Re: [PATCH v6 3/3] multifd: Only flush once each full round of memory

[Peter Xu]

On Wed, Feb 15, 2023 at 07:02:31PM +0100, Juan Quintela wrote:
> We need to add a new flag to mean to flush at that point.
> Notice that we still flush at the end of setup and at the end of
> complete stages.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>

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

One nitpick below.

> 
> ---
> 
> Add missing qemu_fflush(), now it passes all tests always.
> ---
>  qapi/migration.json   |  3 ---
>  migration/migration.c |  6 +-----
>  migration/ram.c       | 28 +++++++++++++++++++++++++++-
>  3 files changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 3afd81174d..34e1657c4e 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -491,9 +491,6 @@
>  #                                    suboptimal (we flush too many
>  #                                    times).
>  #                                    Default value is false.
> -#                                    Setting this capability has no
> -#                                    effect until the patch that
> -#                                    removes this comment.
>  #                                    (since 8.0)
>  #
>  # Features:
> diff --git a/migration/migration.c b/migration/migration.c
> index cfba0da005..74bcc16848 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2706,11 +2706,7 @@ bool migrate_multifd_flush_after_each_section(void)
>  {
>      MigrationState *s = migrate_get_current();
>  
> -    /*
> -     * Until the patch that remove this comment, we always return that
> -     * the capability is enabled.
> -     */
> -    return true || 
> s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_FLUSH_AFTER_EACH_SECTION];
> +    return 
> s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_FLUSH_AFTER_EACH_SECTION];
>  }
>  
>  bool migrate_pause_before_switchover(void)
> diff --git a/migration/ram.c b/migration/ram.c
> index 6191dac9af..bc5eb1640b 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -85,6 +85,7 @@
>  #define RAM_SAVE_FLAG_XBZRLE   0x40
>  /* 0x80 is reserved in qemu-file.h for RAM_SAVE_FLAG_HOOK */
>  #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
> +#define RAM_SAVE_FLAG_MULTIFD_FLUSH    0x200
>  /* We can't use any flag that is bigger than 0x200 */
>  
>  int (*xbzrle_encode_buffer_func)(uint8_t *, uint8_t *, int,
> @@ -1595,6 +1596,7 @@ retry:
>   * associated with the search process.
>   *
>   * Returns:
> + *         <0: An error happened
>   *         PAGE_ALL_CLEAN: no dirty page found, give up
>   *         PAGE_TRY_AGAIN: no dirty page found, retry for next block
>   *         PAGE_DIRTY_FOUND: dirty page found
> @@ -1622,6 +1624,15 @@ static int find_dirty_block(RAMState *rs, 
> PageSearchStatus *pss)
>          pss->page = 0;
>          pss->block = QLIST_NEXT_RCU(pss->block, next);
>          if (!pss->block) {
> +            if (!migrate_multifd_flush_after_each_section()) {
> +                QEMUFile *f = rs->pss[RAM_CHANNEL_PRECOPY].pss_channel;
> +                int ret = multifd_send_sync_main(f);
> +                if (ret < 0) {
> +                    return ret;
> +                }
> +                qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> +                qemu_fflush(f);
> +            }
>              /*
>               * If memory migration starts over, we will meet a dirtied page
>               * which may still exists in compression threads's ring, so we
> @@ -2614,6 +2625,9 @@ static int ram_find_and_save_block(RAMState *rs)
>                      break;
>                  } else if (res == PAGE_TRY_AGAIN) {
>                      continue;
> +                } else if (res < 0) {
> +                    pages = res;
> +                    break;
>                  }
>              }
>          }
> @@ -3300,6 +3314,10 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>          return ret;
>      }
>  
> +    if (!migrate_multifd_flush_after_each_section()) {
> +        qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> +    }
> +
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>      qemu_fflush(f);
>  
> @@ -3485,6 +3503,9 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>          return ret;
>      }
>  
> +    if (!migrate_multifd_flush_after_each_section()) {
> +        qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> +    }
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>      qemu_fflush(f);
>  
> @@ -4169,7 +4190,9 @@ int ram_load_postcopy(QEMUFile *f, int channel)
>              }
>              decompress_data_with_multi_threads(f, page_buffer, len);
>              break;
> -
> +        case RAM_SAVE_FLAG_MULTIFD_FLUSH:
> +            multifd_recv_sync_main();
> +            break;
>          case RAM_SAVE_FLAG_EOS:
>              /* normal exit */
>              if (migrate_multifd_flush_after_each_section()) {

We could have dropped RAM_SAVE_FLAG_MULTIFD_FLUSH and RAM_SAVE_FLAG_EOS for
now until we support postcopy+multifd.

Here it's not only about enabling them together, but it's about running
them in parallel, which I doubt whether it can really be implemented at all
due to the fundamentally concepts difference between multifd & postcopy.. :(

> @@ -4443,6 +4466,9 @@ static int ram_load_precopy(QEMUFile *f)
>                  break;
>              }
>              break;
> +        case RAM_SAVE_FLAG_MULTIFD_FLUSH:
> +            multifd_recv_sync_main();
> +            break;
>          case RAM_SAVE_FLAG_EOS:
>              /* normal exit */
>              if (migrate_multifd_flush_after_each_section()) {
> -- 
> 2.39.1
> 
> 

-- 
Peter Xu