Re: [Qemu-devel] [PATCH v4 4/4] migration: use the free page hint feature from balloon

[Wei Wang]

On 03/15/2018 03:49 AM, Dr. David Alan Gilbert wrote:
* Wei Wang (wei.w.w...@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.w...@intel.com>
CC: Dr. David Alan Gilbert <dgilb...@redhat.com>
CC: Juan Quintela <quint...@redhat.com>
CC: Michael S. Tsirkin <m...@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?


Yes, balloon_free_page_stop will cause the optimization thread to exit 
immediately. It doesn't rely on anything from the guest.
The guest won't keep reporting, since before the optimization thread 
exits, it sends a stop sign to the guest to stop reporting (but not 
waiting for any ACKs as that's not needed actually).

I also applied other comments in the new version, please have check v5 
patches. Thanks.

Best,
Wei