From mboxrd@z Thu Jan  1 00:00:00 1970
From: Cong Wang <xiyou.wangcong@gmail.com>
Subject: Re: [PATCH v3 net-next] net/sched: add skbprio scheduler
Date: Thu, 12 Jul 2018 23:05:45 -0700
Message-ID: <CAM_iQpWakUAx8aLiSg23h4dZHuGxw5DdNacFg3AF944-Q4PzHw@mail.gmail.com>
References: <20180707101351.GA8300@gmail.com> <20180709154409.GC8880@localhost.localdomain>
 <b9f0e292-a29a-3c70-a215-cd6fc2ffbc6a@digirati.com.br> <20180709195319.GD8880@localhost.localdomain>
 <ababf6ae-28f5-359c-8574-a2c3306167d2@digirati.com.br> <20180709214016.GD10923@localhost.localdomain>
 <CAM_iQpW9pqSMZfJsfntZtva8wrfsw8UpQAg0ryumu5pPbgJ2UQ@mail.gmail.com> <20180711193336.GF8880@localhost.localdomain>
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Cc: Michel Machado <michel@digirati.com.br>,
        Nishanth Devarajan <ndev2021@gmail.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>,
        David Miller <davem@davemloft.net>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        Cody Doucette <doucette@bu.edu>
To: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mail-pf0-f193.google.com ([209.85.192.193]:44093 "EHLO
        mail-pf0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1725930AbeGMGTA (ORCPT
        <rfc822;netdev@vger.kernel.org>); Fri, 13 Jul 2018 02:19:00 -0400
Received: by mail-pf0-f193.google.com with SMTP id j3-v6so22003104pfh.11
        for <netdev@vger.kernel.org>; Thu, 12 Jul 2018 23:05:55 -0700 (PDT)
In-Reply-To: <20180711193336.GF8880@localhost.localdomain>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

On Wed, Jul 11, 2018 at 12:33 PM Marcelo Ricardo Leitner
<marcelo.leitner@gmail.com> wrote:
>
> On Tue, Jul 10, 2018 at 07:25:53PM -0700, Cong Wang wrote:
> > On Mon, Jul 9, 2018 at 2:40 PM Marcelo Ricardo Leitner
> > <marcelo.leitner@gmail.com> wrote:
> > >
> > > On Mon, Jul 09, 2018 at 05:03:31PM -0400, Michel Machado wrote:
> > > >    Changing TC_PRIO_MAX from 15 to 63 risks breaking backward compatibility
> > > > with applications.
> > >
> > > If done, it needs to be done carefully, indeed. I don't know if it's
> > > doable, neither I know how hard is your requirement for 64 different
> > > priorities.
> >
> > struct tc_prio_qopt {
> >         int     bands;                  /* Number of bands */
> >         __u8    priomap[TC_PRIO_MAX+1]; /* Map: logical priority -> PRIO band */
> > };
> >
> > How would you do it carefully?
>
> quick shot, multiplex v1 and v2 formats based on bands and sizeof():
>
> #define TCQ_PRIO_BANDS_V1       16
> #define TCQ_PRIO_BANDS_V2       64
> #define TC_PRIO_MAX_V2          64
>
> struct tc_prio_qopt_v2 {
>         int     bands;                  /* Number of bands */
>         __u8    priomap[TC_PRIO_MAX_V2+1]; /* Map: logical priority -> PRIO band */
> };
>

Good try, but:

1. You don't take padding into account, although the difference
between 16 and 64 is big here. If it were 16 and 20, almost certainly
wouldn't work.

2. What if I compile a new iproute2 on an old kernel? The iproute2
will use V2, while old kernel has no knowledge of V2, so it only
copies a part of V2 in the end....