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

[Dumitru Ceara]

On Sat, Sep 14, 2019 at 7:16 PM Han Zhou <zhouhan at gmail.com> wrote:
>
>
>
> 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.

Hi Han,

Will this actually work without a change in OVS? I wonder because in
the ovs-fields man page [1] I see:

"Conjunctive flows must not overlap with each other, at
 a given priority, that is, any given packet must be
 able to match at most one conjunctive flow at a given
 priority. Overlapping conjunctive flows yield
 unpredictable results."

I guess another possibility is to detect flows that have overlapping
sets of matches in ofctrl and change their priorities (along with all
their other conjunction clauses) in order to differentiate the ACLs.
That might turn out to be quite tricky though as we need to maintain
the logical priority of ACLs defined by the user.

Thanks,
Dumitru

[1] http://man7.org/linux/man-pages/man7/ovs-fields.7.html

>
> 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