[PATCH v4 1/2] sched/numa: fix task swap by skipping kernel threads

[Chen Yu <yu.c.chen@intel.com>]

From: Libo Chen <libo.chen@oracle.com>

Task swapping is triggered when there are no idle CPUs in
task A's preferred node. In this case, the NUMA load balancer
chooses a task B on A's preferred node and swaps B with A. This
helps improve NUMA locality without introducing load imbalance
between nodes.

In the current implementation, B's NUMA node preference is not
mandatory, and it aims not to increase load imbalance. That is
to say, a kernel thread might be chosen as B. However, kernel
threads are not supposed to be covered by NUMA balancing because
NUMA balancing only considers user pages via VMAs.

Fix this by not considering kernel threads as swap targets in
task_numa_compare(). This can be extended beyond kernel threads
in the future by checking if a swap candidate has a valid NUMA
preference through checking the candidate's numa_preferred_nid
and numa_faults. For now, keep the code simple.

Suggested-by: Michal Koutny <mkoutny@suse.com>
Tested-by: Ayush Jain <Ayush.jain3@amd.com>
Signed-off-by: Libo Chen <libo.chen@oracle.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fb9bf995a47..d1af2e084a2a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2273,7 +2273,8 @@ static bool task_numa_compare(struct task_numa_env *env,
 
 	rcu_read_lock();
 	cur = rcu_dereference(dst_rq->curr);
-	if (cur && ((cur->flags & PF_EXITING) || is_idle_task(cur)))
+	if (cur && ((cur->flags & PF_EXITING) || is_idle_task(cur) ||
+		    !cur->mm))
 		cur = NULL;
 
 	/*
-- 
2.25.1