If memcg is tracking anything other than plain user memory (swap, tcp
buf mem, or slab memory), it is possible that a reference will be held
by the group after it is dead.
This patch provides a debugging facility in the root memcg, so we can
inspect which memcgs still have pending objects, and what is the cause
of this state.
Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
---
Documentation/cgroups/memory.txt | 13 ++++
mm/memcontrol.c | 156 +++++++++++++++++++++++++++++++++++++--
2 files changed, 162 insertions(+), 7 deletions(-)
diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 8b8c28b..704247eb 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -70,6 +70,7 @@ Brief summary of control files.
memory.move_charge_at_immigrate # set/show controls of moving charges
memory.oom_control # set/show oom controls.
memory.numa_stat # show the number of memory usage per numa node
+ memory.dangling_memcgs # show debugging information about dangling groups
memory.kmem.limit_in_bytes # set/show hard limit for kernel memory
memory.kmem.usage_in_bytes # show current kernel memory allocation
@@ -577,6 +578,18 @@ unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
And we have total = file + anon + unevictable.
+5.7 dangling_memcgs
+
+This file will only be ever present in the root cgroup. When a memcg is
+destroyed, the memory consumed by it may not be immediately freed. This is
+because when some extensions are used, such as swap or kernel memory, objects
+can outlive the group and hold a reference to it.
+
+If this is the case, the dangling_memcgs file will show information about what
+are the memcgs still alive, and which references are still preventing it to be
+freed. This is a debugging facility only, and no guarantees of interface
+stability will be given.
+
6. Hierarchy support
The memory controller supports a deep hierarchy and hierarchical accounting.
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 05b87aa..46f7cfb 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -311,14 +311,31 @@ struct mem_cgroup {
/* thresholds for mem+swap usage. RCU-protected */
struct mem_cgroup_thresholds memsw_thresholds;
- /* For oom notifier event fd */
- struct list_head oom_notify;
+ union {
+ /* For oom notifier event fd */
+ struct list_head oom_notify;
+ /*
+ * we can only trigger an oom event if the memcg is alive.
+ * so we will reuse this field to hook the memcg in the list
+ * of dead memcgs.
+ */
+ struct list_head dead;
+ };
- /*
- * Should we move charges of a task when a task is moved into this
- * mem_cgroup ? And what type of charges should we move ?
- */
- unsigned long move_charge_at_immigrate;
+ union {
+ /*
+ * Should we move charges of a task when a task is moved into
+ * this mem_cgroup ? And what type of charges should we move ?
+ */
+ unsigned long move_charge_at_immigrate;
+
+ /*
+ * We are no longer concerned about moving charges after memcg
+ * is dead. So we will fill this up with its name, to aid
+ * debugging.
+ */
+ char *memcg_name;
+ };
/*
* set > 0 if pages under this cgroup are moving to other cgroup.
*/
@@ -349,6 +366,33 @@ struct mem_cgroup {
#endif
};
+#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MEMCG_SWAP)
+static LIST_HEAD(dangling_memcgs);
+static DEFINE_MUTEX(dangling_memcgs_mutex);
+
+static inline void memcg_dangling_free(struct mem_cgroup *memcg)
+{
+ mutex_lock(&dangling_memcgs_mutex);
+ list_del(&memcg->dead);
+ mutex_unlock(&dangling_memcgs_mutex);
+ kfree(memcg->memcg_name);
+}
+
+static inline void memcg_dangling_add(struct mem_cgroup *memcg)
+{
+
+ memcg->memcg_name = kstrdup(cgroup_name(memcg->css.cgroup), GFP_KERNEL);
+
+ INIT_LIST_HEAD(&memcg->dead);
+ mutex_lock(&dangling_memcgs_mutex);
+ list_add(&memcg->dead, &dangling_memcgs);
+ mutex_unlock(&dangling_memcgs_mutex);
+}
+#else
+static inline void memcg_dangling_free(struct mem_cgroup *memcg) {}
+static inline void memcg_dangling_add(struct mem_cgroup *memcg) {}
+#endif
+
/* internal only representation about the status of kmem accounting. */
enum {
KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
@@ -4868,6 +4912,92 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
return simple_read_from_buffer(buf, nbytes, ppos, str, len);
}
+#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MEMCG_SWAP)
+static void
+mem_cgroup_dangling_swap(struct mem_cgroup *memcg, struct seq_file *m)
+{
+#ifdef CONFIG_MEMCG_SWAP
+ u64 kmem;
+ u64 memsw;
+
+ /*
+ * kmem will also propagate here, so we are only interested in the
+ * difference. See comment in mem_cgroup_reparent_charges for details.
+ *
+ * We could save this value for later consumption by kmem reports, but
+ * there is not a lot of problem if the figures differ slightly.
+ */
+ kmem = res_counter_read_u64(&memcg->kmem, RES_USAGE);
+ memsw = res_counter_read_u64(&memcg->memsw, RES_USAGE) - kmem;
+ seq_printf(m, "\t%llu swap bytes\n", memsw);
+#endif
+}
+
+static void
+mem_cgroup_dangling_kmem(struct mem_cgroup *memcg, struct seq_file *m)
+{
+#ifdef CONFIG_MEMCG_KMEM
+ u64 kmem;
+ struct memcg_cache_params *params;
+
+#ifdef CONFIG_INET
+ struct tcp_memcontrol *tcp = &memcg->tcp_mem;
+ s64 tcp_socks;
+ u64 tcp_bytes;
+
+ tcp_socks = percpu_counter_sum_positive(&tcp->tcp_sockets_allocated);
+ tcp_bytes = res_counter_read_u64(&tcp->tcp_memory_allocated, RES_USAGE);
+ seq_printf(m, "\t%llu tcp bytes, in %lld sockets\n",
+ tcp_bytes, tcp_socks);
+
+#endif
+
+ kmem = res_counter_read_u64(&memcg->kmem, RES_USAGE);
+ seq_printf(m, "\t%llu kmem bytes", kmem);
+
+ /* list below may not be initialized, so not even try */
+ if (!kmem)
+ return;
+
+ seq_printf(m, " in caches");
+ mutex_lock(&memcg->slab_caches_mutex);
+ list_for_each_entry(params, &memcg->memcg_slab_caches, list) {
+ struct kmem_cache *s = memcg_params_to_cache(params);
+
+ seq_printf(m, " %s", s->name);
+ }
+ mutex_unlock(&memcg->slab_caches_mutex);
+ seq_printf(m, "\n");
+#endif
+}
+
+/*
+ * After a memcg is destroyed, it may still be kept around in memory.
+ * Currently, the two main reasons for it are swap entries, and kernel memory.
+ * Because they will be freed assynchronously, they will pin the memcg structure
+ * and its resources until the last reference goes away.
+ *
+ * This root-only file will show information about which users
+ */
+static int mem_cgroup_dangling_read(struct cgroup *cont, struct cftype *cft,
+ struct seq_file *m)
+{
+ struct mem_cgroup *memcg;
+
+ mutex_lock(&dangling_memcgs_mutex);
+
+ list_for_each_entry(memcg, &dangling_memcgs, dead) {
+ seq_printf(m, "%s:\n", memcg->memcg_name);
+
+ mem_cgroup_dangling_swap(memcg, m);
+ mem_cgroup_dangling_kmem(memcg, m);
+ }
+
+ mutex_unlock(&dangling_memcgs_mutex);
+ return 0;
+}
+#endif
+
static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
{
int ret = -EINVAL;
@@ -5831,6 +5961,14 @@ static struct cftype mem_cgroup_files[] = {
},
#endif
#endif
+
+#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MEMCG_SWAP)
+ {
+ .name = "dangling_memcgs",
+ .read_seq_string = mem_cgroup_dangling_read,
+ .flags = CFTYPE_ONLY_ON_ROOT,
+ },
+#endif
{ }, /* terminate */
};
@@ -5933,6 +6071,7 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
* the cgroup_lock.
*/
disarm_static_keys(memcg);
+
if (size < PAGE_SIZE)
kfree(memcg);
else
@@ -5950,6 +6089,8 @@ static void free_work(struct work_struct *work)
struct mem_cgroup *memcg;
memcg = container_of(work, struct mem_cgroup, work_freeing);
+
+ memcg_dangling_free(memcg);
__mem_cgroup_free(memcg);
}
@@ -6139,6 +6280,7 @@ static void mem_cgroup_destroy(struct cgroup *cont)
kmem_cgroup_destroy(memcg);
+ memcg_dangling_add(memcg);
mem_cgroup_put(memcg);
}
--
1.7.11.7
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