Message-ID: <CAM0EoMmohH3VdMGZDNb6zkte774uohp1u0Fzxo24_tPT+PBb=Q@mail.gmail.com>
Date: Sun, 8 Oct 2023 08:38:25 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Jakub Kicinski <kuba@...nel.org>, Victor Nogueira <victor@...atatu.com>, xiyou.wangcong@...il.com, 
	davem@...emloft.net, pabeni@...hat.com, edumazet@...gle.com, 
	mleitner@...hat.com, vladbu@...dia.com, simon.horman@...igine.com, 
	pctammela@...atatu.com, netdev@...r.kernel.org, kernel@...atatu.com
Subject: Re: [PATCH net-next v4 0/3] net/sched: Introduce tc block ports
 tracking and use

On Sat, Oct 7, 2023 at 1:20 PM Jiri Pirko <jiri@...nulli.us> wrote:
>
> Sat, Oct 07, 2023 at 04:09:15PM CEST, jhs@...atatu.com wrote:
> >On Sat, Oct 7, 2023 at 8:43 AM Jiri Pirko <jiri@...nulli.us> wrote:
> >>
> >> Sat, Oct 07, 2023 at 01:06:43PM CEST, jhs@...atatu.com wrote:
> >> >On Sat, Oct 7, 2023 at 6:20 AM Jiri Pirko <jiri@...nulli.us> wrote:
> >> >>
> >> >> Sat, Oct 07, 2023 at 01:00:00AM CEST, jhs@...atatu.com wrote:
> >> >> >On Fri, Oct 6, 2023 at 6:25 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >> >> >>
> >> >> >> On Fri, 6 Oct 2023 15:06:45 -0400 Jamal Hadi Salim wrote:
> >> >> >> > > I don't understand the need for configuration less here. You don't have
> >> >> >> > > it for the rest of the actions. Why this is special?
> >> >> >>
> >> >> >> +1, FWIW
> >> >> >
> >> >> >We dont have any rule that says all actions MUST have parameters ;->
> >> >> >There is nothing speacial about any action that doesnt have a
> >> >> >parameter.
> >> >>
> >> >> You are getting the configuration from the block/device the action is
> >> >> attached to. Can you point me to another action doing that?
> >> >
> >> >We are entering a pedantic road i am afraid. If there is no existing
> >> >action that has zero config then consider this one the first one. We
> >>
> >> Nope, nothing pedantic about it. I was just curious if there's anything
> >> out there I missed.
> >>
> >
> >Not sure if you noticed in the patch: the blockid on which the skb
> >arrived on is now available in the tc_cb[] so when it shows up at the
> >action we can just use it.
>
> I see, but does it has to be? I don't think so with the solution I'm
> proposing.

It's a simplistic use for a broadcast. We should support the one you
suggested as well.

> >
> >>
> >> >use skb->metadata all the time as a source of information for actions,
> >> >classifiers, qdiscs. If i dont need config i dont need to invent one
> >>
> >> skb->metadata is something that is specific to a packet. That has
> >> nothing to do with the actual configuration.
> >
> >Essentially we turned blockid into skb metadata. A user specifying
> >configuration of a different blockid is certainly useful. My point is
> >we can have both worlds: when such a user config is missing we'll
> >assume a default which happens to be in the skb.
> >
> >>
> >> >just because, well, all other actions are using one or more config;->
> >> >Your suggestion to specify an extra config to select a block - which
> >> >may be different than the one the one packet on - is a useful
> >> >feature(it just adds more code) but really should be optional. i.e if
> >> >you dont specify a block id configuration then we pick the metadata
> >> >one.
> >>
> >> My primary point is, this should be mirred redirect to block instead of
> >> what we currently have only for dev. That's it.
> >
> >Agreed (and such a feature should be added regardless of this action).
> >The tc block provides a simple abstraction, but do you think it is
> >enough? Alternative is to use a list of ports given to mirred: it
> >allows us to group ports from different tc blocks or even just a
> >subset of what is in a tc block - but it will require a lot more code
> >to express such functionality.
>
> Again, you attach filter to either dev or block. If you extend mirred
> redirect to accept the same 2 types of target, I think it would be best.
>

We are going to make block work with mirror, it makes sense. I am not
sure about the redirect, what is the semantic? mirror to everyone but
redirect to the last one?

>
> >
> >>
> >>
> >> >
> >> >> >If we can adequately cleanup mirred,  then we can put it there but
> >> >> >certainly now we are adding more buttons to click on mirred. It may
> >> >> >make sense to refactor the mirred code then reuse the refactored code
> >> >> >in a new action.
> >> >>
> >> >> I don't understand why you need any new action. mirred redirect to block
> >> >> instead of dev is exactly what you need. Isn't it?
> >> >
> >> >The actions have different meanings and lumping them together then
> >> >selecting via a knob is not the right approach.
> >> >There's shared code for sure. Infact the sending code was ripped from
> >> >mirred so as not to touch the rest because like i said mirred has
> >> >since grown a couple of horns and tails. In retrospect mirred should
> >> >have been two actions with shared code - but it is too late to change
> >> >now because it is very widely used. If someone like me was afraid of
> >> >touching it is because there's a maintainance challenge. I consider it
> >> >in the same zone as trying to restructure something in the skb.
> >> >I agree mirroring to a group of ports with a simple config is a useful
> >> >feature. Mirroring to a group via a tc block is simpler but adding a
> >> >list of ports instead is more powerful. So this feature is useful to
> >> >have in mirred - just the adding of yet one more button to say "skip
> >> >this port" is my concern.
> >>
> >> Why? Perhaps skb->iif could be used for check in the tx iteration.
> >>
> >
> >We use skb->dev->ifindex to find the exception. Is iif better?.
>
> iif contains ifindex of the actual ingress device. So if the netdev is
> part of bond for example, this still contains the original ifindex.
> So I buess that this depends on what you need. Looks to me that
> skb->dev->ifindex would be probaly better. It contains the netdev that
> the filter is attached on, right?
>

Note: you can use mirred to redirect to either ingress or egress of
other ports - I believe one of these ifindices changes to reflect the
new ifindex. We'll take a closer look.

>
> >Jiri - but why does this have to be part of mirred::mirror? I am
> >asking the same question of why mirror and redirect have to be part
> >mirred instead of separate actions.
>
> You have to maintain the backwards compatibility. Currently mirred is
> one action right? Does not matter how you do it in kernel, user should
> not tell any difference.

I dont mean to break existing mirred. What i meant was in retrospect i
wish i had the insight to separate mirred into two actions(and share
the code instead), it would have simplified the code and its
maintainance. It is for the same reason i am not in favor of is adding
the "skip this port" in mirror. This is in the spirit of unix
philosophy, which we have been mostly adhering to: write small
features/actions which do one thing well and stitch them together to
compose.

cheers,
jamal
>
> >
> >cheers,
> >jamal

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