netdev - Re: [PATCH net-next 0/4] net: mpls: Allow users to configure more labels per route

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Date:   Tue, 28 Mar 2017 10:52:10 +0100
From:   Robert Shearman <rshearma@...cade.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        David Ahern <dsa@...ulusnetworks.com>
CC:     <netdev@...r.kernel.org>, <roopa@...ulusnetworks.com>
Subject: Re: [PATCH net-next 0/4] net: mpls: Allow users to configure more
 labels per route

On 28/03/17 04:08, Eric W. Biederman wrote:
> David Ahern <dsa@...ulusnetworks.com> writes:
>
>> On 3/27/17 4:39 AM, Robert Shearman wrote:
>>> On 25/03/17 19:15, Eric W. Biederman wrote:
>>>> David Ahern <dsa@...ulusnetworks.com> writes:
>>>>
>>>>> Bump the maximum number of labels for MPLS routes from 2 to 12. To keep
>>>>> memory consumption in check the labels array is moved to the end of
>>>>> mpls_nh
>>>>> and mpls_iptunnel_encap structs as a 0-sized array. Allocations use the
>>>>> maximum number of labels across all nexthops in a route for LSR and the
>>>>> number of labels configured for LWT.
>>>>>
>>>>> The mpls_route layout is changed to:
>>>>>
>>>>>    +----------------------+
>>>>>    | mpls_route           |
>>>>>    +----------------------+
>>>>>    | mpls_nh 0            |
>>>>>    +----------------------+
>>>>>    | alignment padding    |   4 bytes for odd number of labels; 0 for
>>>>> even
>>>>>    +----------------------+
>>>>>    | via[rt_max_alen] 0   |
>>>>>    +----------------------+
>>>>>    | alignment padding    |   via's aligned on sizeof(unsigned long)
>>>>>    +----------------------+
>>>>>    | ...                  |
>>>>>
>>>>> Meaning the via follows its mpls_nh providing better locality as the
>>>>> number of labels increases. UDP_RR tests with namespaces shows no impact
>>>>> to a modest performance increase with this layout for 1 or 2 labels and
>>>>> 1 or 2 nexthops.
>>>>>
>>>>> The new limit is set to 12 to cover all currently known segment
>>>>> routing use cases.
>>>>
>>>> How does this compare with running the packet a couple of times through
>>>> the mpls table to get all of the desired labels applied?
>>>
>>> At the moment (i.e setting output interface for a route to the loopback
>>> interface) the TTL would currently be calculated incorrectly since it'll
>>> be decremented each time the packet is run through the input processing.
>>> If that was avoided, then the only issue would be the lower performance.
>>
>> We have the infrastructure to add all the labels on one pass. It does
>> not make sense to recirculate the packet to get the same effect.
>
> I was really asking what are the advantages and disadvantages of this
> change rather than suggesting it was a bad idea.  The information about
> ttl is useful.
>
> Adding that this will route packets with more labels more quickly than
> the recirculation method is also useful to know.

I should also add that not recirculating also avoids having to allocate 
extra local labels, which may be limited in supply in some deployments, 
and avoids the extra control plane/user complexity associated with 
managing the routes associated with the recirculation.

Thanks,
Rob