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

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Date:   Mon, 14 Jan 2019 09:12:50 +0100
From:   Simon Horman <simon.horman@...ronome.com>
To:     Jamal Hadi Salim <jhs@...atatu.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 Sat, Jan 12, 2019 at 07:12:34AM -0500, Jamal Hadi Salim wrote:
> 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.

For all filters are at the same priority with the same mask
Flower will use a hash lookup which I expect that would be better than
O(N). That was the only point I wanted to make.

> 
> > > 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.

So the old feature is permanently broken?

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

Backwards compatibility aside I have no objection.