netdev - Re: Problem with queuing vlan tagged packets after migration from 3.16.0 to 4.9.0

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Message-ID: <013564f9-157c-9a65-781f-2431675c64a9@mojatatu.com>
Date:   Sat, 12 Jan 2019 07:12:34 -0500
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Simon Horman <simon.horman@...ronome.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Bartek Kois <bartek.kois@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Problem with queuing vlan tagged packets after migration from
 3.16.0 to 4.9.0

On 2019-01-10 8:45 a.m., Simon Horman wrote:
> On Sun, Jan 06, 2019 at 09:44:30AM -0500, Jamal Hadi Salim wrote:

>> You can use flower instead of basic but such one offs basic would
>> be more effective.
>> Bartek, if you say you have 20 vlans: worst case scenario
>> here is you are going to do 20 lookups (with basic) before
>> hitting the efficient u32 lookup alg and of course best case
>> is 1 lookup.
> 
> FWIW, flower's lookup is hash-based so I would expect it to do better
> than O(n).
> 

Referring more to a large number of filters(N) in particular:
Flower will perform at O(N) to find a match (not sure
you can make it perform better than O(N)) but u32
can made to perform at _much better_ than O(logN);
i havent looked closely at Bartek's arrangement but cursory inspection
indicates he is using such an arrangement for performance.

>> We can add vlan as a metadata extension to u32 - just
>> like skb->mark is. It will help to remove the need for basic
>> filter but may be pushing it
> 
> That sounds more like fixing a long standing regression than a new feature
> to me. I'm not sure that I understand what it would be pushing.
> 

It will be a new feature in the sense the user will have to specify
something like (adding "mark" for clarify):

tc filter add .... protocol 802.1q .. u32 \
match u32 0 0 \
mark 15 \
vlanid 1234
action vlan pop reclassify

As opposed to what used to work before which would
have matched at packet offset.

If there is agreement (lots of bikeshedding last time)
i or someone else can whip a patch for this.

cheers,
jamal