netdev - Re: [patch net-next 0/9] net: sched: introduce chain templates support with offloading to mlxsw

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Date:   Tue, 26 Jun 2018 14:18:58 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Linux Netdev List <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Simon Horman <simon.horman@...ronome.com>,
        John Hurley <john.hurley@...ronome.com>,
        David Ahern <dsahern@...il.com>, mlxsw@...lanox.com
Subject: Re: [patch net-next 0/9] net: sched: introduce chain templates
 support with offloading to mlxsw

On Tue, 26 Jun 2018 09:12:17 +0200, Jiri Pirko wrote:
> Tue, Jun 26, 2018 at 09:00:45AM CEST, jakub.kicinski@...ronome.com wrote:
> >On Mon, Jun 25, 2018 at 11:43 PM, Jiri Pirko <jiri@...nulli.us> wrote:  
> >> Tue, Jun 26, 2018 at 06:58:50AM CEST, jakub.kicinski@...ronome.com wrote:  
> >>>On Mon, 25 Jun 2018 23:01:39 +0200, Jiri Pirko wrote:  
> >>>> From: Jiri Pirko <jiri@...lanox.com>
> >>>>
> >>>> For the TC clsact offload these days, some of HW drivers need
> >>>> to hold a magic ball. The reason is, with the first inserted rule inside
> >>>> HW they need to guess what fields will be used for the matching. If
> >>>> later on this guess proves to be wrong and user adds a filter with a
> >>>> different field to match, there's a problem. Mlxsw resolves it now with
> >>>> couple of patterns. Those try to cover as many match fields as possible.
> >>>> This aproach is far from optimal, both performance-wise and scale-wise.
> >>>> Also, there is a combination of filters that in certain order won't
> >>>> succeed.
> >>>>
> >>>> Most of the time, when user inserts filters in chain, he knows right away
> >>>> how the filters are going to look like - what type and option will they
> >>>> have. For example, he knows that he will only insert filters of type
> >>>> flower matching destination IP address. He can specify a template that
> >>>> would cover all the filters in the chain.  
> >>>
> >>>Perhaps it's lack of sleep, but this paragraph threw me a little off
> >>>the track.  IIUC the goal of this set is to provide a way to inform the
> >>>HW about expected matches before any rule is programmed into the HW.
> >>>Not before any rule is added to a particular chain.  One can just use
> >>>the first rule in the chain to make a guess about the chain, but thanks
> >>>to this set user can configure *all* chains before any rules are added.  
> >>
> >> The template is per-chain. User can use template for chain x and
> >> not-use it for chain y. Up to him.  
> >
> >Makes sense.
> >
> >I can't help but wonder if it'd be better to associate the
> >constraints/rules with chains instead of creating a new "template"
> >object.  It seems more natural to create a chain with specific
> >constraints in place than add and delete template of which there can
> >be at most one to a chain...  Perhaps that's more about the user space
> >tc command line.  Anyway, not a strong objection, just a thought.  
> 
> Hmm. I don't think it is good idea. User should see the template in a
> "show" command per chain. We would have to have 2 show commands, one to
> list the template objects and one to list templates per chains. It makes
> things more complicated for no good reason. I think that this simple
> chain-lock is easier and serves the purpose.

Hm, I think the dump is fine, what I was thinking about was:

# tc chain add dev dummy0 ingress chain_index 22 \
     ^^^^^
	template proto ip \
	^^^^^^^^
	flower dst_mac 00:00:00:00:00:00/00:00:00:00:FF:FF

instead of:

# tc filter template add dev dummy0 ingress \
     ^^^^^^^^^^^^^^^
	proto ip chain_index 22 \
	flower dst_mac 00:00:00:00:00:00/00:00:00:00:FF:FF

And then delete becomes:

# tc chain del dev dummy0 ingress chain_index 22
Error: The chain is not empty.

The fact that template is very much like a filter is sort of an
implementation detail, from user perspective it may be more intuitive
to model template as an attribute of the chain, not a filter object
added to a chain.

But I could well be the only person who feels that way :)