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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> 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:

I removed that.

> 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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK