[ovs-dev] [PATCH ovn] Exclude inport and outport symbol tables from conjunction

[Han Zhou]

On Sat, Sep 14, 2019 at 9:09 AM Han Zhou <zhouhan at gmail.com> wrote:
>
>
>
> On Sat, Sep 14, 2019 at 12:40 AM Numan Siddique <nusiddiq at redhat.com>
wrote:
> >
> >
> >
> > On Sat, Sep 14, 2019 at 2:41 AM Daniel Alvarez Sanchez <
dalvarez at redhat.com> wrote:
> >>
> >> Acked-by: Daniel Alvarez <dalvarez at redhat.com>
> >>
> >>
> >> On Fri, Sep 13, 2019 at 11:02 PM Mark Michelson <mmichels at redhat.com>
wrote:
> >> >
> >> > Acked-by: Mark Michelson
> >> >
> >> > It sucks that we lose the efficiency of the conjunctive match
altogether
> >> > on port groups because of this error, but I understand this is a huge
> >> > bug and needs fixing.
> >> If I'm not mistaken, from OpenStack standpoint conjunction was *only*
> >> being used when using port groups and ACLs that matched on port ranges
> >> ( e.g tcp.dst >= X && tcp.dst <=Y) which was not working. Therefore
> >> we're not losing performance because it was already broken (given that
> >> there was more than one ACL like that).
> >>
> >> >
> >> > Perhaps it would be good to start up a discussion on this list about
a
> >> > more longterm solution that would allow for conjunctive matches with
no
> >> > ambiguity.
> >> Agreed! We already discussed some ideas on IRC but it'd be awesome to
> >> have a thread and brainstorm there.
> >>
> >
> > Thanks for the reviews. I applied this to master.
> > Agree we can discuss it further and come up with ideas.
> >
> > I know Dumitru has some idea to make use of conjunctions for port
groups.
> > CC'ing Han if he has any comments on ideas.
> >
>
> Hi Numan,
>
> This is a good finding. However, I think it is not specifically a problem
of port group. It seems to be a more general problem and this patch fixes
only a special case.
> For example, would there be similar problem for below ACLs without port
groups:
>
> ip4 && tcp.src >= 1000 && tcp.src <= 1001 && tcp.dst >= 500 && tcp.dst <=
501
> ip4 && tcp.src >= 1000 && tcp.src <= 1001 && tcp.dst >= 600 && tcp.dst <=
601
>
> Another example is with address set:
>
> ip4 && ip4.src == $as1 && tcp.dst >= 500 && tcp.dst <= 501
> ip4 && ip4.src == $as1 && tcp.dst >= 600 && tcp.dst <= 601
>
> Or even without range:
> ip4 && tcp.src == {1000, 1001} && tcp.dst == {500, 501}
> ip4 && tcp.src == {1000, 1001} && tcp.dst == {600, 601}
>
> You may think of more examples. Whenever there are multiple
conjunctionable ACLs with same match as part of the conjunction, it should
result in such problem.
>
> A quick fix to all these problems may be just abandon conjunction, but I
believe there are better ways to address it.
>
> First of all, these matches can be rewritten by combining them in a
single ACL with "OR" operator, e.g.:
>
> outport == @pg1 && ip4 && tcp.dst >= 500 && tcp.dst <= 501
> outport == @pg1 && ip4 && tcp.dst >= 600 && tcp.dst <= 601
>
> can be rewritten as ====>
>
> outport == @pg1 && ip4 && (tcp.dst >= 500 && tcp.dst <= 501 || tcp.dst >=
600 && tcp.dst <= 601)
>
> Similar can be done for all above examples. So, a workaround to the
problem is from user side (e.g. OpenStack) to make sure always combining
ACLs with "OR" if there are common conjunctionable matches between
different ACLs. However, a better way would be in ovn-northd itself to
detect and combine such ACLs internally, before generating the final
logical flows in SB. It may be more convenient to be done in
ovn-controller, because we are not even parsing the ACLs in ovn-northd
today, but the cost of such pre-processing would be duplicated in all HVs.
It surely will increase CPU cost for doing such combination, but I'd not
worry too much if we do it properly at each LS level instead of for all
ACLs.

I just thought a little more about combining the conjunctions. It seems we
can do it without pre-processing by just handling duplicated flows in
ofctrl_add_flow(). Currently we just drop duplicated flows, but we can
check that if the action is conjuncture and the conjuncture ID is
different, we can perform a combination by using existing flow's
conjunction id to update all the flows related to that to-be-added
duplicated flow. This way, the combination is performed on-the-fly, without
introduce too much cost and without introduce parsing in ovn-northd either.

In addition, there are more general cases that can't be handled by
combining ACLs, if there are overlapping sets in different ACLs. E.g.
tcp.src == {1000, 1001} && tcp.dst == {500, 501}
tcp.src == {1001, 1002} && tcp.dst == {600, 601}

In this example, there is no way to combine these 2 ACLs because there is
no common components in the matches, but the first set in each conjunctions
are overlapping. So there will be flows generated something like:
tcp.src=1001: conjunction(1, 1/2)
...
tcp.src=1001: conjunction(2, 1/2)
...
This causes the same duplicated flow problem and combining the two set of
conjunctions is incorrect.

However, although this is valid case in theory, it seems not a real problem
in reality. Usually ACL will be defined with different priorities if there
are overlapping (but not identical) set of matches. (At least they are not
well designed ACLs - I might be wrong)

cc Ben in case he had thought about these problems before.

Thanks,
Han