[dpdk-dev] [PATCH v4 06/24] ethdev: enable hotplug on multi-process

[Burakov, Anatoly]

On 26-Jun-18 3:24 PM, Zhang, Qi Z wrote:
> 
> 
>> -----Original Message-----
>> From: Burakov, Anatoly
>> Sent: Tuesday, June 26, 2018 9:46 PM
>> To: Zhang, Qi Z <qi.z.zhang at intel.com>; thomas at monjalon.net
>> Cc: Ananyev, Konstantin <konstantin.ananyev at intel.com>; dev at dpdk.org;
>> Richardson, Bruce <bruce.richardson at intel.com>; Yigit, Ferruh
>> <ferruh.yigit at intel.com>; Shelton, Benjamin H
>> <benjamin.h.shelton at intel.com>; Vangati, Narender
>> <narender.vangati at intel.com>
>> Subject: Re: [PATCH v4 06/24] ethdev: enable hotplug on multi-process
>>
>> On 26-Jun-18 2:25 PM, Zhang, Qi Z wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Burakov, Anatoly
>>>> Sent: Tuesday, June 26, 2018 9:21 PM
>>>> To: Zhang, Qi Z <qi.z.zhang at intel.com>; thomas at monjalon.net
>>>> Cc: Ananyev, Konstantin <konstantin.ananyev at intel.com>; dev at dpdk.org;
>>>> Richardson, Bruce <bruce.richardson at intel.com>; Yigit, Ferruh
>>>> <ferruh.yigit at intel.com>; Shelton, Benjamin H
>>>> <benjamin.h.shelton at intel.com>; Vangati, Narender
>>>> <narender.vangati at intel.com>
>>>> Subject: Re: [PATCH v4 06/24] ethdev: enable hotplug on multi-process
>>>>
>>>> On 26-Jun-18 1:58 PM, Zhang, Qi Z wrote:
>>>>>
>>>>> my understand is peer is identified by a string (or filename) what I
>>>>> mean is clone the content of the buffer that peer point to , So I
>>>>> don't need to worry if the original peer be used to point to some
>>>>> other data
>>>>>
>>>>
>>>> As far as the application is concerned, peer is an opaque pointer,
>>>> and should be treated as such. Peeking behind a void pointer that is
>>>> not designed for this purpose is not a good idea, even if technically you
>> know what's in there.
>>>
>>> We can expose a clone interface, like MP_PEER_CLONE, so we don't need to
>> know what's inside, just need to know that it can be used on another thread?
>>>
>>
>> Well, that can probably work. Feels like a hacky workaround though.
>>
>> Another way to do the same thing would be to store peer information right in
>> the message, as opposed to providing it separately. Still a hack though, and will
>> require far more changes, but it could be a better solution as (if done right) it
>> would allow identifying which reply came from which peer.
>>
>> Of course, an even better approach would be to devise some kind of
>> addressing scheme (uuid?), so that peer addresses are no longer opaque
>> pointers but rather are valid data types.
>>
>> Thoughts?
> 
> I may not give insight comment from the IPC implementation, but from user's view, what required is a unique token,
>   it can be used for reply at anywhere and at any time, to me, currently implementation looks like missing some mapping management between an abstract token to its real data.
> 

Well, the idea was to not provide that :) We wanted to keep it simple 
and actively discourage any attempts to know which peer you're 
communicating with, as allowing that would imply some kind of addressing 
scheme, peer discovery and things like that, which we don't want to 
bother with. If you want replies, you use callbacks, which provide you 
with a peer address.

On the one hand, i understand the frustration of being forced to deal 
with deliberate simplicity of IPC API and to create workarounds like 
doing sendmsg() and storing requests that you're waiting on inside the 
application and launching interrupt threads, all for things that should 
Just Work in an IPC API. Believe me, i know about its deficiencies more 
than most people :)

On the other hand, we don't want to introduce hacks to solve a specific 
problem. If we are to solve this in IPC, we should do it properly. I 
don't think adding a workaround with "cloning" peer address is the 
solution. A proper solution would've been to implement addressing 
scheme, so that any response could be submitted at any time.

The hard route would be to add peer discovery, etc. The easy route would 
be to just change the peer address to something typed, something that we 
can use to reconstruct the original peer id. A great candidate for this 
is the uuid, but it's an external dependency. A workaround for this 
would be to just use a uint64 value obtained through some magic that is 
part of socket path. For example, replace 
"/var/run/dpdk/rte/mp_socket_pid_rdtsc" with something just as (likely) 
unique, but something that can be put into a uint64.

Without significantly changing the API and the internals, i think this 
is the best we will be able to do.

> Thanks
> Qi.
> 
>>
>> --
>> Thanks,
>> Anatoly


-- 
Thanks,
Anatoly