Re: [RFC PATCH v2 8/9] migration/multifd: Don't send ram data during SYNC

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 22, 2024 at 02:59:13PM -0300, Fabiano Rosas wrote:
> Skip saving and loading any ram data in the packet in the case of a
> SYNC. This fixes a shortcoming of the current code which requires a
> reset of the MultiFDPages_t fields right after the previous
> pending_job finishes, otherwise the very next job might be a SYNC and
> multifd_send_fill_packet() will put the stale values in the packet.
> 
> By not calling multifd_ram_fill_packet(), we can stop resetting
> MultiFDPages_t in the multifd core and leave that to the client code.
> 
> Actually moving the reset function is not yet done because
> pages->num==0 is used by the client code to determine whether the
> MultiFDPages_t needs to be flushed. The subsequent patches will
> replace that with a generic flag that is not dependent on
> MultiFDPages_t.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  migration/multifd.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index d25b8658b2..4394ca6ade 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -438,6 +438,7 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
>  {
>      MultiFDPacket_t *packet = p->packet;
>      uint64_t packet_num;
> +    bool sync_packet = p->flags & MULTIFD_FLAG_SYNC;
>  
>      memset(packet, 0, p->packet_len);
>  
> @@ -452,7 +453,9 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
>  
>      p->packets_sent++;
>  
> -    multifd_ram_fill_packet(p);
> +    if (!sync_packet) {
> +        multifd_ram_fill_packet(p);
> +    }
>  
>      trace_multifd_send(p->id, packet_num,
>                         be32_to_cpu(packet->normal_pages),
> @@ -563,7 +566,12 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
>      p->packet_num = be64_to_cpu(packet->packet_num);
>      p->packets_recved++;
>  
> -    ret = multifd_ram_unfill_packet(p, errp);
> +    if (p->flags & MULTIFD_FLAG_SYNC) {
> +        p->normal_num = 0;
> +        p->zero_num = 0;

Instead of this, I wonder whether we shouldn't touch those fields at all,
but:

diff --git a/migration/multifd.c b/migration/multifd.c
index 0a85951d58..55abd9a1ef 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -1547,7 +1547,9 @@ static void *multifd_recv_thread(void *opaque)
             flags = p->flags;
             /* recv methods don't know how to handle the SYNC flag */
             p->flags &= ~MULTIFD_FLAG_SYNC;
-            has_data = p->normal_num || p->zero_num;
+
+            if (!(flags & MULTIFD_FLAG_SYNC))
+                has_data = p->normal_num || p->zero_num;
             qemu_mutex_unlock(&p->mutex);
         } else {
             /*

> +    } else {
> +        ret = multifd_ram_unfill_packet(p, errp);
> +    }
>  
>      trace_multifd_recv(p->id, p->packet_num, p->normal_num, p->zero_num,
>                         p->flags, p->next_packet_size);
> -- 
> 2.35.3
> 

-- 
Peter Xu