Re: [PATCH v10 6/7] multifd: Send header packet without flags if zero-copy-send is enabled

[Peter Xu]

On Tue, Apr 26, 2022 at 08:06:55PM -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 | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 15fb668e64..07b2e92d8d 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -617,6 +617,7 @@ static void *multifd_send_thread(void *opaque)
>      MultiFDSendParams *p = opaque;
>      Error *local_err = NULL;
>      int ret = 0;
> +    bool use_zero_copy_send = migrate_use_zero_copy_send();
>  
>      trace_multifd_send_thread_start(p->id);
>      rcu_register_thread();
> @@ -639,9 +640,14 @@ static void *multifd_send_thread(void *opaque)
>          if (p->pending_job) {
>              uint64_t packet_num = p->packet_num;
>              uint32_t flags = p->flags;
> -            p->iovs_num = 1;
>              p->normal_num = 0;
>  
> +            if (use_zero_copy_send) {
> +                p->iovs_num = 0;
> +            } else {
> +                p->iovs_num = 1;
> +            }
> +
>              for (int i = 0; i < p->pages->num; i++) {
>                  p->normal[p->normal_num] = p->pages->offset[i];
>                  p->normal_num++;
> @@ -665,8 +671,19 @@ 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 (use_zero_copy_send) {
> +                /* Send header first, without zerocopy */
> +                ret = qio_channel_write_all(p->c, (void *)p->packet,
> +                                            p->packet_len, &local_err);
> +                if (ret != 0) {
> +                    break;
> +                }
> +

Extra but useless newline.. but not worth a repost.  Looks good here:

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

Thanks,

> +            } 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, p->iovs_num,
>                                           &local_err);
> -- 
> 2.36.0
> 

-- 
Peter Xu