[PATCH v5 2/2] 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>
Reviewed-by: Peter Xu <peterx@redhat.com>

---

I am open to use ram_bytes_remaining() in its only use and be more
"optimistic" about the downtime.

Don't use __nocheck() functions.
---
 migration/migration.c | 3 ++-
 migration/ram.c       | 2 +-
 migration/ram.h       | 2 +-
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 712f802962..c81c65bf28 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2754,7 +2754,8 @@ static void migration_update_counters(MigrationState *s,
      */
     if (qatomic_read(&ram_counters.dirty_pages_rate) &&
         transferred > 10000) {
-        s->expected_downtime = ram_counters.remaining / bandwidth;
+        s->expected_downtime =
+            qatomic_read(&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 7c534a41e0..704df661d1 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1223,7 +1223,7 @@ 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(&ram_counters.dirty_bytes_last_sync, ram_bytes_remaining());
     }
     qemu_mutex_unlock(&rs->bitmap_mutex);
 
diff --git a/migration/ram.h b/migration/ram.h
index 3db0a9d65c..11a0fde99b 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;
 
-- 
2.40.0