[PATCH RFC 5/5] vm_cgroup: do not charge tasks in root cgroup

[Vladimir Davydov <vdavydov@parallels.com>]

For the root cgroup (the whole system), we already have overcommit
accounting and control, so we can skip charging tasks in the root cgroup
to avoid overhead.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
 mm/vm_cgroup.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/vm_cgroup.c b/mm/vm_cgroup.c
index 6642f934540a..c871fecaab4c 100644
--- a/mm/vm_cgroup.c
+++ b/mm/vm_cgroup.c
@@ -1,6 +1,7 @@
 #include <linux/cgroup.h>
 #include <linux/res_counter.h>
 #include <linux/mm.h>
+#include <linux/mman.h>
 #include <linux/slab.h>
 #include <linux/rcupdate.h>
 #include <linux/shmem_fs.h>
@@ -65,6 +66,9 @@ static int vm_cgroup_do_charge(struct vm_cgroup *vmcg,
 	unsigned long val = nr_pages << PAGE_SHIFT;
 	struct res_counter *fail_res;
 
+	if (vm_cgroup_is_root(vmcg))
+		return 0;
+
 	return res_counter_charge(&vmcg->res, val, &fail_res);
 }
 
@@ -73,6 +77,9 @@ static void vm_cgroup_do_uncharge(struct vm_cgroup *vmcg,
 {
 	unsigned long val = nr_pages << PAGE_SHIFT;
 
+	if (vm_cgroup_is_root(vmcg))
+		return;
+
 	res_counter_uncharge(&vmcg->res, val);
 }
 
@@ -159,6 +166,9 @@ static u64 vm_cgroup_read_u64(struct cgroup_subsys_state *css,
 	struct vm_cgroup *vmcg = vm_cgroup_from_css(css);
 	int memb = cft->private;
 
+	if (vm_cgroup_is_root(vmcg))
+		return vm_memory_committed() << PAGE_SHIFT;
+
 	return res_counter_read_u64(&vmcg->res, memb);
 }
 
-- 
1.7.10.4

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