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

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> 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.

Just for reference, you don't have to use it, but I have this patch:

https://gitlab.com/farosas/qemu/-/commit/4316e145ae7e7bf378ef7fde64c2b02260362847

>> 
>> >> 
>> >> >
>> >> >   - 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.
>

I see. Makes sense.

>> 
>> >
>> > 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.
>>