[Devel] Re: [PATCH v4 04/14] kmem accounting basic infrastructure

[Michal Hocko]

On Mon 08-10-12 14:06:10, Glauber Costa wrote:
> This patch adds the basic infrastructure for the accounting of the slab
> caches. To control that, the following files are created:
> 
>  * memory.kmem.usage_in_bytes
>  * memory.kmem.limit_in_bytes
>  * memory.kmem.failcnt
>  * memory.kmem.max_usage_in_bytes
> 
> They have the same meaning of their user memory counterparts. They
> reflect the state of the "kmem" res_counter.
> 
> Per cgroup slab memory accounting is not enabled until a limit is set

s/slab/kmem/ right?

> for the group. Once the limit is set the accounting cannot be disabled
> for that group.  This means that after the patch is applied, no
> behavioral changes exists for whoever is still using memcg to control
> their memory usage, until memory.kmem.limit_in_bytes is set for the
> first time.
> 
> We always account to both user and kernel resource_counters. This
> effectively means that an independent kernel limit is in place when the
> limit is set to a lower value than the user memory. A equal or higher
> value means that the user limit will always hit first, meaning that kmem
> is effectively unlimited.
> 
> People who want to track kernel memory but not limit it, can set this
> limit to a very high number (like RESOURCE_MAX - 1page - that no one
> will ever hit, or equal to the user memory)
> 
> [ v4: make kmem files part of the main array;
>       do not allow limit to be set for non-empty cgroups ]
> 
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> ---
>  mm/memcontrol.c | 123 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 122 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 71d259e..ba855cc 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
[...]
> @@ -332,6 +337,26 @@ struct mem_cgroup {
>  #endif
>  };
>  
> +/* internal only representation about the status of kmem accounting. */
> +enum {
> +	KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
> +};
> +
> +/* first bit */
> +#define KMEM_ACCOUNTED_MASK 0x1
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +static void memcg_kmem_set_active(struct mem_cgroup *memcg)
> +{
> +	set_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_accounted);
> +}
> +
> +static bool memcg_kmem_is_accounted(struct mem_cgroup *memcg)
> +{
> +	return test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_accounted);
> +}
> +#endif

set_active vs. is_accounted. Is there any reason for inconsistency here?

> +
>  /* Stuffs for move charges at task migration. */
>  /*
>   * Types of charges to be moved. "move_charge_at_immitgrate" is treated as a
[...]
> @@ -3947,6 +3980,58 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
>  	len = scnprintf(str, sizeof(str), "%llu\n", (unsigned long long)val);
>  	return simple_read_from_buffer(buf, nbytes, ppos, str, len);
>  }
> +
> +static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
> +{
> +	int ret = -EINVAL;
> +#ifdef CONFIG_MEMCG_KMEM
> +	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
> +	/*
> +	 * For simplicity, we won't allow this to be disabled.  It also can't
> +	 * be changed if the cgroup has children already, or if tasks had
> +	 * already joined.
> +	 *
> +	 * If tasks join before we set the limit, a person looking at
> +	 * kmem.usage_in_bytes will have no way to determine when it took
> +	 * place, which makes the value quite meaningless.
> +	 *
> +	 * After it first became limited, changes in the value of the limit are
> +	 * of course permitted.
> +	 *
> +	 * Taking the cgroup_lock is really offensive, but it is so far the only
> +	 * way to guarantee that no children will appear. There are plenty of
> +	 * other offenders, and they should all go away. Fine grained locking
> +	 * is probably the way to go here. When we are fully hierarchical, we
> +	 * can also get rid of the use_hierarchy check.
> +	 */
> +	cgroup_lock();
> +	mutex_lock(&set_limit_mutex);
> +	if (!memcg->kmem_accounted && val != RESOURCE_MAX) {

Just a nit but wouldn't memcg_kmem_is_accounted(memcg) be better than
directly checking kmem_accounted?
Besides that I am not sure I fully understand RESOURCE_MAX test. Say I
want to have kmem accounting for monitoring so I do 
echo -1 > memory.kmem.limit_in_bytes

so you set the value but do not activate it. Isn't this just a reminder
from the time when the accounting could be deactivated?

> +		if (cgroup_task_count(cont) || (memcg->use_hierarchy &&
> +						!list_empty(&cont->children))) {
> +			ret = -EBUSY;
> +			goto out;
> +		}
> +		ret = res_counter_set_limit(&memcg->kmem, val);

VM_BUG_IN(ret) ?
There shouldn't be any usage when you enable it or something bad is
going on.

> +		if (ret)
> +			goto out;
> +
> +		memcg_kmem_set_active(memcg);
> +	} else
> +		ret = res_counter_set_limit(&memcg->kmem, val);
> +out:
> +	mutex_unlock(&set_limit_mutex);
> +	cgroup_unlock();
> +#endif
> +	return ret;
> +}
> +
> +static void memcg_propagate_kmem(struct mem_cgroup *memcg,
> +				 struct mem_cgroup *parent)
> +{
> +	memcg->kmem_accounted = parent->kmem_accounted;
> +}
> +
>  /*
>   * The user of this function is...
>   * RES_LIMIT.
[...]
-- 
Michal Hocko
SUSE Labs