Re: [RFC] Low overhead patches for the memory cgroup controller (v2)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-05-16 02:45:03]:

> I think set/clear flag here adds race condtion....because pc->flags is
> modfied by
>   pc->flags = pcg_dafault_flags[ctype] in commit_charge()
> you have to modify above lines to be
> 
>   SetPageCgroupCache(pc) or some..
>   ...
>   SetPageCgroupUsed(pc)
> 
> Then, you can use set_bit() without lock_page_cgroup().
> (Currently, pc->flags is modified only under lock_page_cgroup(), so,
>  non atomic code is used.)
>

Here is the next version of the patch


Feature: Remove the overhead associated with the root cgroup

From: Balbir Singh <balbir@linux.vnet.ibm.com>

This patch changes the memory cgroup and removes the overhead associated
with accounting all pages in the root cgroup. As a side-effect, we can
no longer set a memory hard limit in the root cgroup.

A new flag is used to track page_cgroup associated with the root cgroup
pages. A new flag to track whether the page has been accounted or not
has been added as well. Flags are now set atomically for page_cgroup,
pcg_default_flags is now obsolete, but I've not removed it yet. It
provides some readability to help the code.

Tests:
1. Tested lightly, previous versions showed good performance improvement 10%.

NOTE:
I haven't got the time right now to run oprofile and get detailed test results,
since I am in the middle of travel.

Please review the code for functional correctness and if you can test
it even better. I would like to push this in, especially if the %
performance difference I am seeing is reproducible elsewhere as well.

Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
---

 include/linux/page_cgroup.h |   12 ++++++++++++
 mm/memcontrol.c             |   42 ++++++++++++++++++++++++++++++++++++++----
 mm/page_cgroup.c            |    1 -
 3 files changed, 50 insertions(+), 5 deletions(-)


diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 7339c7b..ebdae9a 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -26,6 +26,8 @@ enum {
 	PCG_LOCK,  /* page cgroup is locked */
 	PCG_CACHE, /* charged as cache */
 	PCG_USED, /* this object is in use. */
+	PCG_ROOT, /* page belongs to root cgroup */
+	PCG_ACCT, /* page has been accounted for */
 };
 
 #define TESTPCGFLAG(uname, lname)			\
@@ -42,9 +44,19 @@ static inline void ClearPageCgroup##uname(struct page_cgroup *pc)	\
 
 /* Cache flag is set only once (at allocation) */
 TESTPCGFLAG(Cache, CACHE)
+SETPCGFLAG(Cache, CACHE)
 
 TESTPCGFLAG(Used, USED)
 CLEARPCGFLAG(Used, USED)
+SETPCGFLAG(Used, USED)
+
+SETPCGFLAG(Root, ROOT)
+CLEARPCGFLAG(Root, ROOT)
+TESTPCGFLAG(Root, ROOT)
+
+SETPCGFLAG(Acct, ACCT)
+CLEARPCGFLAG(Acct, ACCT)
+TESTPCGFLAG(Acct, ACCT)
 
 static inline int page_cgroup_nid(struct page_cgroup *pc)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9712ef7..35415fc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -43,6 +43,7 @@
 
 struct cgroup_subsys mem_cgroup_subsys __read_mostly;
 #define MEM_CGROUP_RECLAIM_RETRIES	5
+struct mem_cgroup *root_mem_cgroup __read_mostly;
 
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 /* Turned on only when memory cgroup is enabled && really_do_swap_account = 0 */
@@ -196,6 +197,10 @@ enum charge_type {
 #define PCGF_CACHE	(1UL << PCG_CACHE)
 #define PCGF_USED	(1UL << PCG_USED)
 #define PCGF_LOCK	(1UL << PCG_LOCK)
+/* Not used, but added here for completeness */
+#define PCGF_ROOT	(1UL << PCG_ROOT)
+#define PCGF_ACCT	(1UL << PCG_ACCT)
+
 static const unsigned long
 pcg_default_flags[NR_CHARGE_TYPE] = {
 	PCGF_CACHE | PCGF_USED | PCGF_LOCK, /* File Cache */
@@ -420,7 +425,7 @@ void mem_cgroup_del_lru_list(struct page *page, enum lru_list lru)
 		return;
 	pc = lookup_page_cgroup(page);
 	/* can happen while we handle swapcache. */
-	if (list_empty(&pc->lru) || !pc->mem_cgroup)
+	if ((!PageCgroupAcct(pc) && list_empty(&pc->lru)) || !pc->mem_cgroup)
 		return;
 	/*
 	 * We don't check PCG_USED bit. It's cleared when the "page" is finally
@@ -429,6 +434,9 @@ void mem_cgroup_del_lru_list(struct page *page, enum lru_list lru)
 	mz = page_cgroup_zoneinfo(pc);
 	mem = pc->mem_cgroup;
 	MEM_CGROUP_ZSTAT(mz, lru) -= 1;
+	ClearPageCgroupAcct(pc);
+	if (PageCgroupRoot(pc))
+		return;
 	list_del_init(&pc->lru);
 	return;
 }
@@ -452,8 +460,8 @@ void mem_cgroup_rotate_lru_list(struct page *page, enum lru_list lru)
 	 * For making pc->mem_cgroup visible, insert smp_rmb() here.
 	 */
 	smp_rmb();
-	/* unused page is not rotated. */
-	if (!PageCgroupUsed(pc))
+	/* unused or root page is not rotated. */
+	if (!PageCgroupUsed(pc) || PageCgroupRoot(pc))
 		return;
 	mz = page_cgroup_zoneinfo(pc);
 	list_move(&pc->lru, &mz->lists[lru]);
@@ -477,6 +485,9 @@ void mem_cgroup_add_lru_list(struct page *page, enum lru_list lru)
 
 	mz = page_cgroup_zoneinfo(pc);
 	MEM_CGROUP_ZSTAT(mz, lru) += 1;
+	SetPageCgroupAcct(pc);
+	if (PageCgroupRoot(pc))
+		return;
 	list_add(&pc->lru, &mz->lists[lru]);
 }
 
@@ -1114,9 +1125,24 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
 		css_put(&mem->css);
 		return;
 	}
+
 	pc->mem_cgroup = mem;
 	smp_wmb();
-	pc->flags = pcg_default_flags[ctype];
+	switch (ctype) {
+	case MEM_CGROUP_CHARGE_TYPE_CACHE:
+	case MEM_CGROUP_CHARGE_TYPE_SHMEM:
+		SetPageCgroupCache(pc);
+		SetPageCgroupUsed(pc);
+		break;
+	case MEM_CGROUP_CHARGE_TYPE_MAPPED:
+		SetPageCgroupUsed(pc);
+		break;
+	default:
+		break;
+	}
+
+	if (mem == root_mem_cgroup)
+		SetPageCgroupRoot(pc);
 
 	mem_cgroup_charge_statistics(mem, pc, true);
 
@@ -1521,6 +1547,8 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
 	mem_cgroup_charge_statistics(mem, pc, false);
 
 	ClearPageCgroupUsed(pc);
+	if (mem == root_mem_cgroup)
+		ClearPageCgroupRoot(pc);
 	/*
 	 * pc->mem_cgroup is not cleared here. It will be accessed when it's
 	 * freed from LRU. This is safe because uncharged page is expected not
@@ -2038,6 +2066,10 @@ static int mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
 	name = MEMFILE_ATTR(cft->private);
 	switch (name) {
 	case RES_LIMIT:
+		if (memcg == root_mem_cgroup) { /* Can't set limit on root */
+			ret = -EINVAL;
+			break;
+		}
 		/* This function does all necessary parse...reuse it */
 		ret = res_counter_memparse_write_strategy(buffer, &val);
 		if (ret)
@@ -2504,6 +2536,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	if (cont->parent == NULL) {
 		enable_swap_cgroup();
 		parent = NULL;
+		root_mem_cgroup = mem;
 	} else {
 		parent = mem_cgroup_from_cont(cont->parent);
 		mem->use_hierarchy = parent->use_hierarchy;
@@ -2532,6 +2565,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	return &mem->css;
 free_out:
 	__mem_cgroup_free(mem);
+	root_mem_cgroup = NULL;
 	return ERR_PTR(error);
 }
 
diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 09b73c5..6145ff6 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -276,7 +276,6 @@ void __meminit pgdat_page_cgroup_init(struct pglist_data *pgdat)
 
 #endif
 
-
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 
 static DEFINE_MUTEX(swap_cgroup_mutex);
 

-- 
	Balbir