netdev - Re: [PATCH net-next 1/1] net/sched: Disambiguate verdict from return code

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Date: Fri, 6 Oct 2023 11:25:28 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>, Victor Nogueira <victor@...atatu.com>, xiyou.wangcong@...il.com, 
	jiri@...nulli.us, davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com, 
	paulb@...dia.com, netdev@...r.kernel.org, kernel@...atatu.com, 
	martin.lau@...ux.dev, bpf@...r.kernel.org
Subject: Re: [PATCH net-next 1/1] net/sched: Disambiguate verdict from return code

On Fri, Oct 6, 2023 at 10:12 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 6 Oct 2023 15:49:18 +0200 Daniel Borkmann wrote:
> > > Which will no longer work with the "pack multiple values into
> > > the reason" scheme of subsys-specific values :(
> >
> > Too bad, do you happen to know why it won't work?
>
> I'm just guessing but the reason is enum skb_drop_reason
> and the values of subsystem specific reasons won't be part
> of that enum.

IIUC, this would gives us the readability and never require any
changes to bpftrace, whereas the major:minor encoding would require
further logic in bpftrace.

> > Given they went into the
> > length of extending this for subsystems, they presumably would also like to
> > benefit from above. :/
> >
> > > What I'm saying is that there is a trade-off here between providing
> > > as much info as possible vs basic user getting intelligible data..
> >
> > Makes sense. I think we can drop that aspect for the subsys specific error
> > codes. Fwiw, TCP has 22 drop codes in the core section alone, so this should
> > be fine if you think it's better. The rest of the patch shown should still
> > apply the same way. I can tweak it to use the core section for codes, and
> > then it can be successively extended if that looks good to you - unless you
> > are saying from above, that just one error code is better and then going via
> > detailed stats for specific errors is preferred.
>
> No, no, multiple reasons are perfectly fine. The non-technical
> advantage of mac80211 error codes being separate is that there
> are no git conflicts when we add new ones. TC codes can just
> be added to the main enum like TCP 🤷️

We still need to differentiate policy vs error - I suppose we could go
with Daniel's idea of introducing TC_ACT_ABORT/ERROR and ensure all
the callees set the drop_reason.

cheers,
jamal