[RFC PATCH 1/2] mm: rework memcg kernel stack accounting

[Roman Gushchin]

If CONFIG_VMAP_STACK is set, kernel stacks are allocated
using __vmalloc_node_range() with __GFP_ACCOUNT. So kernel
stack pages are charged against corresponding memory cgroups
on allocation and uncharged on releasing them.

The problem is that we do cache kernel stacks in small
per-cpu caches and do reuse them for new tasks, which can
belong to different memory cgroups.

Each stack page still holds a reference to the original cgroup,
so the cgroup can't be released until the vmap area is released.

To make this happen we need more than two subsequent exits
without forks in between on the current cpu, which makes it
very unlikely to happen. As a result, I saw a significant number
of dying cgroups (in theory, up to 2 * number_of_cpu +
number_of_tasks), which can't be released even by significant
memory pressure.

As a cgroup structure can take a significant amount of memory
(first of all, per-cpu data like memcg statistics), it leads
to a noticeable waste of memory.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/fork.c | 44 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 6 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 69b6fea5a181..91872b2b37bd 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -224,9 +224,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 		return s->addr;
 	}
 
+	/*
+	 * Allocated stacks are cached and later reused by new threads,
+	 * so memcg accounting is performed manually on assigning/releasing
+	 * stacks to tasks. Drop __GFP_ACCOUNT.
+	 */
 	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
 				     VMALLOC_START, VMALLOC_END,
-				     THREADINFO_GFP,
+				     THREADINFO_GFP & ~__GFP_ACCOUNT,
 				     PAGE_KERNEL,
 				     0, node, __builtin_return_address(0));
 
@@ -246,12 +251,41 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 #endif
 }
 
+static void memcg_charge_kernel_stack(struct task_struct *tsk)
+{
+#ifdef CONFIG_VMAP_STACK
+	struct vm_struct *vm = task_stack_vm_area(tsk);
+
+	if (vm) {
+		int i;
+
+		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+			memcg_kmem_charge(vm->pages[i], __GFP_NOFAIL,
+					  compound_order(vm->pages[i]));
+
+		/* All stack pages belong to the same memcg. */
+		mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
+				     THREAD_SIZE / 1024);
+	}
+#endif
+}
+
 static inline void free_thread_stack(struct task_struct *tsk)
 {
 #ifdef CONFIG_VMAP_STACK
-	if (task_stack_vm_area(tsk)) {
+	struct vm_struct *vm = task_stack_vm_area(tsk);
+
+	if (vm) {
 		int i;
 
+		/* All stack pages belong to the same memcg. */
+		mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
+				     -(int)(THREAD_SIZE / 1024));
+
+		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
+			memcg_kmem_uncharge(vm->pages[i],
+					  compound_order(vm->pages[i]));
+
 		for (i = 0; i < NR_CACHED_STACKS; i++) {
 			if (this_cpu_cmpxchg(cached_stacks[i],
 					NULL, tsk->stack_vm_area) != NULL)
@@ -352,10 +386,6 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 					    NR_KERNEL_STACK_KB,
 					    PAGE_SIZE / 1024 * account);
 		}
-
-		/* All stack pages belong to the same memcg. */
-		mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
-				     account * (THREAD_SIZE / 1024));
 	} else {
 		/*
 		 * All stack pages are in the same zone and belong to the
@@ -809,6 +839,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	if (!stack)
 		goto free_tsk;
 
+	memcg_charge_kernel_stack(tsk);
+
 	stack_vm_area = task_stack_vm_area(tsk);
 
 	err = arch_dup_task_struct(tsk, orig);
-- 
2.14.4