[PATCH v3 10/12] migration: Make dirty_bytes_last_sync atomic

[Juan Quintela <quintela@redhat.com>]

As we set its value, it needs to be operated with atomics.
We rename it from remaining to better reflect its meaning.

Statistics always return the real reamaining bytes.  This was used to
store how much pages where dirty on the previous generation, so we can
calculate the expected downtime as: dirty_bytes_last_sync /
current_bandwith.

If we use the actual remaining bytes, we would see a very small value
at the end of the iteration.

Signed-off-by: Juan Quintela <quintela@redhat.com>

---

I am open to use ram_bytes_remaining() in its only use and be more
"optimistic" about the downtime.
---
 migration/ram.h       | 2 +-
 migration/migration.c | 4 +++-
 migration/ram.c       | 3 ++-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/migration/ram.h b/migration/ram.h
index 574a604b72..8093ebc210 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -41,6 +41,7 @@
  * one thread).
  */
 typedef struct {
+    aligned_uint64_t dirty_bytes_last_sync;
     aligned_uint64_t dirty_pages_rate;
     Stat64 dirty_sync_count;
     Stat64 dirty_sync_missed_zero_copy;
@@ -51,7 +52,6 @@ typedef struct {
     Stat64 postcopy_bytes;
     Stat64 postcopy_requests;
     Stat64 precopy_bytes;
-    int64_t remaining;
     Stat64 transferred;
 } RAMStats;
 
diff --git a/migration/migration.c b/migration/migration.c
index 50eae2fbcd..83d3bfbf62 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3847,7 +3847,9 @@ static void migration_update_counters(MigrationState *s,
      */
     if (qatomic_read__nocheck(&ram_counters.dirty_pages_rate) &&
         transferred > 10000) {
-        s->expected_downtime = ram_counters.remaining / bandwidth;
+        s->expected_downtime =
+            qatomic_read__nocheck(&ram_counters.dirty_bytes_last_sync) /
+                                  bandwidth;
     }
 
     qemu_file_reset_rate_limit(s->to_dst_file);
diff --git a/migration/ram.c b/migration/ram.c
index 55fabec1fe..771596d377 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1224,7 +1224,8 @@ static void migration_bitmap_sync(RAMState *rs)
         RAMBLOCK_FOREACH_NOT_IGNORED(block) {
             ramblock_sync_dirty_bitmap(rs, block);
         }
-        ram_counters.remaining = ram_bytes_remaining();
+        qatomic_set__nocheck(&ram_counters.dirty_bytes_last_sync,
+                             ram_bytes_remaining());
     }
     qemu_mutex_unlock(&rs->bitmap_mutex);
 
-- 
2.39.2