On 09/21/2011 03:47 PM, Greg Thelen wrote:

On Sun, Sep 18, 2011 at 5:56 PM, Glauber Costa[off-list ref]  wrote:

quoted

We aim to control the amount of kernel memory pinned at any
time by tcp sockets. To lay the foundations for this work,
this patch adds a pointer to the kmem_cgroup to the socket
structure.

Signed-off-by: Glauber Costa<redacted>
CC: David S. Miller<davem@davemloft.net>
CC: Hiroyouki Kamezawa<redacted>
CC: Eric W. Biederman<redacted>

...

quoted

+void sock_update_memcg(struct sock *sk)
+{
+       /* right now a socket spends its whole life in the same cgroup */
+       BUG_ON(sk->sk_cgrp);
+
+       rcu_read_lock();
+       sk->sk_cgrp = mem_cgroup_from_task(current);
+
+       /*
+        * We don't need to protect against anything task-related, because
+        * we are basically stuck with the sock pointer that won't change,
+        * even if the task that originated the socket changes cgroups.
+        *
+        * What we do have to guarantee, is that the chain leading us to
+        * the top level won't change under our noses. Incrementing the
+        * reference count via cgroup_exclude_rmdir guarantees that.
+        */
+       cgroup_exclude_rmdir(mem_cgroup_css(sk->sk_cgrp));

This grabs a css_get() reference, which prevents rmdir (will return
-EBUSY).

Yes.

  How long is this reference held?
For the socket lifetime.

I wonder about the case
where a process creates a socket in memcg M1 and later is moved into
memcg M2.  At that point an admin would expect to be able to 'rmdir
M1'.  I think this rmdir would return -EBUSY and I suspect it would be
difficult for the admin to understand why the rmdir of M1 failed.  It
seems that to rmdir a memcg, an admin would have to kill all processes
that allocated sockets while in M1.  Such processes may not still be
in M1.

quoted

+       rcu_read_unlock();
+}

I agree. But also, don't see too much ways around it without 
implementing full task migration.

Right now I am working under the assumption that tasks are long lived 
inside the cgroup. Migration potentially introduces some nasty locking 
problems in the mem_schedule path.

Also, unless I am missing something, the memcg already has the policy of
not carrying charges around, probably because of this very same complexity.

True that at least it won't EBUSY you... But I think this is at least a 
way to guarantee that the cgroup under our nose won't disappear in the 
middle of our allocations.

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