netdev - Re: [PATCH] memcg/tcp: fix warning caused b res->usage go to negative.

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Date:	Tue, 10 Apr 2012 00:01:49 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	<netdev@...r.kernel.org>, David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] memcg/tcp: fix warning caused b res->usage go to negative.

On 04/09/2012 11:51 PM, Glauber Costa wrote:
> On 04/09/2012 11:37 PM, KAMEZAWA Hiroyuki wrote:
>> 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.
> 
>> 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.
> 

Okay, there is an additional thing to be considered here:

Due to the nature of how jump label works, once they are enabled for one
of the cgroups, they will be enabled for all of them. So the patch I
sent may still break in some scenarios because of the way we record that
the limit was set.

However, if my theory behind what is causing the problem is correct,
this patch should fix the issue for you. Let me know if it does, and
I'll work on the final solution.

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