[ovs-dev] [PATCH] odp-util.c: Fix dp_hash execution with slowpath actions.

[aginwala]

Thanks Han for the fix:

I verified it in the internal ecmp environment and Ping works fine with no
more drop recirc action error messages as per example above.

Maybe it would be good to show if R3 has a VIF too in the diagram and tweak
the commit message a bit stating VIFs under R3 can ping R3 including VIFs
in R0 <-> VIFs in R3 too; with/without this patch and no 'drop recirc
action' error message. What do you think? Just my two cents.

Tested-by: Aliasgar Ginwala <aginwala at ebay.com>


On Fri, May 15, 2020 at 11:55 AM Han Zhou <hzhou at ovn.org> wrote:

> On Fri, May 15, 2020 at 12:18 AM Han Zhou <hzhou at ovn.org> wrote:
> >
> > When dp_hash is executed with slowpath actions, it results in endless
> > recirc loop in kernel datapath, and finally drops the packet, with
> > kernel logs:
> >
> > openvswitch: ovs-system: deferred action limit reached, drop recirc
> action
> >
> > The root cause is that the dp_hash value calculated by slowpath is not
> > passed to datapath when executing the recirc action, thus when the
> recirced
> > packet miss upcall comes to userspace again, it generates the dp_hash
> > and recirc action again, with same recirc_id, which in turn generates
> > a megaflow with recirc action with the recird_id same as the recirc_id in
> > its match condition, which causes a loop in datapath.
> >
> > For example, this can be reproduced with below setup of OVN environment:
> >
> >                          LS1            LS2
> >                           |              |
> >                           |------R1------|
> >         VIF--LS0---R0-----|              |------R3
> >                           |------R2------|
> >
> > Assume there is a route from the VIF to R3: R0 -> R1 -> R3, and there are
> two
> > routes (ECMP) from R3 to the VIF:
> > R3 -> R1 -> R0
> > R3 -> R2 -> R0
> >
> > Now if we ping from the VIF to R3, the OVS flow execution on the HV of
> the VIF
> > will hit the R3's datapath which has flows that responds to the ICMP
> packet
> > by setting ICMP fields, which requires slowpath actions, and in later
> flow
> > tables it will hit the "group" action that selects between the ECMP
> routes.
> >
> > By default OVN uses "dp_hash" method for the "group" action.
> >
> > For the first miss upcall packet, dp_hash value is empty, so the group
> action
> > will be translated to "dp_hash" and "recirc".
> >
> > During action execution, because of the previous actions that sets ICMP
> fields,
> > the whole execution requires slowpath, so it tries to execute all actions
> in
> > userspace in odp_execute_actions(), including dp_hash action, except the
> > recirc action, which can only be executed in datapath. So the dp_hash
> value
> > is calculated in userspace, and then the packet is injected to datapath
> for
> > recirc action execution.
> >
> > However, the dp_hash calculated by the userspace is not passed to
> datapath.
> >
> > Because of this, the packet recirc in datapath doesn't have dp_hash
> value,
> > and the miss upcall for the recirced packet hits the same flow tables and
> > triggers same "dp_hash" and "recirc" action again, with exactly same
> recirc_id!
> >
> > This time, the new upcall doesn't require any slowpath execution, so both
> > the dp_hash and recirc actions are executed in datapath, after creating a
> > datapath megaflow like:
> >
> > recirc_id(XYZ),..., actions:hash(l4(0)),recirc(XYZ)
> >
> > with match recirc_id equals the recirc id in the action, thus creating a
> loop.
> >
> > This patch fixes the problem by passing the calculated dp_hash value to
> > datapath in odp_key_from_dp_packet().
> >
> > Signed-off-by: Han Zhou <hzhou at ovn.org>
> > ---
> >  lib/odp-util.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/lib/odp-util.c b/lib/odp-util.c
> > index b66d266..ac532fe 100644
> > --- a/lib/odp-util.c
> > +++ b/lib/odp-util.c
> > @@ -6392,6 +6392,10 @@ odp_key_from_dp_packet(struct ofpbuf *buf, const
> struct dp_packet *packet)
> >
> >      nl_msg_put_u32(buf, OVS_KEY_ATTR_PRIORITY, md->skb_priority);
> >
> > +    if (md->dp_hash) {
> > +        nl_msg_put_u32(buf, OVS_KEY_ATTR_DP_HASH, md->dp_hash);
> > +    }
> > +
> >      if (flow_tnl_dst_is_set(&md->tunnel)) {
> >          tun_key_to_attr(buf, &md->tunnel, &md->tunnel, NULL, NULL);
> >      }
> > --
> > 2.1.0
> >
>
> Ben and Ilya, this is the fix to the dp_hash problem we discussed in
> yesterday's meeting. The actual fix is simpler that I thought it would be.
> I didn't take the approach of executing dp_hash in datapath because in this
> case since the flow is required to be slowpathed, all the following packets
> for this flow will anyway get upcalled. If all the dp_hash for the flow is
> executed in slowpath then there is no consistency problem. So I think it is
> ok to keep the calculation in userspace and the fix is simple. Let me know
> if you think differently.
>
> Thanks,
> Han
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