mm: memcontrol: revert use of root_mem_cgroup res_counter

Dave Hansen reports a massive scalability regression in an uncontained
page fault benchmark with more than 30 concurrent threads, which he
bisected down to 05b843012335 ("mm: memcontrol: use root_mem_cgroup
res_counter") and pin-pointed on res_counter spinlock contention.

That change relied on the per-cpu charge caches to mostly swallow the
res_counter costs, but it's apparent that the caches don't scale yet.

Revert memcg back to bypassing res_counters on the root level in order
to restore performance for uncontained workloads.

Reported-by: Dave Hansen <dave@sr71.net>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 78 insertions, 25 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ec4dcf1b9562..085dc6d2f876 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2534,6 +2534,8 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
         unsigned long long size;
         int ret = 0;
 
+        if (mem_cgroup_is_root(memcg))
+                goto done;
 retry:
         if (consume_stock(memcg, nr_pages))
                 goto done;
@@ -2611,9 +2613,7 @@ nomem:
         if (!(gfp_mask & __GFP_NOFAIL))
                 return -ENOMEM;
 bypass:
-        memcg = root_mem_cgroup;
-        ret = -EINTR;
-        goto retry;
+        return -EINTR;
 
 done_restock:
         if (batch > nr_pages)
@@ -2626,6 +2626,9 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
         unsigned long bytes = nr_pages * PAGE_SIZE;
 
+        if (mem_cgroup_is_root(memcg))
+                return;
+
         res_counter_uncharge(&memcg->res, bytes);
         if (do_swap_account)
                 res_counter_uncharge(&memcg->memsw, bytes);
@@ -2640,6 +2643,9 @@ static void __mem_cgroup_cancel_local_charge(struct mem_cgroup *memcg,
 {
         unsigned long bytes = nr_pages * PAGE_SIZE;
 
+        if (mem_cgroup_is_root(memcg))
+                return;
+
         res_counter_uncharge_until(&memcg->res, memcg->res.parent, bytes);
         if (do_swap_account)
                 res_counter_uncharge_until(&memcg->memsw,
@@ -4093,6 +4099,46 @@ out:
         return retval;
 }
 
+static unsigned long mem_cgroup_recursive_stat(struct mem_cgroup *memcg,
+                                               enum mem_cgroup_stat_index idx)
+{
+        struct mem_cgroup *iter;
+        long val = 0;
+
+        /* Per-cpu values can be negative, use a signed accumulator */
+        for_each_mem_cgroup_tree(iter, memcg)
+                val += mem_cgroup_read_stat(iter, idx);
+
+        if (val < 0) /* race ? */
+                val = 0;
+        return val;
+}
+
+static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
+{
+        u64 val;
+
+        if (!mem_cgroup_is_root(memcg)) {
+                if (!swap)
+                        return res_counter_read_u64(&memcg->res, RES_USAGE);
+                else
+                        return res_counter_read_u64(&memcg->memsw, RES_USAGE);
+        }
+
+        /*
+         * Transparent hugepages are still accounted for in MEM_CGROUP_STAT_RSS
+         * as well as in MEM_CGROUP_STAT_RSS_HUGE.
+         */
+        val = mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_CACHE);
+        val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_RSS);
+
+        if (swap)
+                val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_SWAP);
+
+        return val << PAGE_SHIFT;
+}
+
+
 static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
                                struct cftype *cft)
 {
@@ -4102,8 +4148,12 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
 
         switch (type) {
         case _MEM:
+                if (name == RES_USAGE)
+                        return mem_cgroup_usage(memcg, false);
                 return res_counter_read_u64(&memcg->res, name);
         case _MEMSWAP:
+                if (name == RES_USAGE)
+                        return mem_cgroup_usage(memcg, true);
                 return res_counter_read_u64(&memcg->memsw, name);
         case _KMEM:
                 return res_counter_read_u64(&memcg->kmem, name);
@@ -4572,10 +4622,7 @@ static void __mem_cgroup_threshold(struct mem_cgroup *memcg, bool swap)
         if (!t)
                 goto unlock;
 
-        if (!swap)
-                usage = res_counter_read_u64(&memcg->res, RES_USAGE);
-        else
-                usage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
+        usage = mem_cgroup_usage(memcg, swap);
 
         /*
          * current_threshold points to threshold just below or equal to usage.
@@ -4673,10 +4720,10 @@ static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
 
         if (type == _MEM) {
                 thresholds = &memcg->thresholds;
-                usage = res_counter_read_u64(&memcg->res, RES_USAGE);
+                usage = mem_cgroup_usage(memcg, false);
         } else if (type == _MEMSWAP) {
                 thresholds = &memcg->memsw_thresholds;
-                usage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
+                usage = mem_cgroup_usage(memcg, true);
         } else
                 BUG();
 
@@ -4762,10 +4809,10 @@ static void __mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg,
 
         if (type == _MEM) {
                 thresholds = &memcg->thresholds;
-                usage = res_counter_read_u64(&memcg->res, RES_USAGE);
+                usage = mem_cgroup_usage(memcg, false);
         } else if (type == _MEMSWAP) {
                 thresholds = &memcg->memsw_thresholds;
-                usage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
+                usage = mem_cgroup_usage(memcg, true);
         } else
                 BUG();
 
@@ -5525,9 +5572,9 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
                  * core guarantees its existence.
                  */
         } else {
-                res_counter_init(&memcg->res, &root_mem_cgroup->res);
-                res_counter_init(&memcg->memsw, &root_mem_cgroup->memsw);
-                res_counter_init(&memcg->kmem, &root_mem_cgroup->kmem);
+                res_counter_init(&memcg->res, NULL);
+                res_counter_init(&memcg->memsw, NULL);
+                res_counter_init(&memcg->kmem, NULL);
                 /*
                  * Deeper hierachy with use_hierarchy == false doesn't make
                  * much sense so let cgroup subsystem know about this
@@ -5969,8 +6016,9 @@ static void __mem_cgroup_clear_mc(void)
         /* we must fixup refcnts and charges */
         if (mc.moved_swap) {
                 /* uncharge swap account from the old cgroup */
-                res_counter_uncharge(&mc.from->memsw,
-                                     PAGE_SIZE * mc.moved_swap);
+                if (!mem_cgroup_is_root(mc.from))
+                        res_counter_uncharge(&mc.from->memsw,
+                                             PAGE_SIZE * mc.moved_swap);
 
                 for (i = 0; i < mc.moved_swap; i++)
                         css_put(&mc.from->css);
@@ -5979,8 +6027,9 @@ static void __mem_cgroup_clear_mc(void)
                  * we charged both to->res and to->memsw, so we should
                  * uncharge to->res.
                  */
-                res_counter_uncharge(&mc.to->res,
-                                     PAGE_SIZE * mc.moved_swap);
+                if (!mem_cgroup_is_root(mc.to))
+                        res_counter_uncharge(&mc.to->res,
+                                             PAGE_SIZE * mc.moved_swap);
                 /* we've already done css_get(mc.to) */
                 mc.moved_swap = 0;
         }
@@ -6345,7 +6394,8 @@ void mem_cgroup_uncharge_swap(swp_entry_t entry)
         rcu_read_lock();
         memcg = mem_cgroup_lookup(id);
         if (memcg) {
-                res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
+                if (!mem_cgroup_is_root(memcg))
+                        res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
                 mem_cgroup_swap_statistics(memcg, false);
                 css_put(&memcg->css);
         }
@@ -6509,12 +6559,15 @@ static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
 {
         unsigned long flags;
 
-        if (nr_mem)
-                res_counter_uncharge(&memcg->res, nr_mem * PAGE_SIZE);
-        if (nr_memsw)
-                res_counter_uncharge(&memcg->memsw, nr_memsw * PAGE_SIZE);
-
-        memcg_oom_recover(memcg);
+        if (!mem_cgroup_is_root(memcg)) {
+                if (nr_mem)
+                        res_counter_uncharge(&memcg->res,
+                                             nr_mem * PAGE_SIZE);
+                if (nr_memsw)
+                        res_counter_uncharge(&memcg->memsw,
+                                             nr_memsw * PAGE_SIZE);
+                memcg_oom_recover(memcg);
+        }
 
         local_irq_save(flags);
         __this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_RSS], nr_anon);