
+
 	/*
 	 * Memory is pinned to only one NUMA node via cpuset.mems, naturally
 	 * no page can be migrated.
@@ -4391,6 +4394,9 @@ static void task_tick_numa(struct rq *rq, struct task_struct *curr)
 	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) || work->next != work)
 		return;
 
+	if (!node_state(task_node(curr), N_MEMORY))
+		return;
+
 	/*
 	 * Using runtime rather than walltime has the dual advantage that
 	 * we (mostly) drive the selection from busy threads and that the
-- 
2.55.0.508.g3f0d502094-goog