[PATCH RFC 10/12] migration: Introduce a helper to return switchover bw estimate

[Peter Xu <peterx@redhat.com>]

Add a helper to be able to return an estimate of switchover bw, called
migration_get_switchover_bw(). Use it to sligitly simplify the current
code.

This will be used in new codes later to remove expected_downtime.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.h |  1 +
 migration/migration.c | 43 ++++++++++++++++++++++---------------------
 2 files changed, 23 insertions(+), 21 deletions(-)

diff --git a/migration/migration.h b/migration/migration.h
index b6888daced..bf3ee6cc07 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -586,6 +586,7 @@ void migration_cancel(void);
 void migration_populate_vfio_info(MigrationInfo *info);
 void migration_reset_vfio_bytes_transferred(void);
 void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page);
+int64_t migration_downtime_calc_expected(MigrationState *s);
 
 /*
  * Migration thread waiting for return path thread.  Return non-zero if an
diff --git a/migration/migration.c b/migration/migration.c
index ad8a824585..56d605ede9 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -984,6 +984,21 @@ void migrate_send_rp_resume_ack(MigrationIncomingState *mis, uint32_t value)
     migrate_send_rp_message(mis, MIG_RP_MSG_RESUME_ACK, sizeof(buf), &buf);
 }
 
+/*
+ * Returns the estimated switchover bandwidth (unit: bytes / seconds)
+ */
+static double migration_get_switchover_bw(MigrationState *s)
+{
+    uint64_t switchover_bw = migrate_avail_switchover_bandwidth();
+
+    if (switchover_bw) {
+        /* If user specified, prioritize this value and don't estimate */
+        return (double)switchover_bw;
+    }
+
+    return s->mbps / 8 * 1000 * 1000;
+}
+
 bool migration_is_running(void)
 {
     MigrationState *s = current_migration;
@@ -3126,37 +3141,22 @@ static void migration_update_counters(MigrationState *s,
 {
     uint64_t transferred, transferred_pages, time_spent;
     uint64_t current_bytes; /* bytes transferred since the beginning */
-    uint64_t switchover_bw;
-    /* Expected bandwidth when switching over to destination QEMU */
-    double expected_bw_per_ms;
-    double bandwidth;
+    double switchover_bw;
 
     if (current_time < s->iteration_start_time + BUFFER_DELAY) {
         return;
     }
 
-    switchover_bw = migrate_avail_switchover_bandwidth();
     current_bytes = migration_transferred_bytes();
     transferred = current_bytes - s->iteration_initial_bytes;
     time_spent = current_time - s->iteration_start_time;
-    bandwidth = (double)transferred / time_spent;
-
-    if (switchover_bw) {
-        /*
-         * If the user specified a switchover bandwidth, let's trust the
-         * user so that can be more accurate than what we estimated.
-         */
-        expected_bw_per_ms = (double)switchover_bw / 1000;
-    } else {
-        /* If the user doesn't specify bandwidth, we use the estimated */
-        expected_bw_per_ms = bandwidth;
-    }
-
-    s->threshold_size = expected_bw_per_ms * migrate_downtime_limit();
-
     s->mbps = (((double) transferred * 8.0) /
                ((double) time_spent / 1000.0)) / 1000.0 / 1000.0;
 
+    /* NOTE: only update this after bandwidth (s->mbps) updated */
+    switchover_bw = migration_get_switchover_bw(s) / 1000;
+    s->threshold_size = switchover_bw * migrate_downtime_limit();
+
     transferred_pages = ram_get_total_transferred_pages() -
                             s->iteration_initial_pages;
     s->pages_per_second = (double) transferred_pages /
@@ -3178,7 +3178,8 @@ static void migration_update_counters(MigrationState *s,
 
     trace_migrate_transferred(transferred, time_spent,
                               /* Both in unit bytes/ms */
-                              bandwidth, switchover_bw / 1000,
+                              (uint64_t)s->mbps,
+                              (uint64_t)switchover_bw,
                               s->threshold_size);
 }
 
-- 
2.50.1