Re: [net-next PATCH v1] net: netprio_cgroup: rework update socket logic

[John Fastabend <john.r.fastabend@intel.com>]

On 7/20/2012 7:00 PM, Neil Horman wrote:
> On Fri, Jul 20, 2012 at 01:39:25PM -0700, John Fastabend wrote:
>> Instead of updating the sk_cgrp_prioidx struct field on every send
>> this only updates the field when a task is moved via cgroup
>> infrastructure.
>>
>> This allows sockets that may be used by a kernel worker thread
>> to be managed. For example in the iscsi case today a user can
>> put iscsid in a netprio cgroup and control traffic will be sent
>> with the correct sk_cgrp_prioidx value set but as soon as data
>> is sent the kernel worker thread isssues a send and sk_cgrp_prioidx
>> is updated with the kernel worker threads value which is the
>> default case.
>>
>> It seems more correct to only update the field when the user
>> explicitly sets it via control group infrastructure. This allows
>> the users to manage sockets that may be used with other threads.
>>
>> Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
> I like the idea, but IIRC last time we tried this I think it caused problems
> with processes that shared sockets.  That is to say, if you have a parent and
> child process that dup an socket descriptior, and put them in separate cgroups,
> you get unpredictable results, as the socket gets assigned a priority based on
> the last processed that moved cgroups.
>
> Neil
>

Shared sockets creates strange behavior as it exists today. If a dup
of the socket fd is created the private data is still shared right. So
in this case the sk_cgrp_prioidx value is going to get updated by both
threads and then it is a race to see what it happens to be set to in
the xmit path.

With this patch at least the behavior is deterministic. Without it
I can create the above scenario but have no way to determine what the
skb priority will actually be set to.

.John