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

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> 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?

They have their own send_cleanup function where p->iov is freed.