Kcompactd is dedicated to a specific node. As such it wants to be
preferrably affine to it, memory and CPUs-wise.

Use the proper kthread API to achieve that. As a bonus it takes care of
CPU-hotplug events and CPU-isolation on its behalf.

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 mm/compaction.c | 43 +++----------------------------------------
 1 file changed, 3 insertions(+), 40 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index a2b16b08cbbf..a31c0f5758cf 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -3154,15 +3154,9 @@ void wakeup_kcompactd(pg_data_t *pgdat, int order, int highest_zoneidx)
 static int kcompactd(void *p)
 {
 	pg_data_t *pgdat = (pg_data_t *)p;
-	struct task_struct *tsk = current;
 	long default_timeout = msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC);
 	long timeout = default_timeout;
 
-	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
-
-	if (!cpumask_empty(cpumask))
-		set_cpus_allowed_ptr(tsk, cpumask);
-
 	set_freezable();
 
 	pgdat->kcompactd_max_order = 0;
@@ -3233,10 +3227,12 @@ void __meminit kcompactd_run(int nid)
 	if (pgdat->kcompactd)
 		return;
 
-	pgdat->kcompactd = kthread_run(kcompactd, pgdat, "kcompactd%d", nid);
+	pgdat->kcompactd = kthread_create_on_node(kcompactd, pgdat, nid, "kcompactd%d", nid);
 	if (IS_ERR(pgdat->kcompactd)) {
 		pr_err("Failed to start kcompactd on node %d\n", nid);
 		pgdat->kcompactd = NULL;
+	} else {
+		wake_up_process(pgdat->kcompactd);
 	}
 }
 
@@ -3254,30 +3250,6 @@ void __meminit kcompactd_stop(int nid)
 	}
 }
 
-/*
- * It's optimal to keep kcompactd 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 kcompactd_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 */
-			if (pgdat->kcompactd)
-				set_cpus_allowed_ptr(pgdat->kcompactd, mask);
-	}
-	return 0;
-}
-
 static int proc_dointvec_minmax_warn_RT_change(const struct ctl_table *table,
 		int write, void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -3337,15 +3309,6 @@ static struct ctl_table vm_compaction[] = {
 static int __init kcompactd_init(void)
 {
 	int nid;
-	int ret;
-
-	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
-					"mm/compaction:online",
-					kcompactd_cpu_online, NULL);
-	if (ret < 0) {
-		pr_err("kcompactd: failed to register hotplug callbacks.\n");
-		return ret;
-	}
 
 	for_each_node_state(nid, N_MEMORY)
 		kcompactd_run(nid);
-- 
2.46.0