(2012/04/10 11:51), Glauber Costa wrote:

On 04/09/2012 11:37 PM, KAMEZAWA Hiroyuki wrote:

quoted

Hm. What happens in following sequence ?

   1. a memcg is created
   2. put a task into the memcg, start tcp steam
   3. set tcp memory limit

The resource used between 2 and 3 will cause the problem finally.

I don't get it. if a task is in memcg, but no limit is set,
that socket will be assigned null memcg, and will stay like that
forever. Only new sockets will have the new memcg pointer.

And previously, we could have the memcg pointer alive, but the jump
labels to be disabled. With the patch I posted, this can't happen
anymore, since the jump labels are guaranteed to live throughout the
whole socket life.

quoted

Then, Dave's request
==
You must either:

1) Integrate the socket's existing usage when the limit is set.

2) Avoid accounting completely for a socket that started before
    the limit was set.
==
are not satisfied. So, we need to have a state per sockets, it's accounted
or not. I'll look into this problem again, today.

Of course they are.

Every socket created before we set the limit is not accounted.
This is 2) that Dave mentioned, and it was *always* this way.

The problem here was the opposite: You could disable the jump labels
with sockets still in flight, because we were disabling it based on
the limit being set back to unlimited.

What this patch does, is defer that until the last socket limited dies.

Thank you for explanation. Hmm, sk->cgrp check ?
Ah, yes it's updated by sock_update_memcg() under jump_label, which is
called by tcp_v4_init_sock().
Hm. and jump_label()'s atomic counter and mutex_lock will be a guard against
set/unset race. Ok.

BTW, what will happen in following case ?

Assume that the last memcg is destroyed and call jump_label_dec. And the
thread waits for jump_label_mutex for a while.

      CPU A                                           CPU B

    jump_label_dec() # mutex will be held        sock_update_memcg() is called
                                                 sk_cgrp is set.
    ...modify instructions                       some accounting is done.
    mutex_unlock()

I wonder you need some serialization somewhere OR disallow turning off accounting.                                              

Thanks,
-Kame