[PATCH 1/8] mm: memcontrol: fix cpuhotplug statistics flushing

[Johannes Weiner <hannes@xxxxxxxxxxx>]

The memcg hotunplug callback erroneously flushes counts on the local
CPU, not the counts of the CPU going away; those counts will be lost.

Flush the CPU that is actually going away.

Also simplify the code a bit by using mod_memcg_state() and
count_memcg_events() instead of open-coding the upward flush - this is
comparable to how vmstat.c handles hotunplug flushing.

Fixes: a983b5ebee572 ("mm: memcontrol: fix excessive complexity in memory.stat reporting")
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reviewed-by: Roman Gushchin <guro@xxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/memcontrol.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ed5cc78a8dbf..8120d565dd79 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2411,45 +2411,52 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 static int memcg_hotplug_cpu_dead(unsigned int cpu)
 {
 	struct memcg_stock_pcp *stock;
-	struct mem_cgroup *memcg, *mi;
+	struct mem_cgroup *memcg;
 
 	stock = &per_cpu(memcg_stock, cpu);
 	drain_stock(stock);
 
 	for_each_mem_cgroup(memcg) {
+		struct memcg_vmstats_percpu *statc;
 		int i;
 
+		statc = per_cpu_ptr(memcg->vmstats_percpu, cpu);
+
 		for (i = 0; i < MEMCG_NR_STAT; i++) {
 			int nid;
-			long x;
 
-			x = this_cpu_xchg(memcg->vmstats_percpu->stat[i], 0);
-			if (x)
-				for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
-					atomic_long_add(x, &memcg->vmstats[i]);
+			if (statc->stat[i]) {
+				mod_memcg_state(memcg, i, statc->stat[i]);
+				statc->stat[i] = 0;
+			}
 
 			if (i >= NR_VM_NODE_STAT_ITEMS)
 				continue;
 
 			for_each_node(nid) {
+				struct batched_lruvec_stat *lstatc;
 				struct mem_cgroup_per_node *pn;
+				long x;
 
 				pn = mem_cgroup_nodeinfo(memcg, nid);
-				x = this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
-				if (x)
+				lstatc = per_cpu_ptr(pn->lruvec_stat_cpu, cpu);
+
+				x = lstatc->count[i];
+				lstatc->count[i] = 0;
+
+				if (x) {
 					do {
 						atomic_long_add(x, &pn->lruvec_stat[i]);
 					} while ((pn = parent_nodeinfo(pn, nid)));
+				}
 			}
 		}
 
 		for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
-			long x;
-
-			x = this_cpu_xchg(memcg->vmstats_percpu->events[i], 0);
-			if (x)
-				for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
-					atomic_long_add(x, &memcg->vmevents[i]);
+			if (statc->events[i]) {
+				count_memcg_events(memcg, i, statc->events[i]);
+				statc->events[i] = 0;
+			}
 		}
 	}
 
-- 
2.30.0