Re: [PATCH v7 08/13] multifd: Prepare to send a packet without the mutex held

[Dr. David Alan Gilbert]

* Juan Quintela (quintela@redhat.com) wrote:
> We do the send_prepare() and the fill of the head packet without the
> mutex held.  It will help a lot for compression and later in the
> series for zero pages.
> 
> Notice that we can use p->pages without holding p->mutex because
> p->pending_job == 1.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/multifd.h |  2 ++
>  migration/multifd.c | 11 ++++++-----
>  2 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/migration/multifd.h b/migration/multifd.h
> index af8ce8921d..d48597a1ea 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -109,7 +109,9 @@ typedef struct {
>      /* array of pages to sent.
>       * The owner of 'pages' depends of 'pending_job' value:
>       * pending_job == 0 -> migration_thread can use it.
> +     *                     No need for mutex lock.
>       * pending_job != 0 -> multifd_channel can use it.
> +     *                     No need for mutex lock.
>       */
>      MultiFDPages_t *pages;
>  
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 69b9d7cf98..056599cbaf 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -661,6 +661,8 @@ static void *multifd_send_thread(void *opaque)
>                  p->flags |= MULTIFD_FLAG_SYNC;
>                  p->sync_needed = false;
>              }
> +            qemu_mutex_unlock(&p->mutex);
> +
>              p->normal_num = 0;
>  
>              if (use_zero_copy_send) {
> @@ -682,11 +684,6 @@ static void *multifd_send_thread(void *opaque)

Looking at my source tree, without the rest of your series, I see an
unlock just before this pair of }'s; doesn't that need to go?

Dave

>                  }
>              }
>              multifd_send_fill_packet(p);
> -            p->num_packets++;
> -            p->total_normal_pages += p->normal_num;
> -            p->pages->num = 0;
> -            p->pages->block = NULL;
> -            qemu_mutex_unlock(&p->mutex);
>  
>              trace_multifd_send(p->id, packet_num, p->normal_num, p->flags,
>                                 p->next_packet_size);
> @@ -711,6 +708,10 @@ static void *multifd_send_thread(void *opaque)
>              }
>  
>              qemu_mutex_lock(&p->mutex);
> +            p->num_packets++;
> +            p->total_normal_pages += p->normal_num;
> +            p->pages->num = 0;
> +            p->pages->block = NULL;
>              p->sent_bytes += p->packet_len;;
>              p->sent_bytes += p->next_packet_size;
>              p->pending_job--;
> -- 
> 2.35.3
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK