Issues around packet capture when secondary process is doing rx/tx

[Konstantin Ananyev]

> -----Original Message-----
> From: Stephen Hemminger <stephen at networkplumber.org>
> Sent: Tuesday, January 9, 2024 11:07 PM
> To: Konstantin Ananyev <konstantin.ananyev at huawei.com>
> Cc: dev at dpdk.org; arshdeep.kaur at intel.com; Gowda, Sandesh <sandesh.gowda at intel.com>; Reshma Pattan
> <reshma.pattan at intel.com>
> Subject: Re: Issues around packet capture when secondary process is doing rx/tx
> 
> On Mon, 8 Jan 2024 15:13:25 +0000
> Konstantin Ananyev <konstantin.ananyev at huawei.com> wrote:
> 
> > > I have been looking at a problem reported by Sandesh
> > > where packet capture does not work if rx/tx burst is done in secondary process.
> > >
> > > The root cause is that existing rx/tx callback model just doesn't work
> > > unless the process doing the rx/tx burst calls is the same one that
> > > registered the callbacks.
> > >
> > > An example sequence would be:
> > > 	1. dumpcap (or pdump) as secondary tells pdump in primary to register callback
> > > 	2. secondary process calls rx_burst.
> > > 	3. rx_burst sees the callback but it has pointer pdump_rx which is not necessarily
> > > 	   at same location in primary and secondary process.
> > > 	4. indirect function call in secondary to bad location likely causes crash.
> >
> > As I remember, RX/TX callbacks were never intended to work over multiple processes.
> > Right now RX/TX callbacks are private for the process, different process simply should not
> > see/execute them.
> > I.E. it callbacks list is part of 'struct rte_eth_dev' itself, not the rte_eth_dev.data that is shared
> > between processes.
> > It should be normal, wehn for the same port/queue you will end-up with different list of callbacks
> > for different processes.
> > So, unless I am missing something, I don't see how we can end-up with 3) and 4) from above:
> > From my understanding secondary process will never see/call primary's callbacks.
> >
> > About pdump itself, it was a while when I looked at it last time, but as I remember to start it to work,
> > server process has to call rte_pdump_init() which in terns register PDUMP_MP handler.
> > I suppose for the secondary process to act as a 'pdump server' it needs to call rte_pdump_init() itself,
> > though I am not sure such option is supported right now.
> >
> 
> Did some more tests with modified testpmd, and reached some conclusions:
> 
> The logical interface would be to allow rte_pdump_init() to be called by
>    the process that would be using rx/tx burst API's.
> 
>   This doesn't work as it should because the multi-process socket API
>   assumes that the it only runs the server in primary.  The secondary
>   can start its own MP thread, but it won't work:
> 
>   Primary EAL: Multi-process socket /var/run/dpdk/rte/mp_socket
>   Secondary: EAL: Multi-process socket /var/run/dpdk/rte/mp_socket_6057_1ccd4157fd5
> 
>   The problem is when client (pdump or dumpcap) tries to run, it uses the mp_socket
>   in the primary which causes: EAL: Cannot find action: mp_pdump
> 
>   Looks like the whole MP socket mechanism is just not up to this.
> 
> Maybe pdump needs to have its own socket and control thread?
> Or MP socket needs to have some multicast fanout to all secondaries?

Might be we can do something simpler: pass to pdump_enable(), where we want to enable it:
on primary (remote_ process or secondary (local) process?
And then for primary send a message over MP socket (as we doing now), and for secondary (itself)
just do actual pdump enablement on it's own (install callbacks, etc.).
Yes, in that way, one secondary would not be able to enable/idable pdump on another secondary,
only on itself, but might be it is not needed?

Konstrantin