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

[David Miller <davem@davemloft.net>]

From: Neil Horman <nhorman@tuxdriver.com>
Date: Sat, 21 Jul 2012 13:18:55 -0400

> On Sat, Jul 21, 2012 at 10:02:03AM -0700, John Fastabend wrote:
>> 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
>> 
> Ok, I can buy that.  Lets give this a try:
> 
> Acked-by: Neil Horman <nhorman@tuxdriver.com>
> 

Applied.