Re: [PATCH v5 19/23] migration/multifd: Prepare multifd sync for mapped-ram migration

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 28, 2024 at 12:21:23PM -0300, Fabiano Rosas wrote:
> The mapped-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 mapped-ram we want to maintain the sync on the sending side
>   because that provides ordering between the rounds of dirty pages when
>   migrating live.

IIUC as I used to comment, we should probably only need that sync after
each full iteration, which is find_dirty_block().

I think keeping the setup/complete sync is fine, and that can be discussed
separately.  However IMHO we should still avoid the sync in
ram_save_iterate() always, or on new qemu + old machine types (where
flush_after_each_section=true) fixed-ram could suffer perf issues, IIUC.

So I assume at a minimum below would still be preferred?

@@ -3257,7 +3257,8 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
 out:
     if (ret >= 0
         && migration_is_setup_or_active(migrate_get_current()->state)) {
-        if (migrate_multifd() && migrate_multifd_flush_after_each_section()) {
+        if (migrate_multifd() && migrate_multifd_flush_after_each_section() &&
+            !migrate_mapped_ram()) {
             ret = multifd_send_sync_main();
             if (ret < 0) {
                 return ret;

> 
> 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 mapped-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 mapped-ram we only need to synchronize the channels at the end of
>   migration to avoid doing cleanup before the channels have finished
>   their IO.

Did you forget to add the sync at parse_ramblocks() for mapped-ram?

> 
> 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 can cause a sync to
> happen at EOS. Since the EOS flag is needed on the stream, allow
> mapped-ram to just ignore it.

Skipping EOS makes sense, but I suggest do that without invalid_flags.  See
below.

> 
> Also emit an error if any other unexpected flags are found on the
> stream.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
> - skipped all FLUSH flags
> - added invalid flags
> - skipped EOS
> ---
>  migration/ram.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 18620784c6..250dcd110c 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1368,8 +1368,11 @@ 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_mapped_ram()) {
> +                    qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> +                    qemu_fflush(f);
> +                }
>              }
>              /*
>               * If memory migration starts over, we will meet a dirtied page
> @@ -3111,7 +3114,8 @@ 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_mapped_ram()) {
>          qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>      }
>  
> @@ -3334,7 +3338,8 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>          }
>      }
>  
> -    if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
> +    if (migrate_multifd() && !migrate_multifd_flush_after_each_section() &&
> +        !migrate_mapped_ram()) {
>          qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>      }
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
> @@ -4137,6 +4142,12 @@ static int ram_load_precopy(QEMUFile *f)
>          invalid_flags |= RAM_SAVE_FLAG_COMPRESS_PAGE;
>      }
>  
> +    if (migrate_mapped_ram()) {
> +        invalid_flags |= (RAM_SAVE_FLAG_EOS | RAM_SAVE_FLAG_HOOK |
> +                          RAM_SAVE_FLAG_MULTIFD_FLUSH | RAM_SAVE_FLAG_PAGE |
> +                          RAM_SAVE_FLAG_XBZRLE | RAM_SAVE_FLAG_ZERO);

IMHO EOS cannot be accounted as "invalid" here because it always exists.
Rather than this trick (then explicitly ignore it below... which is even
hackier, IMHO), we can avoid setting EOS in invalid_flags, but explicitly
ignore EOS in below code to bypass it for mapped-ram:

@@ -4301,7 +4302,12 @@ static int ram_load_precopy(QEMUFile *f)
         case RAM_SAVE_FLAG_EOS:
             /* normal exit */
             if (migrate_multifd() &&
-                migrate_multifd_flush_after_each_section()) {
+                migrate_multifd_flush_after_each_section() &&
+                /*
+                 * Mapped-ram migration flushes once and for all after
+                 * parsing ramblocks.  Always ignore EOS for it.
+                 */
+                !migrate_mapped_ram()) {
                 multifd_recv_sync_main();
             }
             break;

> +    }
> +
>      while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
>          ram_addr_t addr;
>          void *host = NULL, *host_bak = NULL;
> @@ -4158,6 +4169,13 @@ static int ram_load_precopy(QEMUFile *f)
>          addr &= TARGET_PAGE_MASK;
>  
>          if (flags & invalid_flags) {
> +            if (invalid_flags & RAM_SAVE_FLAG_EOS) {
> +                /* EOS is always present, just ignore it */
> +                continue;
> +            }
> +
> +            error_report("Unexpected RAM flags: %d", flags & invalid_flags);
> +
>              if (flags & invalid_flags & RAM_SAVE_FLAG_COMPRESS_PAGE) {
>                  error_report("Received an unexpected compressed page");
>              }
> -- 
> 2.35.3
> 

-- 
Peter Xu