migration: Support periodic RAMBlock dirty bitmap sync

When VM is configured with huge memory, the current throttle logic
doesn't look like to scale, because migration_trigger_throttle()
is only called for each iteration, so it won't be invoked for a long
time if one iteration can take a long time.

The periodic dirty sync aims to fix the above issue by synchronizing
the ramblock from remote dirty bitmap and, when necessary, triggering
the CPU throttle multiple times during a long iteration.

This is a trade-off between synchronization overhead and CPU throttle
impact.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/f61f1b3653f2acf026901103e1c73d157d38b08f.1729146786.git.yong.huang@smartx.com
[peterx: make prev_cnt global, and reset for each migration]
Signed-off-by: Peter Xu <peterx@redhat.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/migration/migration.c b/migration/migration.c
index 64c1d0c1db..9bcc9db4fb 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3297,12 +3297,17 @@ static MigIterateState migration_iteration_run(MigrationState *s)
 
 static void migration_iteration_finish(MigrationState *s)
 {
-    /* If we enabled cpu throttling for auto-converge, turn it off. */
+    bql_lock();
+
+    /*
+     * If we enabled cpu throttling for auto-converge, turn it off.
+     * Stopping CPU throttle should be serialized by BQL to avoid
+     * racing for the throttle_dirty_sync_timer.
+     */
     if (migrate_auto_converge()) {
         cpu_throttle_stop();
     }
 
-    bql_lock();
     switch (s->state) {
     case MIGRATION_STATUS_COMPLETED:
         runstate_set(RUN_STATE_POSTMIGRATE);
@@ -3520,6 +3525,11 @@ static void *migration_thread(void *opaque)
         qemu_savevm_send_colo_enable(s->to_dst_file);
     }
 
+    if (migrate_auto_converge()) {
+        /* Start RAMBlock dirty bitmap sync timer */
+        cpu_throttle_dirty_sync_timer(true);
+    }
+
     bql_lock();
     ret = qemu_savevm_state_setup(s->to_dst_file, &local_err);
     bql_unlock();