netdev - Re: [RFC PATCH 1/1 v2] net: sched: Introduce conndscp action

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Message-ID: <B9E730DA-63AB-4FB2-8509-4FFF83F63EEF@darbyshire-bryant.me.uk>
Date:   Fri, 22 Mar 2019 20:50:52 +0000
From:   Kevin 'ldir' Darbyshire-Bryant <ldir@...byshire-bryant.me.uk>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     "jhs@...atatu.com" <jhs@...atatu.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1 v2] net: sched: Introduce conndscp action



> On 22 Mar 2019, at 20:05, Cong Wang <xiyou.wangcong@...il.com> wrote:
> 
> On Fri, Mar 22, 2019 at 11:26 AM Kevin 'ldir' Darbyshire-Bryant
> <ldir@...byshire-bryant.me.uk> wrote:
>> 
>> Hi Cong,
>> 
>> Thanks for your questions.
>> 
>>> On 22 Mar 2019, at 17:39, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>> 
>>> Hello,
>>> 
>>> On Fri, Mar 22, 2019 at 7:09 AM Kevin 'ldir' Darbyshire-Bryant
>>> <ldir@...byshire-bryant.me.uk> wrote:
>>>> 
>>>> Conndscp is a new tc filter action module.  It is designed to copy DSCPs
>>>> to conntrack marks and the reverse operation of conntrack mark contained
>>>> DSCPs to the diffserv field of suitable skbs.
>>>> 
>>> 
>>> Is it possible and feasible to integrate this into connmark?
>> 
>> I started off coding it that way but quickly ran into my limitations with netlink messaging and became frustrated.  Aside from my own limitations, conndscp ab/uses tcf_qstats requeues & overlimits to indicate DSCP->MARK->DSCP operations and has been useful in proving DSCP/marking operations are occurring in the right times/places.  Integrating with connmark which itself uses overlimits to indicate conntrack mark to skb->mark restoration would lose that differentiation/confirmation/debug ability.  A possibility is to ab/use the drop count instead but I fear that would cause confusion.
> 
> This sounds problematic, why a flag/parameter doesn't work?
> 
I don’t understand the question?

> 
>> 
>>> Both are intended to retrieve information from conntrack and store
>>> it into skb. I know the name "connmark" already says it is a mark,
>>> while yours isn't, I still want to see if we can avoid code duplications.
>> 
>> I understand your quest :-)  I think conndscp does a bit more than connmark.  Conndscp is two way diffserv<-->conntrack mark operation.  connmark is a single way conntrack mark->skb.mark operation.
> 
> I am not sure if it is a good idea to modify conntrack in TC,
> as conntrack doesn't even belong to TC. Retrieving information
> from conntrack and saving it to skb is fine, as we modify skb
> in many different ways.

OK, this is why I wanted to ask as RFC before I went too far implementing stuff.  AFAIUI you’re saying it’s tc is okay to restore stuff from a connmark but not to set/change the conntrack mark.  So I need to find a legal place to store a DSCP into a conntrack mark.

Cheers,

Kevin