On Mon 03-12-12 17:24:08, Glauber Costa wrote:
> If memcg is tracking anything other than plain user memory (swap, tcp
> buf mem, or slab memory), it is possible - and normal - that a reference
> will be held by the group after it is dead. Still, for developers, it
> would be extremely useful to be able to query about those states during
> debugging.
>
> 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>
Thanks!
Acked-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> I am no longer relying on cgroup_name, using the full path instead.
> Therefore, I dropped the first patch from v1. That's all in here, now.
> ---
> Documentation/cgroups/memory.txt | 16 ++++
> init/Kconfig | 17 ++++
> mm/memcontrol.c | 192 +++++++++++++++++++++++++++++++++++++--
> 3 files changed, 218 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> index 8b8c28b..addb1f1 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 +578,21 @@ 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, if the option
> +CONFIG_MEMCG_DEBUG_ASYNC_DESTROY is set. 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. There is nothing wrong with that, but it is very useful when debugging,
> +to know where this memory is being held. This is a developer-oriented debugging
> +facility only, and no guarantees of interface stability will be given. The file
> +is read-only, and has the sole purpose of displaying information.
> +
> 6. Hierarchy support
>
> The memory controller supports a deep hierarchy and hierarchical accounting.
> diff --git a/init/Kconfig b/init/Kconfig
> index 3d26eb9..78234f8 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -750,6 +750,23 @@ config MEMCG_KMEM
> the kmem extension can use it to guarantee that no group of processes
> will ever exhaust kernel resources alone.
>
> +config MEMCG_DEBUG_ASYNC_DESTROY
> + bool "Memory Resource Controller Debug assynchronous object destruction"
> + depends on MEMCG_KMEM || MEMCG_SWAP
> + default n
> + help
> + 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. There is nothing wrong with that, but it is
> + very useful when debugging, to know where this memory is being held.
> + This is a developer-oriented debugging facility only, and no
> + guarantees of interface stability will be given.
> +
> config CGROUP_HUGETLB
> bool "HugeTLB Resource Controller for Control Groups"
> depends on RESOURCE_COUNTERS && HUGETLB_PAGE && EXPERIMENTAL
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e3d805f..b9038a4 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 +366,55 @@ struct mem_cgroup {
> #endif
> };
>
> +#ifdef CONFIG_MEMCG_DEBUG_ASYNC_DESTROY
> +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);
> + free_pages((unsigned long)memcg->memcg_name, 0);
> +}
> +
> +static inline void memcg_dangling_add(struct mem_cgroup *memcg)
> +{
> + /*
> + * cgroup.c will do page-sized allocations most of the time,
> + * so we'll just follow the pattern. Also, __get_free_pages
> + * is a better interface than kmalloc for us here, because
> + * we'd like this memory to be always billed to the root cgroup,
> + * not to the process removing the memcg. While kmalloc would
> + * require us to wrap it into memcg_stop/resume_kmem_account,
> + * with __get_free_pages we just don't pass the memcg flag.
> + */
> + memcg->memcg_name = (char *)__get_free_pages(GFP_KERNEL, 0);
> +
> + /*
> + * we will, in general, just ignore failures. No need to go crazy,
> + * being this just a debugging interface. It is nice to copy a memcg
> + * name over, but if we (unlikely) can't, just the address will do
> + */
> + if (!memcg->memcg_name)
> + goto add_list;
> +
> + if (cgroup_path(memcg->css.cgroup, memcg->memcg_name, PAGE_SIZE) < 0) {
> + free_pages((unsigned long)memcg->memcg_name, 0);
> + memcg->memcg_name = NULL;
> + }
> +
> +add_list:
> + 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 */
> @@ -4871,6 +4937,107 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
> return simple_read_from_buffer(buf, nbytes, ppos, str, len);
> }
>
> +#ifdef CONFIG_MEMCG_DEBUG_ASYNC_DESTROY
> +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_tcp(struct mem_cgroup *memcg, struct seq_file *m)
> +{
> +#if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
> + 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", tcp_bytes);
> + /*
> + * if tcp_bytes == 0, tcp_socks != 0 is a bug. One more reason to print
> + * it!
> + */
> + if (tcp_bytes || tcp_socks)
> + seq_printf(m, ", in %lld sockets", tcp_socks);
> + seq_printf(m, "\n");
> +
> +#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;
> +
> + 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) {
> + if (memcg->memcg_name)
> + seq_printf(m, "%s:\n", memcg->memcg_name);
> + else
> + seq_printf(m, "%p (name lost):\n", memcg);
> +
> + mem_cgroup_dangling_swap(memcg, m);
> + mem_cgroup_dangling_tcp(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;
> @@ -5834,6 +6001,14 @@ static struct cftype mem_cgroup_files[] = {
> },
> #endif
> #endif
> +
> +#ifdef CONFIG_MEMCG_DEBUG_ASYNC_DESTROY
> + {
> + .name = "dangling_memcgs",
> + .read_seq_string = mem_cgroup_dangling_read,
> + .flags = CFTYPE_ONLY_ON_ROOT,
> + },
> +#endif
> { }, /* terminate */
> };
>
> @@ -5953,6 +6128,8 @@ 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);
> }
>
> @@ -6142,6 +6319,7 @@ 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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--
Michal Hocko
SUSE Labs
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