memcg: avoid css_get()

Now, memory cgroup increments css(cgroup subsys state)'s reference count
per a charged page.  And the reference count is kept until the page is
uncharged.  But this has 2 bad effect.

 1. Because css_get/put calls atomic_inc()/dec, heavy call of them
    on large smp will not scale well.
 2. Because css's refcnt cannot be in a state as "ready-to-release",
    cgroup's notify_on_release handler can't work with memcg.
 3. css's refcnt is atomic_t, it means smaller than 32bit. Maybe too small.

This has been a problem since the 1st merge of memcg.

This is a trial to remove css's refcnt per a page. Even if we remove
refcnt, pre_destroy() does enough synchronization as
  - check res->usage == 0.
  - check no pages on LRU.

This patch removes css's refcnt per page.  Even after this patch, at the
1st look, it seems css_get() is still called in try_charge().

But the logic is.

  - If a memcg of mm->owner is cached one, consume_stock() will work.
    At success, return immediately.
  - If consume_stock returns false, css_get() is called and go to
    slow path which may be blocked. At the end of slow path,
    css_put() is called and restart from the start if necessary.

So, in the fast path, we don't call css_get() and can avoid access to
shared counter. This patch can make the most possible case fast.

Here is a result of multi-threaded page fault benchmark.

[Before]
    25.32%  multi-fault-all  [kernel.kallsyms]      [k] clear_page_c
     9.30%  multi-fault-all  [kernel.kallsyms]      [k] _raw_spin_lock_irqsave
     8.02%  multi-fault-all  [kernel.kallsyms]      [k] try_get_mem_cgroup_from_mm <=====(*)
     7.83%  multi-fault-all  [kernel.kallsyms]      [k] down_read_trylock
     5.38%  multi-fault-all  [kernel.kallsyms]      [k] __css_put
     5.29%  multi-fault-all  [kernel.kallsyms]      [k] __alloc_pages_nodemask
     4.92%  multi-fault-all  [kernel.kallsyms]      [k] _raw_spin_lock_irq
     4.24%  multi-fault-all  [kernel.kallsyms]      [k] up_read
     3.53%  multi-fault-all  [kernel.kallsyms]      [k] css_put
     2.11%  multi-fault-all  [kernel.kallsyms]      [k] handle_mm_fault
     1.76%  multi-fault-all  [kernel.kallsyms]      [k] __rmqueue
     1.64%  multi-fault-all  [kernel.kallsyms]      [k] __mem_cgroup_commit_charge

[After]
    28.41%  multi-fault-all  [kernel.kallsyms]      [k] clear_page_c
    10.08%  multi-fault-all  [kernel.kallsyms]      [k] _raw_spin_lock_irq
     9.58%  multi-fault-all  [kernel.kallsyms]      [k] down_read_trylock
     9.38%  multi-fault-all  [kernel.kallsyms]      [k] _raw_spin_lock_irqsave
     5.86%  multi-fault-all  [kernel.kallsyms]      [k] __alloc_pages_nodemask
     5.65%  multi-fault-all  [kernel.kallsyms]      [k] up_read
     2.82%  multi-fault-all  [kernel.kallsyms]      [k] handle_mm_fault
     2.64%  multi-fault-all  [kernel.kallsyms]      [k] mem_cgroup_add_lru_list
     2.48%  multi-fault-all  [kernel.kallsyms]      [k] __mem_cgroup_commit_charge

Then, 8.02% of try_get_mem_cgroup_from_mm() disappears because this patch
removes css_tryget() in it. (But yes, this is an extreme case.)

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 76 insertions, 43 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f52b0a1861c4..a1c3c317a4dd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1714,28 +1714,66 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
          * thread group leader migrates. It's possible that mm is not
          * set, if so charge the init_mm (happens for pagecache usage).
          */
-        if (*memcg) {
+        if (!*memcg && !mm)
+                goto bypass;
+again:
+        if (*memcg) { /* css should be a valid one */
                 mem = *memcg;
+                VM_BUG_ON(css_is_removed(&mem->css));
+                if (mem_cgroup_is_root(mem))
+                        goto done;
+                if (consume_stock(mem))
+                        goto done;
                 css_get(&mem->css);
         } else {
-                mem = try_get_mem_cgroup_from_mm(mm);
-                if (unlikely(!mem))
-                        return 0;
-                *memcg = mem;
-        }
+                struct task_struct *p;
 
-        VM_BUG_ON(css_is_removed(&mem->css));
-        if (mem_cgroup_is_root(mem))
-                goto done;
+                rcu_read_lock();
+                p = rcu_dereference(mm->owner);
+                VM_BUG_ON(!p);
+                /*
+                 * because we don't have task_lock(), "p" can exit while
+                 * we're here. In that case, "mem" can point to root
+                 * cgroup but never be NULL. (and task_struct itself is freed
+                 * by RCU, cgroup itself is RCU safe.) Then, we have small
+                 * risk here to get wrong cgroup. But such kind of mis-account
+                 * by race always happens because we don't have cgroup_mutex().
+                 * It's overkill and we allow that small race, here.
+                 */
+                mem = mem_cgroup_from_task(p);
+                VM_BUG_ON(!mem);
+                if (mem_cgroup_is_root(mem)) {
+                        rcu_read_unlock();
+                        goto done;
+                }
+                if (consume_stock(mem)) {
+                        /*
+                         * It seems dagerous to access memcg without css_get().
+                         * But considering how consume_stok works, it's not
+                         * necessary. If consume_stock success, some charges
+                         * from this memcg are cached on this cpu. So, we
+                         * don't need to call css_get()/css_tryget() before
+                         * calling consume_stock().
+                         */
+                        rcu_read_unlock();
+                        goto done;
+                }
+                /* after here, we may be blocked. we need to get refcnt */
+                if (!css_tryget(&mem->css)) {
+                        rcu_read_unlock();
+                        goto again;
+                }
+                rcu_read_unlock();
+        }
 
         do {
                 bool oom_check;
 
-                if (consume_stock(mem))
-                        goto done; /* don't need to fill stock */
                 /* If killed, bypass charge */
-                if (fatal_signal_pending(current))
+                if (fatal_signal_pending(current)) {
+                        css_put(&mem->css);
                         goto bypass;
+                }
 
                 oom_check = false;
                 if (oom && !nr_oom_retries) {
@@ -1750,30 +1788,36 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
                         break;
                 case CHARGE_RETRY: /* not in OOM situation but retry */
                         csize = PAGE_SIZE;
-                        break;
+                        css_put(&mem->css);
+                        mem = NULL;
+                        goto again;
                 case CHARGE_WOULDBLOCK: /* !__GFP_WAIT */
+                        css_put(&mem->css);
                         goto nomem;
                 case CHARGE_NOMEM: /* OOM routine works */
-                        if (!oom)
+                        if (!oom) {
+                                css_put(&mem->css);
                                 goto nomem;
+                        }
                         /* If oom, we never return -ENOMEM */
                         nr_oom_retries--;
                         break;
                 case CHARGE_OOM_DIE: /* Killed by OOM Killer */
+                        css_put(&mem->css);
                         goto bypass;
                 }
         } while (ret != CHARGE_OK);
 
         if (csize > PAGE_SIZE)
                 refill_stock(mem, csize - PAGE_SIZE);
+        css_put(&mem->css);
 done:
+        *memcg = mem;
         return 0;
 nomem:
-        css_put(&mem->css);
+        *memcg = NULL;
         return -ENOMEM;
 bypass:
-        if (mem)
-                css_put(&mem->css);
         *memcg = NULL;
         return 0;
 }
@@ -1790,11 +1834,7 @@ static void __mem_cgroup_cancel_charge(struct mem_cgroup *mem,
                 res_counter_uncharge(&mem->res, PAGE_SIZE * count);
                 if (do_swap_account)
                         res_counter_uncharge(&mem->memsw, PAGE_SIZE * count);
-                VM_BUG_ON(test_bit(CSS_ROOT, &mem->css.flags));
-                WARN_ON_ONCE(count > INT_MAX);
-                __css_put(&mem->css, (int)count);
         }
-        /* we don't need css_put for root */
 }
 
 static void mem_cgroup_cancel_charge(struct mem_cgroup *mem)
@@ -2155,7 +2195,6 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
                 goto charge_cur_mm;
         *ptr = mem;
         ret = __mem_cgroup_try_charge(NULL, mask, ptr, true);
-        /* drop extra refcnt from tryget */
         css_put(&mem->css);
         return ret;
 charge_cur_mm:
@@ -2325,10 +2364,6 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
                 break;
         }
 
-        if (!mem_cgroup_is_root(mem))
-                __do_uncharge(mem, ctype);
-        if (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
-                mem_cgroup_swap_statistics(mem, true);
         mem_cgroup_charge_statistics(mem, pc, false);
 
         ClearPageCgroupUsed(pc);
@@ -2340,11 +2375,17 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
          */
 
         unlock_page_cgroup(pc);
-
+        /*
+         * even after unlock, we have mem->res.usage here and this memcg
+         * will never be freed.
+         */
         memcg_check_events(mem, page);
-        /* at swapout, this memcg will be accessed to record to swap */
-        if (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
-                css_put(&mem->css);
+        if (do_swap_account && ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT) {
+                mem_cgroup_swap_statistics(mem, true);
+                mem_cgroup_get(mem);
+        }
+        if (!mem_cgroup_is_root(mem))
+                __do_uncharge(mem, ctype);
 
         return mem;
 
@@ -2431,13 +2472,12 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 
         memcg = __mem_cgroup_uncharge_common(page, ctype);
 
-        /* record memcg information */
-        if (do_swap_account && swapout && memcg) {
+        /*
+         * record memcg information,  if swapout && memcg != NULL,
+         * mem_cgroup_get() was called in uncharge().
+         */
+        if (do_swap_account && swapout && memcg)
                 swap_cgroup_record(ent, css_id(&memcg->css));
-                mem_cgroup_get(memcg);
-        }
-        if (swapout && memcg)
-                css_put(&memcg->css);
 }
 #endif
 
@@ -2515,7 +2555,6 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
                          */
                         if (!mem_cgroup_is_root(to))
                                 res_counter_uncharge(&to->res, PAGE_SIZE);
-                        css_put(&to->css);
                 }
                 return 0;
         }
@@ -4214,9 +4253,6 @@ static int mem_cgroup_do_precharge(unsigned long count)
                         goto one_by_one;
                 }
                 mc.precharge += count;
-                VM_BUG_ON(test_bit(CSS_ROOT, &mem->css.flags));
-                WARN_ON_ONCE(count > INT_MAX);
-                __css_get(&mem->css, (int)count);
                 return ret;
         }
 one_by_one:
@@ -4452,7 +4488,6 @@ static void mem_cgroup_clear_mc(void)
         }
         /* 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,
@@ -4466,8 +4501,6 @@ static void mem_cgroup_clear_mc(void)
                          */
                         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) */