Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol handling

[Jinpu Wang]

Hi Gonglei,

On Wed, May 29, 2024 at 10:31 AM Gonglei (Arei) <arei.gonglei@huawei.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Greg Sword [mailto:gregsword0@gmail.com]
> > Sent: Wednesday, May 29, 2024 2:06 PM
> > To: Jinpu Wang <jinpu.wang@ionos.com>
> > Subject: Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol handling
> >
> > On Wed, May 29, 2024 at 12:33 PM Jinpu Wang <jinpu.wang@ionos.com>
> > wrote:
> > >
> > > On Wed, May 29, 2024 at 4:43 AM Gonglei (Arei) <arei.gonglei@huawei.com>
> > wrote:
> > > >
> > > > Hi,
> > > >
> > > > > -----Original Message-----
> > > > > From: Peter Xu [mailto:peterx@redhat.com]
> > > > > Sent: Tuesday, May 28, 2024 11:55 PM
> > > > > > > > Exactly, not so compelling, as I did it first only on
> > > > > > > > servers widely used for production in our data center. The
> > > > > > > > network adapters are
> > > > > > > >
> > > > > > > > Ethernet controller: Broadcom Inc. and subsidiaries
> > > > > > > > NetXtreme
> > > > > > > > BCM5720 2-port Gigabit Ethernet PCIe
> > > > > > >
> > > > > > > Hmm... I definitely thinks Jinpu's Mellanox ConnectX-6 looks
> > > > > > > more
> > > > > reasonable.
> > > > > > >
> > > > > > >
> > > > >
> > https://lore.kernel.org/qemu-devel/CAMGffEn-DKpMZ4tA71MJYdyemg0Zda
> > > > > 15
> > > > > > > wVAqk81vXtKzx-LfJQ@mail.gmail.com/
> > > > > > >
> > > > > > > Appreciate a lot for everyone helping on the testings.
> > > > > > >
> > > > > > > > InfiniBand controller: Mellanox Technologies MT27800 Family
> > > > > > > > [ConnectX-5]
> > > > > > > >
> > > > > > > > which doesn't meet our purpose. I can choose RDMA or TCP for
> > > > > > > > VM migration. RDMA traffic is through InfiniBand and TCP
> > > > > > > > through Ethernet on these two hosts. One is standby while the 
> > > > > > > > other
> > is active.
> > > > > > > >
> > > > > > > > Now I'll try on a server with more recent Ethernet and
> > > > > > > > InfiniBand network adapters. One of them has:
> > > > > > > > BCM57414 NetXtreme-E 10Gb/25Gb RDMA Ethernet Controller (rev
> > > > > > > > 01)
> > > > > > > >
> > > > > > > > The comparison between RDMA and TCP on the same NIC could
> > > > > > > > make more
> > > > > > > sense.
> > > > > > >
> > > > > > > It looks to me NICs are powerful now, but again as I mentioned
> > > > > > > I don't think it's a reason we need to deprecate rdma,
> > > > > > > especially if QEMU's rdma migration has the chance to be 
> > > > > > > refactored
> > using rsocket.
> > > > > > >
> > > > > > > Is there anyone who started looking into that direction?
> > > > > > > Would it make sense we start some PoC now?
> > > > > > >
> > > > > >
> > > > > > My team has finished the PoC refactoring which works well.
> > > > > >
> > > > > > Progress:
> > > > > > 1.  Implement io/channel-rdma.c, 2.  Add unit test
> > > > > > tests/unit/test-io-channel-rdma.c and verifying it is
> > > > > > successful, 3.  Remove the original code from migration/rdma.c, 4.
> > > > > > Rewrite the rdma_start_outgoing_migration and
> > > > > > rdma_start_incoming_migration logic, 5.  Remove all rdma_xxx
> > > > > > functions from migration/ram.c. (to prevent RDMA live migration
> > > > > > from polluting the
> > > > > core logic of live migration), 6.  The soft-RoCE implemented by
> > > > > software is used to test the RDMA live migration. It's successful.
> > > > > >
> > > > > > We will be submit the patchset later.
> > > > >
> > > > > That's great news, thank you!
> > > > >
> > > > > --
> > > > > Peter Xu
> > > >
> > > > For rdma programming, the current mainstream implementation is to use
> > rdma_cm to establish a connection, and then use verbs to transmit data.
> > > >
> > > > rdma_cm and ibverbs create two FDs respectively. The two FDs have
> > > > different responsibilities. rdma_cm fd is used to notify connection
> > > > establishment events, and verbs fd is used to notify new CQEs. When
> > poll/epoll monitoring is directly performed on the rdma_cm fd, only a pollin
> > event can be monitored, which means that an rdma_cm event occurs. When
> > the verbs fd is directly polled/epolled, only the pollin event can be 
> > listened,
> > which indicates that a new CQE is generated.
> > > >
> > > > Rsocket is a sub-module attached to the rdma_cm library and provides
> > > > rdma calls that are completely similar to socket interfaces.
> > > > However, this library returns only the rdma_cm fd for listening to link
> > setup-related events and does not expose the verbs fd (readable and writable
> > events for listening to data). Only the rpoll interface provided by the 
> > RSocket
> > can be used to listen to related events. However, QEMU uses the ppoll
> > interface to listen to the rdma_cm fd (gotten by raccept API).
> > > > And cannot listen to the verbs fd event.
I'm confused, the rs_poll_arm
:https://github.com/linux-rdma/rdma-core/blob/master/librdmacm/rsocket.c#L3290
For STREAM, rpoll setup fd for both cq fd and cm fd.

> > > >
> > > > Do you guys have any ideas? Thanks.
> > > +cc linux-rdma
> >
> > Why include rdma community?
> >
>
> Can rdma/rsocket provide an API to expose the verbs fd?
Why do we need verbs fd? looks rsocket during rsend/rrecv is handling
the new completion if any via rs_get_comp

Another question to my mind is Daniel suggested a bit different way of
using rsocket: ZjtOreamN8xF9FDE@redhat.com/">https://lore.kernel.org/qemu-devel/ZjtOreamN8xF9FDE@redhat.com/
Have you considered that?

Thx!
Jinpu




>
>
> Regards,
> -Gonglei
>
> > > +cc Sean
> > >
> > >
> > >
> > > >
> > > >
> > > > Regards,
> > > > -Gonglei
> > >