RE: [PATCH 0/6] refactor RDMA live migration based on rsocket API

["Gonglei (Arei)" via <qemu-devel@nongnu.org>]

> -----Original Message-----
> From: Peter Xu [mailto:peterx@redhat.com]
> Sent: Wednesday, June 5, 2024 10:19 PM
> To: Gonglei (Arei) <arei.gonglei@huawei.com>
> Cc: qemu-devel@nongnu.org; yu.zhang@ionos.com; mgalaxy@akamai.com;
> elmar.gerdes@ionos.com; zhengchuan <zhengchuan@huawei.com>;
> berrange@redhat.com; armbru@redhat.com; lizhijian@fujitsu.com;
> pbonzini@redhat.com; mst@redhat.com; Xiexiangyou
> <xiexiangyou@huawei.com>; linux-rdma@vger.kernel.org; lixiao (H)
> <lixiao91@huawei.com>; jinpu.wang@ionos.com; Wangjialin
> <wangjialin23@huawei.com>; Fabiano Rosas <farosas@suse.de>
> Subject: Re: [PATCH 0/6] refactor RDMA live migration based on rsocket API
> 
> On Wed, Jun 05, 2024 at 10:09:43AM +0000, Gonglei (Arei) wrote:
> > Hi Peter,
> >
> > > -----Original Message-----
> > > From: Peter Xu [mailto:peterx@redhat.com]
> > > Sent: Wednesday, June 5, 2024 3:32 AM
> > > To: Gonglei (Arei) <arei.gonglei@huawei.com>
> > > Cc: qemu-devel@nongnu.org; yu.zhang@ionos.com;
> mgalaxy@akamai.com;
> > > elmar.gerdes@ionos.com; zhengchuan <zhengchuan@huawei.com>;
> > > berrange@redhat.com; armbru@redhat.com; lizhijian@fujitsu.com;
> > > pbonzini@redhat.com; mst@redhat.com; Xiexiangyou
> > > <xiexiangyou@huawei.com>; linux-rdma@vger.kernel.org; lixiao (H)
> > > <lixiao91@huawei.com>; jinpu.wang@ionos.com; Wangjialin
> > > <wangjialin23@huawei.com>; Fabiano Rosas <farosas@suse.de>
> > > Subject: Re: [PATCH 0/6] refactor RDMA live migration based on
> > > rsocket API
> > >
> > > Hi, Lei, Jialin,
> > >
> > > Thanks a lot for working on this!
> > >
> > > I think we'll need to wait a bit on feedbacks from Jinpu and his
> > > team on RDMA side, also Daniel for iochannels.  Also, please
> > > remember to copy Fabiano Rosas in any relevant future posts.  We'd
> > > also like to know whether he has any comments too.  I have him copied in
> this reply.
> > >
> > > On Tue, Jun 04, 2024 at 08:14:06PM +0800, Gonglei wrote:
> > > > From: Jialin Wang <wangjialin23@huawei.com>
> > > >
> > > > Hi,
> > > >
> > > > This patch series attempts to refactor RDMA live migration by
> > > > introducing a new QIOChannelRDMA class based on the rsocket API.
> > > >
> > > > The /usr/include/rdma/rsocket.h provides a higher level rsocket
> > > > API that is a 1-1 match of the normal kernel 'sockets' API, which
> > > > hides the detail of rdma protocol into rsocket and allows us to
> > > > add support for some modern features like multifd more easily.
> > > >
> > > > Here is the previous discussion on refactoring RDMA live migration
> > > > using the rsocket API:
> > > >
> > > > https://lore.kernel.org/qemu-devel/20240328130255.52257-1-philmd@l
> > > > inar
> > > > o.org/
> > > >
> > > > We have encountered some bugs when using rsocket and plan to
> > > > submit them to the rdma-core community.
> > > >
> > > > In addition, the use of rsocket makes our programming more
> > > > convenient, but it must be noted that this method introduces
> > > > multiple memory copies, which can be imagined that there will be a
> > > > certain performance degradation, hoping that friends with RDMA
> > > > network cards can help verify,
> > > thank you!
> > >
> > > It'll be good to elaborate if you tested it in-house. What people
> > > should expect on the numbers exactly?  Is that okay from Huawei's POV?
> > >
> > > Besides that, the code looks pretty good at a first glance to me.
> > > Before others chim in, here're some high level comments..
> > >
> > > Firstly, can we avoid using coroutine when listen()?  Might be
> > > relevant when I see that rdma_accept_incoming_migration() runs in a
> > > loop to do raccept(), but would that also hang the qemu main loop
> > > even with the coroutine, before all channels are ready?  I'm not a
> > > coroutine person, but I think the hope is that we can make dest QEMU
> > > run in a thread in the future just like the src QEMU, so the less coroutine
> the better in this path.
> > >
> >
> > Because rsocket is set to non-blocking, raccept will return EAGAIN
> > when no connection is received, coroutine will yield, and will not hang the
> qemu main loop.
> 
> Ah that's ok.  And also I just noticed it may not be a big deal either as long as
> we're before migration_incoming_process().
> 
> I'm wondering whether it can do it similarly like what we do with sockets in
> qio_net_listener_set_client_func_full().  After all, rsocket wants to mimic the
> socket API.  It'll make sense if rsocket code tries to match with socket, or
> even reuse.
> 

Actually we tried this solution, but it didn't work. Pls see patch 3/6

Known limitations: 
  For a blocking rsocket fd, if we use io_create_watch to wait for
  POLLIN or POLLOUT events, since the rsocket fd is blocking, we
  cannot determine when it is not ready to read/write as we can with
  non-blocking fds. Therefore, when an event occurs, it will occurs
  always, potentially leave the qemu hanging. So we need be cautious
  to avoid hanging when using io_create_watch .


Regards,
-Gonglei

> >
> > > I think I also left a comment elsewhere on whether it would be
> > > possible to allow iochannels implement their own poll() functions to
> > > avoid the per-channel poll thread that is proposed in this series.
> > >
> > > https://lore.kernel.org/r/ZldY21xVExtiMddB@x1n
> > >
> >
> > We noticed that, and it's a big operation. I'm not sure that's a better way.
> >
> > > Personally I think even with the thread proposal it's better than
> > > the old rdma code, but I just still want to double check with you
> > > guys.  E.g., maybe that just won't work at all?  Again, that'll also
> > > be based on the fact that we move migration incoming into a thread
> > > first to keep the dest QEMU main loop intact, I think, but I hope we
> > > will reach that irrelevant of rdma, IOW it'll be nice to happen even earlier if
> possible.
> > >
> > Yep. This is a fairly big change, I wonder what other people's suggestions
> are?
> 
> Yes we can wait for others' opinions.  And btw I'm not asking for it and I don't
> think it'll be a blocker for this approach to land, as I said this is better than the
> current code so it's definitely an improvement to me.
> 
> I'm purely curious, because if you're not going to do it for rdma, maybe
> someday I'll try to do that, and I want to know what "big change" could be as I
> didn't dig further.  It may help me by sharing what issues you've found.
> 
> Thanks,
> 
> --
> Peter Xu