Re: [PATCH 1/5] migration/multifd: Separate compression ops from non-compression

[Peter Xu <peterx@redhat.com>]

On Tue, Jan 30, 2024 at 12:11:47PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Mon, Jan 29, 2024 at 09:42:24AM -0300, Fabiano Rosas wrote:
> >> Peter Xu <peterx@redhat.com> writes:
> >> 
> >> > On Fri, Jan 26, 2024 at 07:19:39PM -0300, Fabiano Rosas wrote:
> >> >> +static MultiFDMethods multifd_socket_ops = {
> >> >> +    .send_setup = multifd_socket_send_setup,
> >> >> +    .send_cleanup = multifd_socket_send_cleanup,
> >> >> +    .send_prepare = multifd_socket_send_prepare,
> >> >
> >> > Here it's named with "socket", however not all socket-based multifd
> >> > migrations will go into this route, e.g., when zstd compression enabled it
> >> > will not go via this route, even if zstd also uses sockets as transport.
> >> > From that pov, this may be slightly confusing.  Maybe it suites more to be
> >> > called "socket_plain" / "socket_no_comp"?
> >> >
> >> > One step back, I had a feeling that the current proposal tried to provide a
> >> > single ->ops to cover a model where we may need more than one layer of
> >> > abstraction.
> >> >
> >> > Since it might be helpful to allow multifd send arbitrary data (e.g. for
> >> > VFIO?  Avihai might have an answer there..), I'll try to even consider that
> >> > into the picture.
> >> >
> >> > Let's consider the ultimate goal of multifd, where the simplest model could
> >> > look like this in my mind (I'm only discussing sender side, but it'll be
> >> > similar on recv side):
> >> >
> >> >                prepare()           send()
> >> >   Input   ----------------> IOVs ------------> iochannels
> >> >
> >> > [I used prepare/send, but please think them as generic terms, not 100%
> >> >  aligned with what we have with existing multifd_ops, or what you proposed
> >> >  later]
> >> >
> >> > Here what are sure, IMHO, is:
> >> >
> >> >   - We always can have some input data to dump; I didn't use "guest pages"
> >> >     just to say we may allow arbitrary data.  For any multifd user that
> >> >     would like to dump arbitrary data, they can already provide IOVs, so
> >> >     here input can be either "MultiFDPages_t" or "IOVs".
> >> 
> >> Or anything else, since the client code also has control over send(),
> >> no? So it could give multifd a pointer to some memory and then use
> >> send() to do whatever it wants with it. Multifd is just providing worker
> >> threads and "scheduling".
> >
> > IOVs contain the case of one single buffer, where n_iovs==1.  Here I
> > mentioned IOVs explicitly because I want to make it part of the protocol so
> > that the interface might be clearer, on what is not changing, and what can
> > change for a multifd client.
> 
> Got it. I agree.
> 
> >> 
> >> Also note that multifd clients currently _do not_ provide IOVs. They
> >> merely provide data to multifd (p->pages) and then convert that data
> >> into IOVs at prepare(). This is different, because multifd currently
> >> holds that p->pages (and turns that into p->normal), which means the
> >> client code does not need to store the data across iterations (in the
> >> case of RAM which is iterative).
> >
> > They provide?  AFAIU that's exactly MultiFDSendParams.iov as of now, while
> > iov_nums is the length.
> 
> Before that, the ram code needs to pass in the p->pages->offset array
> first. Then, that gets put into p->normal. Then, that gets put into
> p->iov at prepare(). So it's not a simple "fill p->iov and pass it to
> multifd".
> 
> Hmm, could we just replace multifd_send_state->pages with a
> multifd_send_state->iov? I don't really understand why do we need to
> carry that pages->offset around.

I am thinking the p->normal is mostly redundant.. at least on the sender
side that I just read.  Since I'll be preparing a new spin of the multifd
cleanup series I posted, maybe I can append one more to try dropping
p->normal[] completely.

> 
> >> 
> >> >
> >> >   - We may always want to have IOVs to represent the buffers at some point,
> >> >     no matter what the input it
> >> >
> >> >   - We always flush the IOVs to iochannels; basically I want to say we can
> >> >     always assume the last layer is connecting to QIOChannel APIs, while I
> >> >     don't think there's outliers here so far, even if the send() may differ.
> >> >
> >> > Then _maybe_ it's clearer that we can have two layers of OPs?
> >> >
> >> >   - prepare(): it tells how the "input" will be converted into a scatter
> >> >     gatter list of buffers.  All compression methods fall into this afaiu.
> >> >     This has _nothing_ to do on how the buffers will be sent.  For
> >> >     arbitrary-typed input, this can already be a no-op since the IOVs
> >> >     provided can already be passed over to send().
> >> >
> >> >   - send(): how to dump the IOVs to the iochannels.  AFAIU this is motly
> >> >     only useful for fixed-ram migrations.
> >> >
> >> > Would this be clearer, rather than keep using a single multifd_ops?
> >> 
> >> Sorry, I don't see how what you describe is any different than what we
> >> have. And I don't see how any of this would mean more than one
> >> multifd_ops. We already have multifd_ops->prepare() and
> >> multifd_ops->send(). What am I missing?
> >
> > I meant instead of having a single MultiFDMethods, we can have
> > MultifdPrepareOps and MultifdSendOps separately.
> >
> > Now with single MultiFDMethods, it must always provide e.g. both prepare()
> > and send() in one set of OPs for one use case.  What I wanted to say is
> > maybe it is cleaner we split it into two OPs, then all the socket-based
> > scenarios can already stick with the same send() method, even though they
> > can prepare() differently.
> 
> Hmm, so zlib/zstd implement all ops except for the send one. And
> socket_plain and file implement all prepare hooks plus the send. So we'd
> have sort of a data handling layer and a transport layer. I'll see how
> it looks.

Yeah something like that if you agree; I'd think socket_plain can also use
the same socket send() with all the rest?  Again, I don't see its specialty
except the zero copy possibility, while the latter should be able to be
covered by proper setup of p->write_flags.

> 
> >
> > IOW, for this base patchset to pave way for compression accelerators, IIUC
> > we don't need a send() yet so far?  Should they still work pretty well with
> > qio_channel_writev_full_all() with proper touchups on p->write_flags just
> > for zero copy purposes?
> 
> Yes. The point here is to just give everyone a heads-up so we avoid
> changing the code in incompatible ways.
> 
> >
> > I'll have a read again to your previous multifd-packet-cleanups branch I
> > guess.  but this series definitely doesn't apply there already.
> 
> multifd-packet-cleanups attempts to replace MultiFDPages_t with a
> generic data structure. That's a separate issue.
> 

-- 
Peter Xu