Re: [Qemu-devel] [PATCH v3 3/3] migration: use the free page hint feature from balloon

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wei Wang (wei.w.wang@intel.com) wrote:
> Start the free page optimization when the bulk stage starts. In case the
> guest is slow in reporting, actively stops it when the bulk stage ends.
> The optimization avoids sending guest free pages during the bulk stage.
> Currently, the optimization is added to precopy only.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Michael S. Tsirkin <mst@redhat.com>

I think this is OK, but with the only problem being that postcopy will
break;  we need to disable the mechanism if postcopy is enabled.

Dave

> ---
>  migration/ram.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 769a0f6..f6af7e6 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -51,6 +51,7 @@
>  #include "qemu/rcu_queue.h"
>  #include "migration/colo.h"
>  #include "migration/block.h"
> +#include "sysemu/balloon.h"
>  
>  /***********************************************************/
>  /* ram save/restore */
> @@ -208,6 +209,8 @@ struct RAMState {
>      uint32_t last_version;
>      /* We are in the first round */
>      bool ram_bulk_stage;
> +    /* The free pages optimization feature is supported */
> +    bool free_page_support;
>      /* How many times we have dirty too many pages */
>      int dirty_rate_high_cnt;
>      /* these variables are used for bitmap sync */
> @@ -775,7 +778,7 @@ unsigned long migration_bitmap_find_dirty(RAMState *rs, RAMBlock *rb,
>      unsigned long *bitmap = rb->bmap;
>      unsigned long next;
>  
> -    if (rs->ram_bulk_stage && start > 0) {
> +    if (rs->ram_bulk_stage && start > 0 && !rs->free_page_support) {
>          next = start + 1;
>      } else {
>          next = find_next_bit(bitmap, size, start);
> @@ -1225,6 +1228,10 @@ static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss, bool *again)
>              /* Flag that we've looped */
>              pss->complete_round = true;
>              rs->ram_bulk_stage = false;
> +            if (rs->free_page_support) {
> +                balloon_free_page_stop();
> +                rs->free_page_support = false;
> +            }
>              if (migrate_use_xbzrle()) {
>                  /* If xbzrle is on, stop using the data compression at this
>                   * point. In theory, xbzrle can do better than compression.
> @@ -1656,6 +1663,8 @@ static void ram_state_reset(RAMState *rs)
>      rs->last_page = 0;
>      rs->last_version = ram_list.version;
>      rs->ram_bulk_stage = true;
> +    rs->free_page_support = balloon_free_page_support() &
> +                            !migration_in_postcopy();
>  }
>  
>  #define MAX_WAIT 50 /* ms, half buffered_file limit */
> @@ -2235,6 +2244,11 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>              return -1;
>          }
>      }
> +
> +    if ((*rsp)->free_page_support) {
> +        balloon_free_page_start();
> +    }
> +
>      (*rsp)->f = f;
>  
>      rcu_read_lock();
> @@ -2329,6 +2343,9 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>  
>      ret = qemu_file_get_error(f);
>      if (ret < 0) {
> +        if (rs->ram_bulk_stage && rs->free_page_support) {
> +                balloon_free_page_stop();
> +        }
>          return ret;
>      }
>  
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK