mm/vmscan.c: remove cpu online notification for now

kswapd kernel thread starts either with a CPU affinity set to the full cpu
mask of its target node or without any affinity at all if the node is
CPUless.  There is a cpu hotplug callback (kswapd_cpu_online) that
implements an elaborate way to update this mask when a cpu is onlined.

It is not really clear whether there is any actual benefit from this
scheme. Completely CPU-less NUMA nodes rarely gain a new CPU during
runtime. Drop the code for that reason. If there is a real usecase then
we can resurrect and simplify the code.

[mhocko@suse.com rewrite changelog]
Suggested-by: Michal Hocko <mhocko@suse.org>
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/20200218224422.3407-1-richardw.yang@linux.intel.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -4030,27 +4030,6 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
 }
 #endif /* CONFIG_HIBERNATION */
 
-/* It's optimal to keep kswapds on the same CPUs as their memory, but
-   not required for correctness.  So if the last cpu in a node goes
-   away, we get changed to run anywhere: as the first one comes back,
-   restore their cpu bindings. */
-static int kswapd_cpu_online(unsigned int cpu)
-{
-	int nid;
-
-	for_each_node_state(nid, N_MEMORY) {
-		pg_data_t *pgdat = NODE_DATA(nid);
-		const struct cpumask *mask;
-
-		mask = cpumask_of_node(pgdat->node_id);
-
-		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
-			/* One of our CPUs online: restore mask */
-			set_cpus_allowed_ptr(pgdat->kswapd, mask);
-	}
-	return 0;
-}
-
 /*
  * This kswapd start function will be called by init and node-hot-add.
  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
@@ -4090,15 +4069,11 @@ void kswapd_stop(int nid)
 
 static int __init kswapd_init(void)
 {
-	int nid, ret;
+	int nid;
 
 	swap_setup();
 	for_each_node_state(nid, N_MEMORY)
  		kswapd_run(nid);
-	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
-					"mm/vmscan:online", kswapd_cpu_online,
-					NULL);
-	WARN_ON(ret < 0);
 	return 0;
 }