Still RFC, added lkml to CC: list.
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
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 atoimic_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.
This is a trial to remove css's refcnt per a page. Even if we remove
refcnt, pre_destroy() does enough synchronization.
After this patch, it seems css_get() is still called in try_charge().
But the logic is.
1. task_lock(mm->owner)
2. If a memcg of mm->owner is cached one, consume_stock() will work.
If it returns true, task_unlock and return immediately.
3. If a memcg doesn't hit cached one, css_get() and access res_counter
and do charge.
So, in the fast path, we don't call css_get() and can avoid access to
shared counter.
The point is that while there are used resource, memcg can't be freed.
So, if it's cached, we don't have to take css_get().
Brief test result on mmotm-2.6.32-Jul-06 + Nishimura-san's cleanup.
test with multi-threaded page fault program on 8cpu system.
(Each threads touch 2M of pages and release them by madvice().)
[Before]
39522457 page-faults
549387860 cache-misses (cache-miss/page-fault 13.90)
60.003715887 seconds time elapsed
25.75% multi-fault-all [kernel.kallsyms] [k] clear_page_c
8.59% multi-fault-all [kernel.kallsyms] [k] _raw_spin_lock_irqsave
8.22% multi-fault-all [kernel.kallsyms] [k] try_get_mem_cgroup_from_mm
8.01% multi-fault-all [kernel.kallsyms] [k] down_read_trylock
5.41% multi-fault-all [kernel.kallsyms] [k] __alloc_pages_nodemask
5.41% multi-fault-all [kernel.kallsyms] [k] __css_put
4.48% multi-fault-all [kernel.kallsyms] [k] _raw_spin_lock_irq
4.41% multi-fault-all [kernel.kallsyms] [k] up_read
3.60% multi-fault-all [kernel.kallsyms] [k] css_put
1.93% multi-fault-all [kernel.kallsyms] [k] handle_mm_fault
1.91% multi-fault-all [kernel.kallsyms] [k] _raw_spin_lock
1.89% multi-fault-all [kernel.kallsyms] [k] _cond_resched
1.78% multi-fault-all [kernel.kallsyms] [k] __rmqueue
1.69% multi-fault-all [kernel.kallsyms] [k] mem_cgroup_add_lru_list
1.63% multi-fault-all [kernel.kallsyms] [k] page_fault
1.45% multi-fault-all [kernel.kallsyms] [k] __mem_cgroup_commit_charge
1.34% multi-fault-all [kernel.kallsyms] [k] find_vma
[After]
43253862 page-faults
505750203 cache-misses (cache-miss/page-fault 11.69)
60.004123555 seconds time elapsed
27.98% multi-fault-all [kernel.kallsyms] [k] clear_page_c
9.89% multi-fault-all [kernel.kallsyms] [k] down_read_trylock
9.88% multi-fault-all [kernel.kallsyms] [k] _raw_spin_lock_irq
9.37% multi-fault-all [kernel.kallsyms] [k] _raw_spin_lock_irqsave
5.91% multi-fault-all [kernel.kallsyms] [k] __alloc_pages_nodemask
5.69% multi-fault-all [kernel.kallsyms] [k] up_read
2.94% multi-fault-all [kernel.kallsyms] [k] _raw_spin_lock
2.70% multi-fault-all [kernel.kallsyms] [k] handle_mm_fault
2.66% multi-fault-all [kernel.kallsyms] [k] mem_cgroup_add_lru_list
2.25% multi-fault-all [kernel.kallsyms] [k] __mem_cgroup_commit_charge
1.90% multi-fault-all [kernel.kallsyms] [k] __rmqueue
1.74% multi-fault-all [kernel.kallsyms] [k] page_fault
1.52% multi-fault-all [kernel.kallsyms] [k] find_vma
Then, overhead is reduced.
Note: Because this is an extreme, not-realistic, test, I'm not sure how
this change will be benefits for usual applications.
Changelog 20100609:
- clean up try_charge().
- fixed mem_cgroup_clear_mc
- removed unnecessary warnings.
- removed task_lock, added more comments.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
mm/memcontrol.c | 96 ++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 63 insertions(+), 33 deletions(-)
Index: mmotm-2.6.34-Jun6/mm/memcontrol.c
===================================================================
--- mmotm-2.6.34-Jun6.orig/mm/memcontrol.c
+++ mmotm-2.6.34-Jun6/mm/memcontrol.c
@@ -1717,28 +1717,61 @@ static int __mem_cgroup_try_charge(struc
* 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);
+ /*
+ * It seems there are some races when 'p' exits. But, at exit(),
+ * p->cgroups will be reset to init_css_set. So, we'll just
+ * find the root cgroup even if 'p' gets to be obsolete.
+ */
+ 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;
+ }
+ 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) {
@@ -1753,30 +1786,36 @@ static int __mem_cgroup_try_charge(struc
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;
}
@@ -1793,11 +1832,7 @@ static void __mem_cgroup_cancel_charge(s
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)
@@ -2158,7 +2193,6 @@ int mem_cgroup_try_charge_swapin(struct
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:
@@ -2345,9 +2379,6 @@ __mem_cgroup_uncharge_common(struct page
unlock_page_cgroup(pc);
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);
return mem;
@@ -2432,15 +2463,18 @@ mem_cgroup_uncharge_swapcache(struct pag
if (!swapout) /* this was a swap cache but the swap is unused ! */
ctype = MEM_CGROUP_CHARGE_TYPE_DROP;
- memcg = __mem_cgroup_uncharge_common(page, ctype);
+ memcg = try_get_mem_cgroup_from_page(page);
+ if (!memcg)
+ return;
+
+ __mem_cgroup_uncharge_common(page, ctype);
/* record memcg information */
- if (do_swap_account && swapout && memcg) {
+ if (do_swap_account && swapout) {
swap_cgroup_record(ent, css_id(&memcg->css));
mem_cgroup_get(memcg);
}
- if (swapout && memcg)
- css_put(&memcg->css);
+ css_put(&memcg->css);
}
#endif
@@ -2518,7 +2552,6 @@ static int mem_cgroup_move_swap_account(
*/
if (!mem_cgroup_is_root(to))
res_counter_uncharge(&to->res, PAGE_SIZE);
- css_put(&to->css);
}
return 0;
}
@@ -4219,7 +4252,6 @@ static int mem_cgroup_do_precharge(unsig
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:
@@ -4469,8 +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) */
--
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