sched/numa: fix task swap by skipping kernel threads

Patch series "sched/numa: add statistics of numa balance task migration",
v6.

Introduce task migration and swap statistics in the following places:
/sys/fs/cgroup/{GROUP}/memory.stat
/proc/{PID}/sched
/proc/vmstat

These statistics facilitate a rapid evaluation of the performance and
resource utilization of the target workload.


This patch (of 2):

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. 
That is to say, a kernel thread might be incorrectly chosen as B. 
However, kernel thread and user space thread that does not have mm are
not supposed to be covered by NUMA balancing because NUMA balancing
only considers user pages via VMAs.

According to Peter's suggestion for fixing this issue, we use
PF_KTHREAD to skip the kernel thread.  curr->mm is also checked because
it is possible that user_mode_thread() might create a user thread
without an mm.  As per Prateek's analysis, after adding the PF_KTHREAD
check, there is no need to further check the PF_IDLE flag:

: - play_idle_precise() already ensures PF_KTHREAD is set before adding
:   PF_IDLE
: 
: - cpu_startup_entry() is only called from the startup thread which
:   should be marked with PF_KTHREAD (based on my understanding looking at
:   commit cff9b23 ("kernel/sched: Modify initial boot task idle
:   setup"))

In summary, the check in task_numa_compare() now aligns with
task_tick_numa().

Link: https://lkml.kernel.org/r/cover.1748493462.git.yu.c.chen@intel.com
Link: https://lkml.kernel.org/r/43d68b356b25d124f0d222ebedf3859e86eefb9f.1748493462.git.yu.c.chen@intel.com
Link: https://lkml.kernel.org/r/cover.1748002400.git.yu.c.chen@intel.com
Link: https://lkml.kernel.org/r/eaacc9c9bd37bac92d43a671867d85b2fdad3b06.1748002400.git.yu.c.chen@intel.com
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Libo Chen <libo.chen@oracle.com>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Tested-by: Ayush Jain <Ayush.jain3@amd.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Aubrey Li <aubrey.li@intel.com>
Cc: "Chen, Tim C" <tim.c.chen@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Cc: Mel Gorman <mgorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Libo Chen

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 | PF_KTHREAD)) ||
+		    !cur->mm))
 		cur = NULL;
 
 	/*