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 00:00:45 -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 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.

>>And that's needed because once any rule is added the tcam config can no
>>longer be easily modified?
>
> Yes.