[PATCH v6 3/3] 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.
Use stat64_get() now that it exists.
---
 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 900dfb6855..cc09a88aac 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2754,7 +2754,8 @@ static void migration_update_counters(MigrationState *s,
      */
     if (stat64_get(&ram_counters.dirty_pages_rate) &&
         transferred > 10000) {
-        s->expected_downtime = ram_counters.remaining / bandwidth;
+        s->expected_downtime =
+            stat64_get(&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 fa94c99766..fcdcf78ec7 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();
+        stat64_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 f189cc79f8..04b05e1b2c 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -41,6 +41,7 @@
  * one thread).
  */
 typedef struct {
+    Stat64 dirty_bytes_last_sync;
     Stat64 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