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

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Date:   Fri, 22 Mar 2019 22:06:26 +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 21:31, Cong Wang <xiyou.wangcong@...il.com> wrote:
> 
> On Fri, Mar 22, 2019 at 1:50 PM Kevin 'ldir' Darbyshire-Bryant
> <ldir@...byshire-bryant.me.uk> wrote:
>> 
>> 
>> 
>>> 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?
> 
> You said conndscp uses some stat to save some configuration
> information, that is, DSCP->MARK->DSCP operations. But
> configuration information is usually saved in a parameter struct
> or some priviate flag. So, I have to ask why a flag/parameter doesn't
> work for this case?
> 
> And, you also implied this is a barrier for you to reuse connmark
> action.
> 
> Am I misunderstanding anything here?

Ahh!  I understand the question, apologies if I was not clear.  conndscp like connmark reports some status information back to tc via tcf_qstats structure.  connmark uses ‘overlimits’ to report the number of marks restored from conntrack->mark to skb->mark.  conndscp uses ‘overlimits’ and ‘requeues’ to report status about how many marks it has restored/set. e.g.

root@...ter:~# tc -s filter show dev eth0
filter parent cacf: protocol all pref 10 u32 chain 0 
filter parent cacf: protocol all pref 10 u32 chain 0 fh 800: ht divisor 1 
filter parent cacf: protocol all pref 10 u32 chain 0 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:1 not_in_hw 
  match 00000000/00000000 at 0
	action order 1: conndscp zone 0 pipe
	 index 1 ref 1 bind 1 mask 0xfc000000 statemask 0x01000000 mode both installed 119695 sec used 0 sec
	Action statistics:
	Sent 944294567 bytes 4390248 pkt (dropped 0, overlimits 2366576 requeues 50157) <<— here
	backlog 0b 0p requeues 50157

I explained (badly) that merging ‘connmark’ and ‘conndscp’ would present an issue (to me) of how to report both types of statistics (connmark skb->mark restores & conndscp connmark->skb->ip-diffserv restores & skb->ipdiffserv->connmark->mark stores)


> 
>> 
>>> 
>>>> 
>>>>> 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.
> 
> Yes.
> 
> I guess you should look into netfilter to modify any conntrack attribute,
> it is at least where conntrack belongs to. :)

So I wonder if an XT_CONNMARK_SAVEDSCP option in xt_connmark would be more acceptable?

Your patience & advice appreciated.

Cheers,

Kevin D-B

gpg: 012C ACB2 28C6 C53E 9775  9123 B3A2 389B 9DE2 334A