Re: [PATCH v6 19/19] migration/multifd: Add documentation for multifd methods

[Peter Xu]

On Tue, Aug 27, 2024 at 03:54:51PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Tue, Aug 27, 2024 at 02:46:06PM -0300, Fabiano Rosas wrote:
> >> Add documentation clarifying the usage of the multifd methods. The
> >> general idea is that the client code calls into multifd to trigger
> >> send/recv of data and multifd then calls these hooks back from the
> >> worker threads at opportune moments so the client can process a
> >> portion of the data.
> >> 
> >> Suggested-by: Peter Xu <peterx@redhat.com>
> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> >> ---
> >> Note that the doc is not symmetrical among send/recv because the recv
> >> side is still wonky. It doesn't give the packet to the hooks, which
> >> forces the p->normal, p->zero, etc. to be processed at the top level
> >> of the threads, where no client-specific information should be.
> >> ---
> >>  migration/multifd.h | 76 +++++++++++++++++++++++++++++++++++++++++----
> >>  1 file changed, 70 insertions(+), 6 deletions(-)
> >> 
> >> diff --git a/migration/multifd.h b/migration/multifd.h
> >> index 13e7a88c01..ebb17bdbcf 100644
> >> --- a/migration/multifd.h
> >> +++ b/migration/multifd.h
> >> @@ -229,17 +229,81 @@ typedef struct {
> >>  } MultiFDRecvParams;
> >>  
> >>  typedef struct {
> >> -    /* Setup for sending side */
> >> +    /*
> >> +     * The send_setup, send_cleanup, send_prepare are only called on
> >> +     * the QEMU instance at the migration source.
> >> +     */
> >> +
> >> +    /*
> >> +     * Setup for sending side. Called once per channel during channel
> >> +     * setup phase.
> >> +     *
> >> +     * Must allocate p->iov. If packets are in use (default), one
> >
> > Pure thoughts: wonder whether we can assert(p->iov) that after the hook
> > returns in code to match this line.
> 
> Not worth the extra instructions in my opinion. It would crash
> immediately once the thread touches p->iov anyway.

It might still be good IMHO to have that assert(), not only to abort
earlier, but also as a code-styled comment.  Your call when resend.

PS: feel free to queue existing patches into your own tree without
resending the whole series!

> 
> >
> >> +     * extra iovec must be allocated for the packet header. Any memory
> >> +     * allocated in this hook must be released at send_cleanup.
> >> +     *
> >> +     * p->write_flags may be used for passing flags to the QIOChannel.
> >> +     *
> >> +     * p->compression_data may be used by compression methods to store
> >> +     * compression data.
> >> +     */
> >>      int (*send_setup)(MultiFDSendParams *p, Error **errp);
> >> -    /* Cleanup for sending side */
> >> +
> >> +    /*
> >> +     * Cleanup for sending side. Called once per channel during
> >> +     * channel cleanup phase. May be empty.
> >
> > Hmm, if we require p->iov allocation per-ops, then they must free it here?
> > I wonder whether we leaked it in most compressors.
> 
> Sorry, this one shouldn't have that text.

I still want to double check with you: we leaked iov[] in most compressors
here, or did I overlook something?

That's definitely more important than the doc update itself..

> 
> >
> > With that, I wonder whether we should also assert(p->iov == NULL) after
> > this one returns (squash in this same patch).
> >
> >> +     */
> >>      void (*send_cleanup)(MultiFDSendParams *p, Error **errp);
> >> -    /* Prepare the send packet */
> >> +
> >> +    /*
> >> +     * Prepare the send packet. Called from multifd_send(), with p
> >
> > multifd_send_thread()?
> 
> No, I meant called as a result of multifd_send(), which is the function
> the client uses to trigger a send on the thread.

OK, but it's confusing.  Some rewords you mentioned below could work.

> 
> >
> >> +     * pointing to the MultiFDSendParams of a channel that is
> >> +     * currently idle.
> >> +     *
> >> +     * Must populate p->iov with the data to be sent, increment
> >> +     * p->iovs_num to match the amount of iovecs used and set
> >> +     * p->next_packet_size with the amount of data currently present
> >> +     * in p->iov.
> >> +     *
> >> +     * Must indicate whether this is a compression packet by setting
> >> +     * p->flags.
> >
> > Sigh.. I wonder whether we could avoid mentioning this, and also we avoid
> > adding new flags for new compressors, relying on libvirt guarding things.
> > Then when we have the handshakes that's something we verify there.
> >
> 
> I understand that part is not in the best shape, but we must document
> the current state. There's no problem changing this later.
> 
> Besides, there's the whole "the migration stream should be considered
> hostile" which might mean we should really be keeping these sanity check
> flags around in case something really weird happens so we don't carry on
> with a bad stream.

Yep, it's OK.

> 
> >> +     *
> >> +     * As a last step, if packets are in use (default), must prepare
> >> +     * the packet by calling multifd_send_fill_packet().
> >> +     */
> >>      int (*send_prepare)(MultiFDSendParams *p, Error **errp);
> >> -    /* Setup for receiving side */
> >> +
> >> +    /*
> >> +     * The recv_setup, recv_cleanup, recv are only called on the QEMU
> >> +     * instance at the migration destination.
> >> +     */
> >> +
> >> +    /*
> >> +     * Setup for receiving side. Called once per channel during
> >> +     * channel setup phase. May be empty.
> >> +     *
> >> +     * May allocate data structures for the receiving of data. May use
> >> +     * p->iov. Compression methods may use p->compress_data.
> >> +     */
> >>      int (*recv_setup)(MultiFDRecvParams *p, Error **errp);
> >> -    /* Cleanup for receiving side */
> >> +
> >> +    /*
> >> +     * Cleanup for receiving side. Called once per channel during
> >> +     * channel cleanup phase. May be empty.
> >> +     */
> >>      void (*recv_cleanup)(MultiFDRecvParams *p);
> >> -    /* Read all data */
> >> +
> >> +    /*
> >> +     * Data receive method. Called from multifd_recv(), with p
> >
> > multifd_recv_thread()?
> 
> Same as before. I'll reword this somehow.
> 
> >
> >> +     * pointing to the MultiFDRecvParams of a channel that is
> >> +     * currently idle. Only called if there is data available to
> >> +     * receive.
> >> +     *
> >> +     * Must validate p->flags according to what was set at
> >> +     * send_prepare.
> >> +     *
> >> +     * Must read the data from the QIOChannel p->c.
> >> +     */
> >>      int (*recv)(MultiFDRecvParams *p, Error **errp);
> >>  } MultiFDMethods;
> >>  
> >> -- 
> >> 2.35.3
> >> 
> 

-- 
Peter Xu