Re: [PATCH v4 22/34] migration/multifd: Prepare multifd sync for fixed-ram migration

[Peter Xu <peterx@redhat.com>]

On Tue, Feb 20, 2024 at 07:41:26PM -0300, Fabiano Rosas wrote:
> The fixed-ram migration can be performed live or non-live, but it is
> always asynchronous, i.e. the source machine and the destination
> machine are not migrating at the same time. We only need some pieces
> of the multifd sync operations.
> 
> multifd_send_sync_main()
> ------------------------
>   Issued by the ram migration code on the migration thread, causes the
>   multifd send channels to synchronize with the migration thread and
>   makes the sending side emit a packet with the MULTIFD_FLUSH flag.
> 
>   With fixed-ram we want to maintain the sync on the sending side
>   because that provides ordering between the rounds of dirty pages when
>   migrating live.
> 
> MULTIFD_FLUSH
> -------------
>   On the receiving side, the presence of the MULTIFD_FLUSH flag on a
>   packet causes the receiving channels to start synchronizing with the
>   main thread.
> 
>   We're not using packets with fixed-ram, so there's no MULTIFD_FLUSH
>   flag and therefore no channel sync on the receiving side.
> 
> multifd_recv_sync_main()
> ------------------------
>   Issued by the migration thread when the ram migration flag
>   RAM_SAVE_FLAG_MULTIFD_FLUSH is received, causes the migration thread
>   on the receiving side to start synchronizing with the recv
>   channels. Due to compatibility, this is also issued when
>   RAM_SAVE_FLAG_EOS is received.
> 
>   For fixed-ram we only need to synchronize the channels at the end of
>   migration to avoid doing cleanup before the channels have finished
>   their IO.
> 
> Make sure the multifd syncs are only issued at the appropriate
> times. Note that due to pre-existing backward compatibility issues, we
> have the multifd_flush_after_each_section property that enables an
> older behavior of synchronizing channels more frequently (and
> inefficiently). Fixed-ram should always run with that property
> disabled (default).

What if the user enables multifd_flush_after_each_section=true?

IMHO we don't necessarily need to attach the fixed-ram loading flush to any
flag in the stream.  For fixed-ram IIUC all the loads will happen in one
shot of ram_load() anyway when parsing the ramblock list, so.. how about we
decouple the fixed-ram load flush from the stream by always do a sync in
ram_load() unconditionally?

@@ -4368,6 +4367,15 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
             ret = ram_load_precopy(f);
         }
     }
+
+    /*
+     * Fixed-ram migration may queue load tasks to multifd threads; make
+     * sure they're all done.
+     */
+    if (migrate_fixed_ram() && migrate_multifd()) {
+        multifd_recv_sync_main();
+    }
+
     trace_ram_load_complete(ret, seq_iter);
 
     return ret;

Then ram_load() always guarantees synchronous loading of pages, and
fixed-ram will completely ignore multifd flushes (then we also skip it for
the ram_save_complete() like what this patch does for the rest).

> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  migration/ram.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 5932e1b8e1..c7050f6f68 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1369,8 +1369,11 @@ static int find_dirty_block(RAMState *rs, PageSearchStatus *pss)
>                  if (ret < 0) {
>                      return ret;
>                  }
> -                qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> -                qemu_fflush(f);
> +
> +                if (!migrate_fixed_ram()) {
> +                    qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> +                    qemu_fflush(f);
> +                }
>              }
>              /*
>               * If memory migration starts over, we will meet a dirtied page
> @@ -3112,7 +3115,8 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>          return ret;
>      }
>  
> -    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
> +    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()
> +        && !migrate_fixed_ram()) {
>          qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>      }
>  
> @@ -4253,6 +4257,15 @@ static int ram_load_precopy(QEMUFile *f)
>              break;
>          case RAM_SAVE_FLAG_EOS:
>              /* normal exit */
> +            if (migrate_fixed_ram()) {
> +                /*
> +                 * The EOS flag appears multiple times on the
> +                 * stream. Fixed-ram needs only one sync at the
> +                 * end. It will be done on the flush flag above.
> +                 */
> +                break;
> +            }
> +
>              if (migrate_multifd() &&
>                  migrate_multifd_flush_after_each_section()) {
>                  multifd_recv_sync_main();
> -- 
> 2.35.3
> 

-- 
Peter Xu