[PATCH v5 7/7] mm/memcontrol: add stock to the memsw page_counter

[Joshua Hahn]

Before this series, each memcg had one stock shared by all its
page_counters (memory + memsw). Now that the memcg stock was folded
into the page_counter level, give memsw its own page_counter_stock
so that it can benefit from caching charges as well.

Note that while the allocation is conditional on do_memsw_account(),
the freeing is not; the freer will only free non-NULL stocks. This
matters because do_memsw_account() could have changed in between the
allocation and the free.

This narrows the memsw skew introduced by the previous patch. memsw is
now charged in the same batches as memory, so the two no longer diverge
systematically, but can still see transient drifts since each keeps
its own per-cpu stock. The drift is bound by
MEMCG_CHARGE_BATCH * nr_possible_cpus.

In the unlikely scenario that one of the stocks does not get allocated,
there will be different granularities of charging for the cgroup's
lifetime (one charging in batch granularity, the other just in
nr_pages).

Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
---
 mm/memcontrol.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5678486cc55b0..33e4ffbd48a8b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2127,8 +2127,10 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 	if (!mutex_trylock(&percpu_charge_mutex))
 		return;
 
-	for_each_mem_cgroup_tree(memcg, root_memcg)
+	for_each_mem_cgroup_tree(memcg, root_memcg) {
 		page_counter_drain_stock_async(&memcg->memory);
+		page_counter_drain_stock_async(&memcg->memsw);
+	}
 
 	/* Hotplug races are OK; workers only touch their own cpu's obj_stock */
 	migrate_disable();
@@ -2159,8 +2161,10 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 	/* no need for the local lock */
 	drain_obj_stock(obj_st);
 
-	for_each_mem_cgroup_tree(memcg, NULL)
+	for_each_mem_cgroup_tree(memcg, NULL) {
 		page_counter_drain_cpu_stock(&memcg->memory, cpu);
+		page_counter_drain_cpu_stock(&memcg->memsw, cpu);
+	}
 
 	/*
 	 * A drain work queued before the CPU went away is executed by an
@@ -2467,7 +2471,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		goto check_high;
 
 	if (do_memsw_account())
-		page_counter_uncharge(&memcg->memsw, nr_pages);
+		page_counter_refill_stock(&memcg->memsw, nr_pages);
 	mem_over_limit = mem_cgroup_from_counter(counter, memory);
 
 reclaim:
@@ -2926,7 +2930,7 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 	if (!mem_cgroup_is_root(memcg)) {
 		page_counter_refill_stock(&memcg->memory, nr_pages);
 		if (do_memsw_account())
-			page_counter_uncharge(&memcg->memsw, nr_pages);
+			page_counter_refill_stock(&memcg->memsw, nr_pages);
 	}
 
 	css_put(&memcg->css);
@@ -3930,6 +3934,8 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 static void mem_cgroup_free(struct mem_cgroup *memcg)
 {
 	page_counter_free_stock(&memcg->memory);
+	/* memsw and swap are the same counter; only memsw is ever stocked */
+	page_counter_free_stock(&memcg->memsw);
 	lru_gen_exit_memcg(memcg);
 	memcg_wb_domain_exit(memcg);
 	__mem_cgroup_free(memcg);
@@ -4098,8 +4104,12 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	css_get(css);
 
 	/* stock allocation failure is nonfatal; fall back to direct charges */
-	if (!mem_cgroup_is_root(memcg))
+	if (!mem_cgroup_is_root(memcg)) {
 		page_counter_alloc_stock(&memcg->memory, MEMCG_CHARGE_BATCH);
+		if (do_memsw_account())
+			page_counter_alloc_stock(&memcg->memsw,
+						 MEMCG_CHARGE_BATCH);
+	}
 
 	/*
 	 * Ensure mem_cgroup_from_private_id() works once we're fully online.
-- 
2.53.0-Meta