Re: CBQ and all other qdiscs now REALLY completely documented (almost!)

[bert hubert]

On Sat, Dec 08, 2001 at 02:20:20PM -0500, jamal wrote:

> Linux remaps packets incoming with different values to some internal
> value; the colum "mapped to" shows the internal mapping
> 
> 8value(hex)   TOS(dec) mapped to(dec)
> ----------------------------------
> 0x0              0      0
>                  1      7
>                  2      0
>                  3      0
>                  4      2
>                  5      2
>                  6      2
>                  7      2
> 0x10             8      6
>                  9      6
>                 10      6
>                 11      6
>                 12      2
>                 13      2
>                 14      2
>                 15      2

I find this tos2prio table in the kernel (2.5.x), which is somewhat
different than your table:

0        TC_PRIO_BESTEFFORT,     0
1        TC_PRIO_(FILLER),       1
2        TC_PRIO_BESTEFFORT,     0
3        TC_PRIO_(BESTEFFORT),   0
4        TC_PRIO_BULK,           2
5        TC_PRIO_(BULK),         2
6        TC_PRIO_BULK,           2
7        TC_PRIO_(BULK),         2
8        TC_PRIO_INTERACTIVE,    6
9        TC_PRIO_(INTERACTIVE),  6
10       TC_PRIO_INTERACTIVE,    6
11       TC_PRIO_(INTERACTIVE),  6
12       TC_PRIO_INTERACTIVE_BULK,       4
13       TC_PRIO_(INTERACTIVE_BULK),     4
14       TC_PRIO_INTERACTIVE_BULK,       4
15       TC_PRIO_(INTERACTIVE_BULK)      4

> Fill in the  "8value(hex)" column gaps using the bitmap from RFC1349 for
> the 8 bits; These are the values ou would see with tcpdump -vvv
> I filled the two easiest ones i could compute in my head.
> 
> Second step:
> 
> Take the default priority map:
>  1, 2, 2, 2, 1, 2, 0, 0 , 1, 1, 1, 1, 1, 1, 1, 1
> This applies for both default prio and the 3-band FIFO queue.
> Note the queue map fitted on the last column
> 
> 8 but value     TOS     mapped to   queue map
> ---------------------------------------------
> 0x0              0      0              1
>                  1      7              2
>                  2      0              2
>                  3      0              2
>                  4      2              1
>                  5      2              2
>                  6      2              0
>                  7      2              0
> 0x10             8      6              1
>                  9      6              1
>                 10      6              1
>                 11      6              1
>                 12      2              1
>                 13      2              1
>                 14      2              1
>                 15      2              1

I've changed this table to:
TOS     Bits  Means                    Linux Priority    Band
------------------------------------------------------------
0x0     0     Normal Service           0 Best Effort     1
0x2     1     Minimize Monetary Cost   1 Filler          2
0x4     2     Maximize Reliability     0 Best Effort     1
0x6     3     mmc+mr                   0 Best Effort     1
0x8     4     Maximize Throughput      2 Bulk            2
0xa     5     mmc+mt                   2 Bulk            2
0xc     6     mr+mt                    2 Bulk            2
0xe     7     mmc+mr+mt                2 Bulk            2
0x10    8     Minimize Delay           6 Interactive     0
0x12    9     mmc+md                   6 Interactive     0
0x14    10    mr+md                    6 Interactive     0
0x16    11    mmc+mr+md                6 Interactive     0
0x18    12    mt+md                    4 Int. Bulk       1
0x1a    13    mmc+mt+md                4 Int. Bulk       1
0x1c    14    mr+mt+md                 4 Int. Bulk       1
0x1e    15    mmc+mr+mt+md             4 Int. Bulk       1

http://ds9a.nl/lartc/HOWTO/cvs/2.4routing/output/2.4routing-9.html#ss9.2

Your table appears to imply that a Maximum Reliability, Mininum Delay
packet, TOS bits=9, gets mapped to band 1, not 0, which would not make
sense.

Laying it out like this, which does appear how it works, does mean that you
can specify priorities in the priomap which do not correspond to possible
TOS values.

Is it possible at all to set skb->priority from userspace without going
through the tos2prio mapping?

CBQ can use the skb->priority to classify:

        /*
         *  Step 1. If skb->priority points to one of our classes, use it.
         */
        if (TC_H_MAJ(prio^sch->handle) == 0 &&
            (cl = cbq_class_lookup(q, prio)) != NULL)
                        return cl;

But to do this, you would need to be able to set skb->priority to a 32bit
number:

include/linux/pkt_sched.h:#define TC_H_MAJ_MASK (0xFFFF0000U)
include/linux/pkt_sched.h:#define TC_H_MAJ(h) ((h)&TC_H_MAJ_MASK)

I can't find where you would do this, any clues?

Thanks again for taking the time to help me.

Regards,

bert

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