Even after mm/vmstat:online teardown, shepherd may still queue work for
the dying cpu until the cpu is removed from online mask. While it's
quite rare, this means that after unbind_workers() unbinds a per-cpu
kworker, it potentially runs vmstat_update for the dying CPU on an
irrelevant cpu before entering atomic AP states.
When CONFIG_DEBUG_PREEMPT=y, it results in the following error with the
backtrace.

  BUG: using smp_processor_id() in preemptible [00000000] code: \
                                               kworker/7:3/1702
  caller is refresh_cpu_vm_stats+0x235/0x5f0
  CPU: 0 UID: 0 PID: 1702 Comm: kworker/7:3 Tainted: G
  Tainted: [N]=TEST
  Workqueue: mm_percpu_wq vmstat_update
  Call Trace:
   <TASK>
   dump_stack_lvl+0x8d/0xb0
   check_preemption_disabled+0xce/0xe0
   refresh_cpu_vm_stats+0x235/0x5f0
   vmstat_update+0x17/0xa0
   process_one_work+0x869/0x1aa0
   worker_thread+0x5e5/0x1100
   kthread+0x29e/0x380
   ret_from_fork+0x2d/0x70
   ret_from_fork_asm+0x1a/0x30
   </TASK>

So, for mm/vmstat:online, disable vmstat_work reliably on teardown and
symmetrically enable it on startup.

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
---
v1: https://lore.kernel.org/all/20241220134234.3809621-1-koichiro.den@canonical.com/
---
 mm/vmstat.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 4d016314a56c..0889b75cef14 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -2148,13 +2148,14 @@ static int vmstat_cpu_online(unsigned int cpu)
 	if (!node_state(cpu_to_node(cpu), N_CPU)) {
 		node_set_state(cpu_to_node(cpu), N_CPU);
 	}
+	enable_delayed_work(&per_cpu(vmstat_work, cpu));
 
 	return 0;
 }
 
 static int vmstat_cpu_down_prep(unsigned int cpu)
 {
-	cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+	disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
 	return 0;
 }
 
-- 
2.43.0