memcg: improve performance in moving swap charge

Try to reduce overheads in moving swap charge by:

- Adds a new function(__mem_cgroup_put), which takes "count" as a arg and
  decrement mem->refcnt by "count".
- Removed res_counter_uncharge, css_put, and mem_cgroup_put from the path
  of moving swap account, and consolidate all of them into mem_cgroup_clear_mc.
  We cannot do that about mc.to->refcnt.

These changes reduces the overhead from 1.35sec to 0.9sec to move charges
of 1G anonymous memory(including 500MB swap) in my test environment.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 60 insertions, 15 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e883198baf81..b00ec74a4c18 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -255,6 +255,7 @@ static struct move_charge_struct {
         struct mem_cgroup *to;
         unsigned long precharge;
         unsigned long moved_charge;
+        unsigned long moved_swap;
         struct task_struct *moving_task;        /* a task moving charges */
         wait_queue_head_t waitq;                /* a waitq for other context */
 } mc = {
@@ -2277,6 +2278,7 @@ void mem_cgroup_uncharge_swap(swp_entry_t ent)
  * @entry: swap entry to be moved
  * @from:  mem_cgroup which the entry is moved from
  * @to:  mem_cgroup which the entry is moved to
+ * @need_fixup: whether we should fixup res_counters and refcounts.
  *
  * It succeeds only when the swap_cgroup's record for this entry is the same
  * as the mem_cgroup's id of @from.
@@ -2287,7 +2289,7 @@ void mem_cgroup_uncharge_swap(swp_entry_t ent)
  * both res and memsw, and called css_get().
  */
 static int mem_cgroup_move_swap_account(swp_entry_t entry,
-                                struct mem_cgroup *from, struct mem_cgroup *to)
+                struct mem_cgroup *from, struct mem_cgroup *to, bool need_fixup)
 {
         unsigned short old_id, new_id;
 
@@ -2295,27 +2297,36 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
         new_id = css_id(&to->css);
 
         if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
-                if (!mem_cgroup_is_root(from))
-                        res_counter_uncharge(&from->memsw, PAGE_SIZE);
                 mem_cgroup_swap_statistics(from, false);
-                mem_cgroup_put(from);
+                mem_cgroup_swap_statistics(to, true);
                 /*
-                 * we charged both to->res and to->memsw, so we should uncharge
-                 * to->res.
+                 * This function is only called from task migration context now.
+                 * It postpones res_counter and refcount handling till the end
+                 * of task migration(mem_cgroup_clear_mc()) for performance
+                 * improvement. But we cannot postpone mem_cgroup_get(to)
+                 * because if the process that has been moved to @to does
+                 * swap-in, the refcount of @to might be decreased to 0.
                  */
-                if (!mem_cgroup_is_root(to))
-                        res_counter_uncharge(&to->res, PAGE_SIZE);
-                mem_cgroup_swap_statistics(to, true);
                 mem_cgroup_get(to);
-                css_put(&to->css);
-
+                if (need_fixup) {
+                        if (!mem_cgroup_is_root(from))
+                                res_counter_uncharge(&from->memsw, PAGE_SIZE);
+                        mem_cgroup_put(from);
+                        /*
+                         * we charged both to->res and to->memsw, so we should
+                         * uncharge to->res.
+                         */
+                        if (!mem_cgroup_is_root(to))
+                                res_counter_uncharge(&to->res, PAGE_SIZE);
+                        css_put(&to->css);
+                }
                 return 0;
         }
         return -EINVAL;
 }
 #else
 static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
-                                struct mem_cgroup *from, struct mem_cgroup *to)
+                struct mem_cgroup *from, struct mem_cgroup *to, bool need_fixup)
 {
         return -EINVAL;
 }
@@ -3398,9 +3409,9 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
         atomic_inc(&mem->refcnt);
 }
 
-static void mem_cgroup_put(struct mem_cgroup *mem)
+static void __mem_cgroup_put(struct mem_cgroup *mem, int count)
 {
-        if (atomic_dec_and_test(&mem->refcnt)) {
+        if (atomic_sub_and_test(count, &mem->refcnt)) {
                 struct mem_cgroup *parent = parent_mem_cgroup(mem);
                 __mem_cgroup_free(mem);
                 if (parent)
@@ -3408,6 +3419,11 @@ static void mem_cgroup_put(struct mem_cgroup *mem)
         }
 }
 
+static void mem_cgroup_put(struct mem_cgroup *mem)
+{
+        __mem_cgroup_put(mem, 1);
+}
+
 /*
  * Returns the parent mem_cgroup in memcgroup hierarchy with hierarchy enabled.
  */
@@ -3789,6 +3805,29 @@ static void mem_cgroup_clear_mc(void)
                 __mem_cgroup_cancel_charge(mc.from, mc.moved_charge);
                 mc.moved_charge = 0;
         }
+        /* we must fixup refcnts and charges */
+        if (mc.moved_swap) {
+                WARN_ON_ONCE(mc.moved_swap > INT_MAX);
+                /* uncharge swap account from the old cgroup */
+                if (!mem_cgroup_is_root(mc.from))
+                        res_counter_uncharge(&mc.from->memsw,
+                                                PAGE_SIZE * mc.moved_swap);
+                __mem_cgroup_put(mc.from, mc.moved_swap);
+
+                if (!mem_cgroup_is_root(mc.to)) {
+                        /*
+                         * 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);
+                        VM_BUG_ON(test_bit(CSS_ROOT, &mc.to->css.flags));
+                        __css_put(&mc.to->css, mc.moved_swap);
+                }
+                /* we've already done mem_cgroup_get(mc.to) */
+
+                mc.moved_swap = 0;
+        }
         mc.from = NULL;
         mc.to = NULL;
         mc.moving_task = NULL;
@@ -3818,11 +3857,13 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
                         VM_BUG_ON(mc.to);
                         VM_BUG_ON(mc.precharge);
                         VM_BUG_ON(mc.moved_charge);
+                        VM_BUG_ON(mc.moved_swap);
                         VM_BUG_ON(mc.moving_task);
                         mc.from = from;
                         mc.to = mem;
                         mc.precharge = 0;
                         mc.moved_charge = 0;
+                        mc.moved_swap = 0;
                         mc.moving_task = current;
 
                         ret = mem_cgroup_precharge_mc(mm);
@@ -3883,8 +3924,12 @@ put:			/* is_target_pte_for_mc() gets the page */
                         break;
                 case MC_TARGET_SWAP:
                         ent = target.ent;
-                        if (!mem_cgroup_move_swap_account(ent, mc.from, mc.to))
+                        if (!mem_cgroup_move_swap_account(ent,
+                                                mc.from, mc.to, false)) {
                                 mc.precharge--;
+                                /* we fixup refcnts and charges later. */
+                                mc.moved_swap++;
+                        }
                         break;
                 default:
                         break;