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Date:   Mon, 16 Jul 2018 23:41:47 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Michel Machado <michel@...irati.com.br>,
        Nishanth Devarajan <ndev2021@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cody Doucette <doucette@...edu>
Subject: Re: [PATCH v3 net-next] net/sched: add skbprio scheduler

On Fri, Jul 13, 2018 at 9:39 PM Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
>
> On Fri, Jul 13, 2018 at 11:26:28AM -0700, Cong Wang wrote:
> > On Fri, Jul 13, 2018 at 6:04 AM Marcelo Ricardo Leitner
> > <marcelo.leitner@...il.com> wrote:
> > >
> > > On Thu, Jul 12, 2018 at 11:05:45PM -0700, Cong Wang wrote:
> > > > On Wed, Jul 11, 2018 at 12:33 PM Marcelo Ricardo Leitner
> > > > <marcelo.leitner@...il.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@...il.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.
> > >
> > > It still would work, no matter how much padding you have, as currently
> > > you can't use more than 3 bands.
> >
> > I am lost.
> >
> > With your proposal above, you have 16 bands for V1 and 64 bands
> > for V2, where does 3 come from???
>
> My bad. s/3/16/


Ah, thanks for clarifying it! Please see below.


>
> >
> >
> > >
> > > >
> > > > 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....
> > >
> > > Yes, and that's not a problem:
> > > - Either bands is > 3 and it will return EINVAL, protecting from
> > >   reading beyond the buffer.
> > > - Or 2 <= bands <= 3 and it will handle it as a _v1 struct, and use
> > >   only the original size.
> >
> > Again why 3 not 16 or 64 ??
>
> Again, s/3/16/
>
> >
> > Also, why does an old kernel has the logic in its binary to determine
> > this?
>
> It won't, and it doesn't need to. If you use bands > 16 with an old
> kernel, it will reject per current code (that I already pasted):
>
>         if (qopt->bands > TCQ_PRIO_BANDS || qopt->bands < 2)
>                 return -EINVAL;
>
> Simple as that. If you try to use more bands than it supports, it will
> reject it.

Hmm, I see, But in your demo code, you miss the following pieces:

        for (i = 0; i <= TC_PRIO_MAX; i++) {
                if (qopt->priomap[i] >= qopt->bands)
                        return -EINVAL;
        }

        memcpy(q->prio2band, qopt->priomap, TC_PRIO_MAX+1);


I guess you want to change TC_PRIO_MAX to qopt->bands
too.

With this together, your suggestion actually looks reasonable.

Do I understand it correctly?


>
> >
> > >
> > > iproute2 (or other app) may still use _v1 if it wants, btw.
> >
> > Yes, old iproute2 must still have v1, what's point? Are you
>
> ??
>
> > suggesting new iproute2 should still have v1 after you propose
> > v1 and v2 for kernel?
>
> I'm only saying that both versions will be accepted by a new kernel.

I see, I thought you suggest to completely move to V2 for new
kernel.

Nice compatibility trick!

Thanks.

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