Re: [PATCH] migration/multifd: Remove redundant copy of page offsets during send

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

Copying in the two migration maintainers.

On Wed, Aug 10, 2022 at 01:39:42PM +0300, Nikolay Borisov wrote:
> All pages which are going to be migrated are first added to
> MultiFDSendParams::MultiFDPages_t::offset array by the main migration
> thread and are subsequently copied to MultiFDSendParams::normal by the
> multifd thread. This is really unnecessary as the MultiFDPages_t is
> guaranteed to be stable since its mutex is being held. Additionally,
> this somewhat simplifies the code as the migration pages are now kept
> in only 1 place during send, also the name 'offset' coupled with the
> context it's being used - usually added to the host pages makes it
> obvious that this is an offset.
> 
> With this change normal/normal_num are no longer used in the multifd
> send path.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  migration/multifd-zlib.c |  6 +++---
>  migration/multifd-zstd.c |  6 +++---
>  migration/multifd.c      | 25 ++++++++++---------------
>  3 files changed, 16 insertions(+), 21 deletions(-)
> 
> diff --git a/migration/multifd-zlib.c b/migration/multifd-zlib.c
> index 18213a951302..363b64e95922 100644
> --- a/migration/multifd-zlib.c
> +++ b/migration/multifd-zlib.c
> @@ -122,11 +122,11 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
>      int ret;
>      uint32_t i;
> 
> -    for (i = 0; i < p->normal_num; i++) {
> +    for (i = 0; i < p->pages->num; i++) {
>          uint32_t available = z->zbuff_len - out_size;
>          int flush = Z_NO_FLUSH;
> 
> -        if (i == p->normal_num - 1) {
> +        if (i == p->pages->num - 1) {
>              flush = Z_SYNC_FLUSH;
>          }
> 
> @@ -135,7 +135,7 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
>           * with compression. zlib does not guarantee that this is safe,
>           * therefore copy the page before calling deflate().
>           */
> -        memcpy(z->buf, p->pages->block->host + p->normal[i], page_size);
> +        memcpy(z->buf, p->pages->block->host + p->pages->offset[i], page_size);
>          zs->avail_in = page_size;
>          zs->next_in = z->buf;
> 
> diff --git a/migration/multifd-zstd.c b/migration/multifd-zstd.c
> index d788d309f22e..4daec8366727 100644
> --- a/migration/multifd-zstd.c
> +++ b/migration/multifd-zstd.c
> @@ -121,13 +121,13 @@ static int zstd_send_prepare(MultiFDSendParams *p, Error **errp)
>      z->out.size = z->zbuff_len;
>      z->out.pos = 0;
> 
> -    for (i = 0; i < p->normal_num; i++) {
> +    for (i = 0; i < p->pages->num; i++) {
>          ZSTD_EndDirective flush = ZSTD_e_continue;
> 
> -        if (i == p->normal_num - 1) {
> +        if (i == p->pages->num - 1) {
>              flush = ZSTD_e_flush;
>          }
> -        z->in.src = p->pages->block->host + p->normal[i];
> +        z->in.src = p->pages->block->host + p->pages->offset[i];
>          z->in.size = page_size;
>          z->in.pos = 0;
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 586ddc9d657a..d70662406490 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -89,13 +89,13 @@ static int nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
>      MultiFDPages_t *pages = p->pages;
>      size_t page_size = qemu_target_page_size();
> 
> -    for (int i = 0; i < p->normal_num; i++) {
> -        p->iov[p->iovs_num].iov_base = pages->block->host + p->normal[i];
> +    for (int i = 0; i < pages->num; i++) {
> +        p->iov[p->iovs_num].iov_base = pages->block->host + pages->offset[i];
>          p->iov[p->iovs_num].iov_len = page_size;
>          p->iovs_num++;
>      }
> 
> -    p->next_packet_size = p->normal_num * page_size;
> +    p->next_packet_size = pages->num * page_size;
>      p->flags |= MULTIFD_FLAG_NOCOMP;
>      return 0;
>  }
> @@ -262,7 +262,7 @@ static void multifd_send_fill_packet(MultiFDSendParams *p)
> 
>      packet->flags = cpu_to_be32(p->flags);
>      packet->pages_alloc = cpu_to_be32(p->pages->allocated);
> -    packet->normal_pages = cpu_to_be32(p->normal_num);
> +    packet->normal_pages = cpu_to_be32(p->pages->num);
>      packet->next_packet_size = cpu_to_be32(p->next_packet_size);
>      packet->packet_num = cpu_to_be64(p->packet_num);
> 
> @@ -270,9 +270,9 @@ static void multifd_send_fill_packet(MultiFDSendParams *p)
>          strncpy(packet->ramblock, p->pages->block->idstr, 256);
>      }
> 
> -    for (i = 0; i < p->normal_num; i++) {
> +    for (i = 0; i < p->pages->num; i++) {
>          /* there are architectures where ram_addr_t is 32 bit */
> -        uint64_t temp = p->normal[i];
> +        uint64_t temp = p->pages->offset[i];
> 
>          packet->offset[i] = cpu_to_be64(temp);
>      }
> @@ -668,7 +668,7 @@ static void *multifd_send_thread(void *opaque)
>          if (p->pending_job) {
>              uint64_t packet_num = p->packet_num;
>              uint32_t flags = p->flags;
> -            p->normal_num = 0;
> +            uint32_t num_pages = p->pages->num;
> 
>              if (use_zero_copy_send) {
>                  p->iovs_num = 0;
> @@ -676,12 +676,7 @@ static void *multifd_send_thread(void *opaque)
>                  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++;
> -            }
> -
> -            if (p->normal_num) {
> +            if (num_pages) {
>                  ret = multifd_send_state->ops->send_prepare(p, &local_err);
>                  if (ret != 0) {
>                      qemu_mutex_unlock(&p->mutex);
> @@ -691,12 +686,12 @@ static void *multifd_send_thread(void *opaque)
>              multifd_send_fill_packet(p);
>              p->flags = 0;
>              p->num_packets++;
> -            p->total_normal_pages += p->normal_num;
> +            p->total_normal_pages += num_pages;
>              p->pages->num = 0;
>              p->pages->block = NULL;
>              qemu_mutex_unlock(&p->mutex);
> 
> -            trace_multifd_send(p->id, packet_num, p->normal_num, flags,
> +            trace_multifd_send(p->id, packet_num, num_pages, flags,
>                                 p->next_packet_size);
> 
>              if (use_zero_copy_send) {
> --
> 2.25.1
> 
> 

With regards,
Daniel
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