block/mirror: Sleep periodically during bitmap scanning

Before, we only yield after initializing dirty bitmap, where the QMP
command would return. That may take very long, and guest IO will be
blocked.

Add sleep points like the later mirror iterations.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1431486673-19280-1-git-send-email-famz@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>

block/mirror.c

diff --git a/block/mirror.c b/block/mirror.c
index d409337..a2700ca 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -444,11 +444,23 @@
     sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
     mirror_free_init(s);
 
+    last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     if (!s->is_none_mode) {
         /* First part, loop on the sectors and initialize the dirty bitmap.  */
         BlockDriverState *base = s->base;
         for (sector_num = 0; sector_num < end; ) {
             int64_t next = (sector_num | (sectors_per_chunk - 1)) + 1;
+            int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+
+            if (now - last_pause_ns > SLICE_TIME) {
+                last_pause_ns = now;
+                block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, 0);
+            }
+
+            if (block_job_is_cancelled(&s->common)) {
+                goto immediate_exit;
+            }
+
             ret = bdrv_is_allocated_above(bs, base,
                                           sector_num, next - sector_num, &n);
 
@@ -467,7 +479,6 @@
     }
 
     bdrv_dirty_iter_init(s->dirty_bitmap, &s->hbi);
-    last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     for (;;) {
         uint64_t delay_ns = 0;
         int64_t cnt;