netdev - Re: [PATCH net-next v3 4/5] net/tc: introduce TC_ACT

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Message-ID: <CAM_iQpV5zbhnG161QN-7zU-8LXy_J-5YMCCBt=MWMyOevRia2g@mail.gmail.com>
Date:   Thu, 26 Jul 2018 16:25:46 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...nulli.us>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eyal Birger <eyal.birger@...il.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next v3 4/5] net/tc: introduce TC_ACT_REINJECT.

On Thu, Jul 26, 2018 at 5:52 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On 25/07/18 01:09 PM, Marcelo Ricardo Leitner wrote:
> > On Wed, Jul 25, 2018 at 09:48:16AM -0700, Cong Wang wrote:
> >> On Wed, Jul 25, 2018 at 5:27 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> >>>
> >>> Those changes were there from the beginning (above patch did
> >>> not introduce them).
> >>> IIRC, the reason was to distinguish between policy intended
> >>> drops and drops because of errors.
> >>
> >> There must be a limit for "overlimit" to make sense. There is
> >> no limit in mirred action's context, probably there is only
> >> such a limit in act_police. So, all rest should not touch overlimit.
> >
> > +1
> >
>
> I agree we should at least record drop count(unrelated patch though).
> we should keep overlimit (for no other reason other than this
> has been around for at least 15 years).
>
> On why "overlimit"? It is just a name for a counter that is useless
> for most actions (but was still being transfered to user space).
> It is the closest counter to counting "this failed because of
> runtime errors" as opposed to "user asked us to drop this".
>
> Probably a good alternative is to make a very small stats v3 structure
> (we have migrated stats structures before) and extend for
> each action/classifier/qdisc to add its extra counters using XSTATS.

Agreed.

>
> Note:
> If you are _mirroring_ packets - incrementing the drop counter is
> misleading because the packet is not dropped by the system.
> i.e the qdisc will not record it as dropped; it should for
> redirect policy.  It is useful to be able to tell
> them apart when you are collecting  analytics just for actions.

Sounds like we just need another counter rather than re-using overlimit
or drops.


> (if youve worked on a massive amount of machines you'll appreciate
> being able to debug by looking at counters that reduce ambiguity).
>

Yes, this is how I found out the overlimit of htb qdisc is inaccurate
or misleading, instead the one in htb class is accurate, see:

commit 3c75f6ee139d464351f8ab77a042dd3635769d98
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Mon Sep 18 12:36:22 2017 -0700

    net_sched: sch_htb: add per class overlimits counter