Re: [Qemu-devel] [PATCH v4 4/4] 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 after the migration bitmap is
> synchronized. This can't be used in the stop&copy phase since the guest
> is paused. Make sure the guest reporting has stopped before
> synchronizing the migration dirty bitmap. 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>
> ---
>  migration/ram.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index e172798..7b4c9b1 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -51,6 +51,8 @@
>  #include "qemu/rcu_queue.h"
>  #include "migration/colo.h"
>  #include "migration/block.h"
> +#include "sysemu/balloon.h"
> +#include "sysemu/sysemu.h"
>  
>  /***********************************************************/
>  /* ram save/restore */
> @@ -208,6 +210,8 @@ 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 +779,7 @@ 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;

An easier thing is just to clear the ram_bulk_stage flag (and if you're
doing it in the middle of the migration you need to reset some of the
pointers; see postcopy_start for an example).

>      } else {
>          next = find_next_bit(bitmap, size, start);
> @@ -833,6 +837,10 @@ static void migration_bitmap_sync(RAMState *rs)
>      int64_t end_time;
>      uint64_t bytes_xfer_now;
>  
> +    if (rs->free_page_support) {
> +        balloon_free_page_stop();

Does balloon_free_page_stop cause it to immediately stop, or does it
just ask nicely?   Could a slow guest keep pumping advice to us even
when it was stopped?

> +    }
> +
>      ram_counters.dirty_sync_count++;
>  
>      if (!rs->time_last_bitmap_sync) {
> @@ -899,6 +907,10 @@ static void migration_bitmap_sync(RAMState *rs)
>      if (migrate_use_events()) {
>          qapi_event_send_migration_pass(ram_counters.dirty_sync_count, NULL);
>      }
> +
> +    if (rs->free_page_support && runstate_is_running()) {
> +        balloon_free_page_start();
> +    }
>  }
>  
>  /**
> @@ -1656,6 +1668,8 @@ 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();

That's probably the wrong test for postcopy; I think it'll always
be false there.  Using migrate_postcopy_ram() tells you whether
postcopy-ram is enabled; although not necessarily in use at that
point.

Dave

>  }
>  
>  #define MAX_WAIT 50 /* ms, half buffered_file limit */
> @@ -2330,6 +2344,9 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>  
>      ret = qemu_file_get_error(f);
>      if (ret < 0) {
> +        if (rs->free_page_support) {
> +            balloon_free_page_stop();
> +        }
>          return ret;
>      }
>  
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK