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Subject[PATCH 4/6] memcg: new force_empty and move_account
This patch provides a function to move account information of a page between
mem_cgroups and rewrite force_empty to make use of this.

This moving of page_cgroup is done under
- lru_lock of source/destination mem_cgroup is held.
- lock_page_cgroup() is held.

Then, a routine which touches pc->mem_cgroup without lock_page_cgroup() should
confirm pc->mem_cgroup is still valid or not. Typlical code can be following.

(while page is not under lock_page())
mem = pc->mem_cgroup;
mz = page_cgroup_zoneinfo(pc)
spin_lock_irqsave(&mz->lru_lock);
if (pc->mem_cgroup == mem)
...../* some list handling */
spin_unlock_irq(&mz->lru_lock);
Of course, better way is
lock_page_cgroup(pc);
....
unlock_page_cgroup(pc);
But you should confirm the nest of lock and avoid deadlock.

If you treats page_cgroup from mem_cgroup's LRU under mz->lru_lock,
you don't have to worry about what pc->mem_cgroup points to.
moved pages are added to head of lru, not to tail.

Expected users of this routine is:
- force_empty (rmdir)
- moving tasks between cgroup (for moving account information.)
- hierarchy (maybe useful.)

force_empty(rmdir) uses this move_account and move pages to its parent.
This "move" will not cause OOM (I added "oom" parameter to try_charge().)

If the parent is busy (not enough memory), force_empty calls try_to_free_page()
and reduce usage.

Purpose of this behavior is
- Fix "forget all" behavior of force_empty and avoid leak of accounting.
- By "moving first, free if necessary", keep pages on memory as much as
possible.

Adding a switch to change behavior of force_empty to
- free first, move if necessary
- free all, if there is mlocked/busy pages, return -EBUSY.
is under consideration.

Changelog: (v5) -> (v6)
- removed unnecessary check.
- do all under lock_page_cgroup().
- removed res_counter_charge() from move function itself.
(and modifies try_charge() function.)
- add argument to add_list() to specify to add page_cgroup head or tail.
- merged with force_empty patch. (to answer who is user? question)

Changelog: (v4) -> (v5)
- check for lock_page() is removed.
- rewrote description.

Changelog: (v2) -> (v4)
- added lock_page_cgroup().
- splitted out from new-force-empty patch.
- added how-to-use text.
- fixed race in __mem_cgroup_uncharge_common().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Documentation/controllers/memory.txt | 10 -
mm/memcontrol.c | 267 +++++++++++++++++++++++++++--------
2 files changed, 216 insertions(+), 61 deletions(-)
Index: mmotm-2.6.27-rc7+/mm/memcontrol.c
===================================================================
--- mmotm-2.6.27-rc7+.orig/mm/memcontrol.c
+++ mmotm-2.6.27-rc7+/mm/memcontrol.c
@@ -257,7 +257,7 @@ static void __mem_cgroup_remove_list(str
}

static void __mem_cgroup_add_list(struct mem_cgroup_per_zone *mz,
- struct page_cgroup *pc)
+ struct page_cgroup *pc, bool hot)
{
int lru = LRU_BASE;

@@ -271,7 +271,10 @@ static void __mem_cgroup_add_list(struct
}

MEM_CGROUP_ZSTAT(mz, lru) += 1;
- list_add(&pc->lru, &mz->lists[lru]);
+ if (hot)
+ list_add(&pc->lru, &mz->lists[lru]);
+ else
+ list_add_tail(&pc->lru, &mz->lists[lru]);

mem_cgroup_charge_statistics(pc->mem_cgroup, pc, true);
}
@@ -467,21 +470,12 @@ unsigned long mem_cgroup_isolate_pages(u
return nr_taken;
}

-
-/**
- * mem_cgroup_try_charge - get charge of PAGE_SIZE.
- * @mm: an mm_struct which is charged against. (when *memcg is NULL)
- * @gfp_mask: gfp_mask for reclaim.
- * @memcg: a pointer to memory cgroup which is charged against.
- *
- * charge aginst memory cgroup pointed by *memcg. if *memcg == NULL, estimated
- * memory cgroup from @mm is got and stored in *memcg.
- *
- * Retruns 0 if success. -ENOMEM at failure.
+/*
+ * Unlike exported interface, "oom" parameter is added. if oom==true,
+ * oom-killer can be invoked.
*/
-
-int mem_cgroup_try_charge(struct mm_struct *mm,
- gfp_t gfp_mask, struct mem_cgroup **memcg)
+static int __mem_cgroup_try_charge(struct mm_struct *mm,
+ gfp_t gfp_mask, struct mem_cgroup **memcg, bool oom)
{
struct mem_cgroup *mem;
int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
@@ -528,7 +522,8 @@ int mem_cgroup_try_charge(struct mm_stru
continue;

if (!nr_retries--) {
- mem_cgroup_out_of_memory(mem, gfp_mask);
+ if (oom)
+ mem_cgroup_out_of_memory(mem, gfp_mask);
goto nomem;
}
}
@@ -538,6 +533,24 @@ nomem:
return -ENOMEM;
}

+/**
+ * mem_cgroup_try_charge - get charge of PAGE_SIZE.
+ * @mm: an mm_struct which is charged against. (when *memcg is NULL)
+ * @gfp_mask: gfp_mask for reclaim.
+ * @memcg: a pointer to memory cgroup which is charged against.
+ *
+ * charge aginst memory cgroup pointed by *memcg. if *memcg == NULL, estimated
+ * memory cgroup from @mm is got and stored in *memcg.
+ *
+ * Retruns 0 if success. -ENOMEM at failure.
+ */
+
+int mem_cgroup_try_charge(struct mm_struct *mm,
+ gfp_t mask, struct mem_cgroup **memcg)
+{
+ return __mem_cgroup_try_charge(mm, mask, memcg, false);
+}
+
/*
* commit a charge got by mem_cgroup_try_charge() and makes page_cgroup to be
* USED state. If already USED, uncharge and return.
@@ -567,11 +580,109 @@ static void __mem_cgroup_commit_charge(s
mz = page_cgroup_zoneinfo(pc);

spin_lock_irqsave(&mz->lru_lock, flags);
- __mem_cgroup_add_list(mz, pc);
+ __mem_cgroup_add_list(mz, pc, true);
spin_unlock_irqrestore(&mz->lru_lock, flags);
unlock_page_cgroup(pc);
}

+/**
+ * mem_cgroup_move_account - move account of the page
+ * @pc ... page_cgroup of the page.
+ * @from ... mem_cgroup which the page is moved from.
+ * @to ... mem_cgroup which the page is moved to. @from != @to.
+ *
+ * The caller must confirm following.
+ * 1. disable irq.
+ * 2. lru_lock of old mem_cgroup(@from) should be held.
+ *
+ * returns 0 at success,
+ * returns -EBUSY when lock is busy or "pc" is unstable.
+ *
+ * This function do "uncharge" from old cgroup but doesn't do "charge" to
+ * new cgroup. It should be done by a caller.
+ */
+
+static int mem_cgroup_move_account(struct page_cgroup *pc,
+ struct mem_cgroup *from, struct mem_cgroup *to)
+{
+ struct mem_cgroup_per_zone *from_mz, *to_mz;
+ int nid, zid;
+ int ret = -EBUSY;
+
+ VM_BUG_ON(!irqs_disabled());
+ VM_BUG_ON(from == to);
+
+ nid = page_cgroup_nid(pc);
+ zid = page_cgroup_zid(pc);
+ from_mz = mem_cgroup_zoneinfo(from, nid, zid);
+ to_mz = mem_cgroup_zoneinfo(to, nid, zid);
+
+
+ if (!trylock_page_cgroup(pc))
+ return ret;
+
+ if (!PageCgroupUsed(pc))
+ goto out;
+
+ if (pc->mem_cgroup != from)
+ goto out;
+
+ if (spin_trylock(&to_mz->lru_lock)) {
+ __mem_cgroup_remove_list(from_mz, pc);
+ css_put(&from->css);
+ res_counter_uncharge(&from->res, PAGE_SIZE);
+ pc->mem_cgroup = to;
+ css_get(&to->css);
+ __mem_cgroup_add_list(to_mz, pc, false);
+ ret = 0;
+ spin_unlock(&to_mz->lru_lock);
+ }
+out:
+ unlock_page_cgroup(pc);
+ return ret;
+}
+
+/*
+ * move charges to its parent.
+ */
+
+static int mem_cgroup_move_parent(struct page_cgroup *pc,
+ struct mem_cgroup *child,
+ gfp_t gfp_mask)
+{
+ struct cgroup *cg = child->css.cgroup;
+ struct cgroup *pcg = cg->parent;
+ struct mem_cgroup *parent;
+ struct mem_cgroup_per_zone *mz;
+ unsigned long flags;
+ int ret;
+
+ /* Is ROOT ? */
+ if (!pcg)
+ return -EINVAL;
+
+ parent = mem_cgroup_from_cont(pcg);
+
+ ret = __mem_cgroup_try_charge(NULL, gfp_mask, &parent, false);
+ if (ret)
+ return ret;
+
+ mz = mem_cgroup_zoneinfo(child,
+ page_cgroup_nid(pc), page_cgroup_zid(pc));
+
+ spin_lock_irqsave(&mz->lru_lock, flags);
+ ret = mem_cgroup_move_account(pc, child, parent);
+ spin_unlock_irqrestore(&mz->lru_lock, flags);
+
+ /* drop extra refcnt */
+ css_put(&parent->css);
+ /* uncharge if move fails */
+ if (ret)
+ res_counter_uncharge(&parent->res, PAGE_SIZE);
+
+ return ret;
+}
+
/*
* Charge the memory controller for page usage.
* Return
@@ -593,7 +704,7 @@ static int mem_cgroup_charge_common(stru
prefetchw(pc);

mem = memcg;
- ret = mem_cgroup_try_charge(mm, gfp_mask, &mem);
+ ret = __mem_cgroup_try_charge(mm, gfp_mask, &mem, true);
if (ret)
return ret;

@@ -896,46 +1007,52 @@ int mem_cgroup_resize_limit(struct mem_c
* This routine traverse page_cgroup in given list and drop them all.
* *And* this routine doesn't reclaim page itself, just removes page_cgroup.
*/
-#define FORCE_UNCHARGE_BATCH (128)
-static void mem_cgroup_force_empty_list(struct mem_cgroup *mem,
+static int mem_cgroup_force_empty_list(struct mem_cgroup *mem,
struct mem_cgroup_per_zone *mz,
enum lru_list lru)
{
- struct page_cgroup *pc;
- struct page *page;
- int count = FORCE_UNCHARGE_BATCH;
+ struct page_cgroup *pc, *busy;
unsigned long flags;
+ unsigned long loop;
struct list_head *list;
+ int ret = 0;

list = &mz->lists[lru];

- spin_lock_irqsave(&mz->lru_lock, flags);
- while (!list_empty(list)) {
- pc = list_entry(list->prev, struct page_cgroup, lru);
- page = pc->page;
- if (!PageCgroupUsed(pc))
+ loop = MEM_CGROUP_ZSTAT(mz, lru);
+ /* give some margin against EBUSY etc...*/
+ loop += 256;
+ busy = NULL;
+ while (loop--) {
+ ret = 0;
+ spin_lock_irqsave(&mz->lru_lock, flags);
+ if (list_empty(list)) {
+ spin_unlock_irqrestore(&mz->lru_lock, flags);
break;
- get_page(page);
+ }
+ pc = list_entry(list->prev, struct page_cgroup, lru);
+ if (busy == pc) {
+ list_move(&pc->lru, list);
+ busy = 0;
+ spin_unlock_irqrestore(&mz->lru_lock, flags);
+ continue;
+ }
spin_unlock_irqrestore(&mz->lru_lock, flags);
- /*
- * Check if this page is on LRU. !LRU page can be found
- * if it's under page migration.
- */
- if (PageLRU(page)) {
- __mem_cgroup_uncharge_common(page,
- MEM_CGROUP_CHARGE_TYPE_FORCE);
- put_page(page);
- if (--count <= 0) {
- count = FORCE_UNCHARGE_BATCH;
- cond_resched();
- }
- } else {
- spin_lock_irqsave(&mz->lru_lock, flags);
+
+ ret = mem_cgroup_move_parent(pc, mem, GFP_HIGHUSER_MOVABLE);
+ if (ret == -ENOMEM)
break;
- }
- spin_lock_irqsave(&mz->lru_lock, flags);
+
+ if (ret == -EBUSY || ret == -EINVAL) {
+ /* found lock contention or "pc" is obsolete. */
+ busy = pc;
+ cond_resched();
+ } else
+ busy = NULL;
}
- spin_unlock_irqrestore(&mz->lru_lock, flags);
+ if (!ret && !list_empty(list))
+ return -EBUSY;
+ return ret;
}

/*
@@ -944,32 +1061,66 @@ static void mem_cgroup_force_empty_list(
*/
static int mem_cgroup_force_empty(struct mem_cgroup *mem)
{
- int ret = -EBUSY;
- int node, zid;
+ int ret;
+ int node, zid, shrink;
+ int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;

css_get(&mem->css);
- /*
- * page reclaim code (kswapd etc..) will move pages between
- * active_list <-> inactive_list while we don't take a lock.
- * So, we have to do loop here until all lists are empty.
- */
+
+ shrink = 0;
+move_account:
while (mem->res.usage > 0) {
+ ret = -EBUSY;
if (atomic_read(&mem->css.cgroup->count) > 0)
goto out;
- for_each_node_state(node, N_POSSIBLE)
- for (zid = 0; zid < MAX_NR_ZONES; zid++) {
+
+ ret = 0;
+ for_each_node_state(node, N_POSSIBLE) {
+ for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
struct mem_cgroup_per_zone *mz;
enum lru_list l;
mz = mem_cgroup_zoneinfo(mem, node, zid);
- for_each_lru(l)
- mem_cgroup_force_empty_list(mem, mz, l);
+ for_each_lru(l) {
+ ret = mem_cgroup_force_empty_list(mem,
+ mz, l);
+ if (ret)
+ break;
+ }
}
+ if (ret)
+ break;
+ }
+ /* it seems parent cgroup doesn't have enough mem */
+ if (ret == -ENOMEM)
+ goto try_to_free;
cond_resched();
}
ret = 0;
out:
css_put(&mem->css);
return ret;
+
+try_to_free:
+ /* returns EBUSY if we come here twice. */
+ if (shrink) {
+ ret = -EBUSY;
+ goto out;
+ }
+ /* try to free all pages in this cgroup */
+ shrink = 1;
+ while (nr_retries && mem->res.usage > 0) {
+ int progress;
+ progress = try_to_free_mem_cgroup_pages(mem,
+ GFP_HIGHUSER_MOVABLE);
+ if (!progress)
+ nr_retries--;
+
+ }
+ /* try move_account...there may be some *locked* pages. */
+ if (mem->res.usage)
+ goto move_account;
+ ret = 0;
+ goto out;
}

static u64 mem_cgroup_read(struct cgroup *cont, struct cftype *cft)
Index: mmotm-2.6.27-rc7+/Documentation/controllers/memory.txt
===================================================================
--- mmotm-2.6.27-rc7+.orig/Documentation/controllers/memory.txt
+++ mmotm-2.6.27-rc7+/Documentation/controllers/memory.txt
@@ -211,7 +211,9 @@ The memory.force_empty gives an interfac

# echo 1 > memory.force_empty

-will drop all charges in cgroup. Currently, this is maintained for test.
+Will move account to parent. if parenet is full, will try to free pages.
+If both of a parent and a child are busy, return -EBUSY;
+This file, memory.force_empty, is just for debug purpose.

4. Testing

@@ -242,8 +244,10 @@ reclaimed.

A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
cgroup might have some charge associated with it, even though all
-tasks have migrated away from it. Such charges are automatically dropped at
-rmdir() if there are no tasks.
+tasks have migrated away from it.
+Such charges are moved to its parent as mush as possible and freed if parent
+seems to be full. (see force_empty)
+If both of them are busy, rmdir() returns -EBUSY.

5. TODO



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