Re: [PATCH v9 6/7] multifd: Send header packet without flags if zero-copy-send is enabled

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Apr 25, 2022 at 06:50:55PM -0300, Leonardo Bras wrote:
> Since d48c3a0445 ("multifd: Use a single writev on the send side"),
> sending the header packet and the memory pages happens in the same
> writev, which can potentially make the migration faster.
> 
> Using channel-socket as example, this works well with the default copying
> mechanism of sendmsg(), but with zero-copy-send=true, it will cause
> the migration to often break.
> 
> This happens because the header packet buffer gets reused quite often,
> and there is a high chance that by the time the MSG_ZEROCOPY mechanism get
> to send the buffer, it has already changed, sending the wrong data and
> causing the migration to abort.
> 
> It means that, as it is, the buffer for the header packet is not suitable
> for sending with MSG_ZEROCOPY.
> 
> In order to enable zero copy for multifd, send the header packet on an
> individual write(), without any flags, and the remanining pages with a
> writev(), as it was happening before. This only changes how a migration
> with zero-copy-send=true works, not changing any current behavior for
> migrations with zero-copy-send=false.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/multifd.c | 29 ++++++++++++++++++++++++++---
>  1 file changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 15fb668e64..6c940aaa98 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -639,6 +639,8 @@ static void *multifd_send_thread(void *opaque)
>          if (p->pending_job) {
>              uint64_t packet_num = p->packet_num;
>              uint32_t flags = p->flags;
> +            int iov_offset = 0;
> +

No need for this if you change:

>              p->iovs_num = 1;

   if (!migrate_use_zero_copy_send()) {
      p->iovs_num = 1;
   }


>              p->normal_num = 0;
>  
> @@ -665,15 +667,36 @@ static void *multifd_send_thread(void *opaque)
>              trace_multifd_send(p->id, packet_num, p->normal_num, flags,
>                                 p->next_packet_size);
>  
> -            p->iov[0].iov_len = p->packet_len;
> -            p->iov[0].iov_base = p->packet;
> +            if (migrate_use_zero_copy_send()) {
> +                /* Send header without zerocopy */
> +                ret = qio_channel_write_all(p->c, (void *)p->packet,
> +                                            p->packet_len, &local_err);
> +                if (ret != 0) {
> +                    break;
> +                }
> +
> +                if (!p->normal_num) {
> +                    /* No pages will be sent */
> +                    goto skip_send;
> +                }

Don't need this AFAIK, because the qio_channel_writev_all
call will be a no-op if  iovs_num is zero

>  
> -            ret = qio_channel_writev_all(p->c, p->iov, p->iovs_num,
> +                /* Skip first iov : header */
> +                iov_offset = 1;

Don't need to set this

> +            } else {
> +                /* Send header using the same writev call */
> +                p->iov[0].iov_len = p->packet_len;
> +                p->iov[0].iov_base = p->packet;
> +            }
> +
> +            ret = qio_channel_writev_all(p->c, p->iov + iov_offset,
> +                                         p->iovs_num - iov_offset,
>                                           &local_err);

This wouldn't need changing if we don't reserve iovs[0] when
not required.

> +
>              if (ret != 0) {
>                  break;
>              }
>  
> +skip_send:
>              qemu_mutex_lock(&p->mutex);
>              p->pending_job--;
>              qemu_mutex_unlock(&p->mutex);
> -- 
> 2.36.0
> 

With regards,
Daniel
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