On 2013/1/21 17:27, Daniel Wagner wrote:
> On 21.01.2013 10:01, Li Zefan wrote:
>> On 2013/1/21 16:50, Daniel Wagner wrote:
>>> Hi Li,
>>>
>>> On 21.01.2013 07:08, Li Zefan wrote:
>>>> I'm not a network developer, so correct me if I'm wrong.
>>>>
>>>> Since commit 7955490f732c2b8
>>>> ("net: netprio_cgroup: rework update socket logic"),
>>>> sock->sk->sk_cgrp_prioidx
>>>> is set when the socket is created, and won't be updated unless the task is
>>>> moved to another cgroup.
>>>>
>>>> Now the problem is, a socket can be _shared_ by multiple processes (fork,
>>>> SCM_RIGHT).
>>>> If we place those processes in different cgroups, and each cgroup has
>>>> different configs, but all of the processes will send data via this socket
>>>> with the same network priority.
>>>
>>> Wouldn't that be addressed by 48a87cc26c13b68f6cce4e9d769fcb17a6b3e4b8
>>>
>>> net: netprio: fd passed in SCM_RIGHTS datagram not set correctly
>>>
>>> A socket fd passed in a SCM_RIGHTS datagram was not getting
>>> updated with the new tasks cgrp prioidx. This leaves IO on
>>> the socket tagged with the old tasks priority.
>>>
>>> To fix this add a check in the scm recvmsg path to update the
>>> sock cgrp prioidx with the new tasks value.
>>>
>>> As I read this this should work for net_prio.
>>>
>>
>> But after process A passed the socket fd to B, both A and B can use the
>> same socket to send data, right? Then if A and B were placed in different
>> cgroups with differnt configs, A's config won't take effect anymore.
>>
>> Am I missing something?
>
> I don't know. I guess at one point the socket resources are shared and then
> one configuration is taking preference. As you can see I am far away of
> being
> an expert in this field. Hopefully someone who understands this bits
> can chip in.
>
> BTW, isn't this a similar to what should happen with the block io cgroup?
> What is the behavior with a fd writing to a file in the scenario you
> describe above?
>
It forbids task moving in this case:
/*
* We cannot support shared io contexts, as we have no mean to support
* two tasks with the same ioc in two different groups without major rework
* of the main cic data structures. For now we allow a task to change
* its cgroup only if it's the only owner of its ioc.
*/
static int blkcg_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
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