Re: [PATCH v6 19/23] migration/multifd: Prepare multifd sync for mapped-ram migration

[Peter Xu]

On Thu, Feb 29, 2024 at 12:30:13PM -0300, Fabiano Rosas wrote:
> The mapped-ram migration can be performed live or non-live, but it is
> always asynchronous, i.e. the source machine and the destination
> machine are not migrating at the same time. We only need some pieces
> of the multifd sync operations.
> 
> multifd_send_sync_main()
> ------------------------
>   Issued by the ram migration code on the migration thread, causes the
>   multifd send channels to synchronize with the migration thread and
>   makes the sending side emit a packet with the MULTIFD_FLUSH flag.
> 
>   With mapped-ram we want to maintain the sync on the sending side
>   because that provides ordering between the rounds of dirty pages when
>   migrating live.
> 
> MULTIFD_FLUSH
> -------------
>   On the receiving side, the presence of the MULTIFD_FLUSH flag on a
>   packet causes the receiving channels to start synchronizing with the
>   main thread.
> 
>   We're not using packets with mapped-ram, so there's no MULTIFD_FLUSH
>   flag and therefore no channel sync on the receiving side.
> 
> multifd_recv_sync_main()
> ------------------------
>   Issued by the migration thread when the ram migration flag
>   RAM_SAVE_FLAG_MULTIFD_FLUSH is received, causes the migration thread
>   on the receiving side to start synchronizing with the recv
>   channels. Due to compatibility, this is also issued when
>   RAM_SAVE_FLAG_EOS is received.
> 
>   For mapped-ram we only need to synchronize the channels at the end of
>   migration to avoid doing cleanup before the channels have finished
>   their IO.
> 
> Make sure the multifd syncs are only issued at the appropriate times.
> 
> Note that due to pre-existing backward compatibility issues, we have
> the multifd_flush_after_each_section property that can cause a sync to
> happen at EOS. Since the EOS flag is needed on the stream, allow
> mapped-ram to just ignore it.
> 
> Also emit an error if any other unexpected flags are found on the
> stream.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu