Re: Qdisc->u32_node - licence to kill

Mon, Aug 07, 2017 at 07:47:14PM CEST, john.fastabend@gmail.com wrote:

On 08/07/2017 09:41 AM, Jiri Pirko wrote:

quoted

Hi Jamal/Cong/David/all.

Digging in the u32 code deeper now. I need to get rid of tp->q for shared
blocks, but I found out about this:

struct Qdisc {
	......
        void                    *u32_node;
	......
};

Yeah, ugly. u32 uses it to store some shared data, tp_c. It actually
stores a linked list of all hashtables added to one qdiscs.

So basically what you have is, you have 1 root ht per prio/pref. Then
you can have multiple hts, linked from any other ht, does not matter in
which prio/pref they are.

We can create arbitrary hash tables here independent of prio/pref via
TCA_U32_DIVISOR. Then these can be linked to other hash tables via
TCA_U32_LINK commands.

Yeah, that's what I thought.

prio/pref does not really play any part here from my reading, except as
a further specifier in the walk callbacks. Making it a useful filter on
dump operations.

Not correct. prio/pref is one level up priority, independent on specific
cls implementation. You can have cls_u32 instance on prio 10 and
cls_flower instance on prio 20. Both work.

In fact, the current u32 "linking" ignores the upper level
prio/pref and breakes user assumptions when he inserts rules with
specific prio.

quoted

Do I understand that correctly that prio/pref only has meaning if
linking does not take place, because if there is linking, the prio/pref
of inserted rule is simply ignored?

I think even then the prio/pref meaning is dubious, from u32_change,

Please see tc_ctl_tfilter. That is where prio/pref is processed. What
you describe is one level down.

               for (pins = rtnl_dereference(*ins); pins;
                    ins = &pins->next, pins = rtnl_dereference(*ins))
                       if (TC_U32_NODE(handle) < TC_U32_NODE(pins->handle))
                               break;

I think the list insert is done via handle not via prio/pref.

quoted

That is the most confusing thing I saw in net/sched/ so far. 
Is this a bug? Sounds like one.

I don't think this is a bug at very least I don't see how we can
change it without breaking users. I know people depend on the hash map
capabilities and linking logic.

Do they insert rules into multiple hashtables with different prio? Why?
What is the usecase?

quoted

Did someone introduce *u32_node (formerly static struct tc_u_common
*u32_list;) just to allow this weirdness?

Can I just remove this shared tp_c and make the linking to other
hashtables only possible within the same prio/pref? That would make
sense to me.

The idea to make linking hash tables only possible within the same
prio/pref will break existing programs. We can't do this its part of
UAPI now and people depend on it.

That's why I asked if that is a bug. I still feel it is. But I
definitelly understand your concern. I'm just trying to figure out how
to resolve this misdesign :(