On 10/11/2012 02:11 PM, Michal Hocko wrote:

On Mon 08-10-12 14:06:10, Glauber Costa wrote:

quoted

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?

right.

quoted

+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?

No, not at all.

I see you have talked about that in other e-mails, (I was on sick leave
yesterday), so let me consolidate it all here:

What we discussed before, regarding to echo -1 > ... was around the
disable code, something that we no longer allow. So now, if you will
echo -1 to that file *after* it is limited, you get in track only mode.

But for you to start that, you absolutely have to write something
different than -1.

Just one example: libcgroup, regardless of how lame we think it is in
this regard, will write to all cgroup files by default when a file is
updated. If you haven't written anything, it will still write the same
value that the file had before.

This means that an already deployed libcg-managed installation will
suddenly enable kmem for every cgroup. Sure this can be fixed in
userspace, but:

1) There is no reason to break it, if we can
2) It is perfectly reasonable to expect that if you write to a file the
same value that was already there, nothing happens.

I'll update the docs to say that you can just write -1 *after* it is
limited, but i believe enabling it has to be a very clear transition,
for sanity's sake.

quoted

+		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.

Good point, this is indeed an impossible scenario I was just being
overcautious about.