Subject: Re: Expose bond_xmit_hash function

From: Jay Vosburgh <[email-masked]>
Date: Mon, 20 Jan 2020 10:43:40 -0800

Maor Gottlieb <[email-masked]> wrote:

>
>On 1/16/2020 6:00 PM, Jay Vosburgh wrote:
>> Maor Gottlieb <[email-masked]> wrote:
>>
>>> On 1/16/2020 4:42 PM, Andy Gospodarek wrote:
>>>> On Wed, Jan 15, 2020 at 03:15:35PM +0100, Jiri Pirko wrote:
>>>>> Wed, Jan 15, 2020 at 02:04:49PM CET, [email-masked] wrote:
>>>>>> On 1/15/2020 11:45 AM, Jiri Pirko wrote:
>>>>>>> Wed, Jan 15, 2020 at 09:01:43AM CET, [email-masked] wrote:
>>>>>>>> RDMA over Converged Ethernet (RoCE) is a standard protocol which enables
>>>>>>>> RDMA’s efficient data transfer over Ethernet networks allowing transport
>>>>>>>> offload with hardware RDMA engine implementation.
>>>>>>>> The RoCE v2 protocol exists on top of either the UDP/IPv4 or the
>>>>>>>> UDP/IPv6 protocol:
>>>>>>>>
>>>>>>>> --------------------------------------------------------------
>>>>>>>> | L2 | L3 | UDP |IB BTH | Payload| ICRC | FCS |
>>>>>>>> --------------------------------------------------------------
>>>>>>>>
>>>>>>>> When a bond LAG netdev is in use, we would like to have the same hash
>>>>>>>> result for RoCE packets as any other UDP packets, for this purpose we
>>>>>>>> need to expose the bond_xmit_hash function to external modules.
>>>>>>>> If no objection, I will push a patch that export this symbol.
>>>>>>> I don't think it is good idea to do it. It is an internal bond function.
>>>>>>> it even accepts "struct bonding *bond". Do you plan to push netdev
>>>>>>> struct as an arg instead? What about team? What about OVS bonding?
>>>>>> No, I am planning to pass the bond struct as an arg. Currently, team
>>>>> Hmm, that would be ofcourse wrong, as it is internal bonding driver
>>>>> structure.
>>>>>
>>>>>
>>>>>> bonding is not supported in RoCE LAG and I don't see how OVS is related.
>>>>> Should work for all. OVS is related in a sense that you can do bonding
>>>>> there too.
>>>>>
>>>>>
>>>>>>> Also, you don't really need a hash, you need a slave that is going to be
>>>>>>> used for a packet xmit.
>>>>>>>
>>>>>>> I think this could work in a generic way:
>>>>>>>
>>>>>>> struct net_device *master_xmit_slave_get(struct net_device *master_dev,
>>>>>>> 					 struct sk_buff *skb);
>>>>>> The suggestion is to put this function in the bond driver and call it
>>>>>> instead of bond_xmit_hash? is it still necessary if I have the bond pointer?
>>>>> No. This should be in a generic code. No direct calls down to bonding
>>>>> driver please. Or do you want to load bonding module every time your
>>>>> module loads?
>>>>>
>>>>> I thinks this can be implemented with ndo with "master_xmit_slave_get()"
>>>>> as a wrapper. Masters that support this would just implement the ndo.
>>>> In general I think this is a good idea (though maybe not with an skb as
>>>> an arg so we can use it easily within BPF), but I'm not sure if solves
>>>> the problem that Maor et al were setting out to solve.
>>>>
>>>> Maor, if you did export bond_xmit_hash() to be used by another driver,
>>>> you would presumably have a check in place so if the RoCE and UDP
>>>> packets had a different hash function output you would make a change and
>>>> be sure that the UDP frames would go out on the same device that the
>>>> RoCE traffic would normally use.  Is this correct?  Would you also send
>>>> the frames directly on the interface using dev_queue_xmit() and bypass
>>>> the bonding driver completely?
>>> RoCE packets are UDP. The idea is that the same UDP header (RoCE as
>>> well) will get the same hash result so they will be transmitted from the
>>> same port.
>>> The frames will be sent by using the RDMA send API and bypass the
>>> bonding driver completely.
>>> Is it answer your question?
>> 	If the RDMA send bypasses bonding, how will you insure that the
>> same hash result maps to the same underlying interface for both bonding
>> and RDMA?
>>
>> 	-J
>
>In RoCE, the affinity is determined while the HW resources are created 
>and will be modified globally in run time to track the active salves.
>If we get the slave result, all the UDP packets will have the same 
>affinity as RoCE.

	How do you "track the active slaves?"

	What I want to know is whether or not the RoCE code is going to
peek into the bonding internal data structures to look at, e.g.,
bond->slave_arr.  I don't see an obvious way to insure a duplication of
bonding's hash to slave mapping without knowing the placement of the
slaves in slave_arr.

>The downside is that all RoCE HW resources will be stuck with the 
>original affinity port and not move to the re-activate slave once it 
>goes up. Another disadvantage, when both ports are down, we still need 
>to create the RoCE HW resources with a given port affinity. Both 
>problems are solved by exporting the hash.

	This sounds like a different explanation; above, you said things
are "modified globally in run time" but here the mappings are static.  I
don't see an explanation of how having the hash solves these problems,
or why it would be better than Jiri's suggestion of an .ndo or generic
wrapper.

	-J

---
	-Jay Vosburgh, [email-masked]