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

[Ilya Maximets]

On 5/15/20 8:55 PM, Han Zhou wrote:
> 
> 
> On Fri, May 15, 2020 at 12:18 AM Han Zhou <hzhou at ovn.org <mailto: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 <mailto: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.

Hi.  Sorry for it took so long to reply.

I understand that this patch fixes this particular case, however I still
think it's dangerous to pass the hash calculated in userspace to kernel
since it might cause mismatch for the later packets in case where the
flow doesn't have actions that requires sending to userspace.

One more thing.  We have such a comment in odp-execute.c:

        /* Calculate a hash value directly. This might not match the         
         * value computed by the datapath, but it is much less expensive,    
         * and the current use case (bonding) does not require a strict      
         * match to work properly. */

But we're using dp_hash not only for bonding for a long time now.
And this doesn't look correct.  Even for bonding I'm not sure if that
is a fully correct assumption.

>From the other side, AFAIU, OVS is not able to execute any "non-terminal"
actions in datapath, get results and continue to execute further actions.
The option here is to assume that dp_hash + recirc always goes together.

I'm not sure how to proceed here.
Ben, what do you think?

Best regards, Ilya Maximets.