[Qemu-devel] [PATCH 07/16] block: change drain to look only at one child at a time

[Paolo Bonzini]

bdrv_requests_pending is checking children to also wait until internal
requests (such as metadata writes) have completed.  However, checking
children is in general overkill because, apart from this special case,
the parent's in_flight count will always be incremented by at least one
for every request in the child.

Since internal requests are only generated by the parent in the child,
instead visit the tree parent first, and then wait for internal I/O in
the children to complete.

Signed-off-by: Paolo Bonzini <address@hidden>
---
 block/io.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/block/io.c b/block/io.c
index a9a23a6..e0c9215 100644
--- a/block/io.c
+++ b/block/io.c
@@ -249,6 +249,23 @@ static void bdrv_drain_recurse(BlockDriverState *bs)
     }
 }
 
+static bool bdrv_drain_io_recurse(BlockDriverState *bs)
+{
+    BdrvChild *child;
+    bool waited = false;
+
+    while (atomic_read(&bs->in_flight) > 0) {
+        aio_poll(bdrv_get_aio_context(bs), true);
+        waited = true;
+    }
+
+    QLIST_FOREACH(child, &bs->children, next) {
+        waited |= bdrv_drain_io_recurse(child->bs);
+    }
+
+    return waited;
+}
+
 /*
  * Wait for pending requests to complete on a single BlockDriverState subtree,
  * and suspend block driver's internal I/O until next request arrives.
@@ -265,10 +282,7 @@ void bdrv_drain(BlockDriverState *bs)
     bdrv_no_throttling_begin(bs);
     bdrv_io_unplugged_begin(bs);
     bdrv_drain_recurse(bs);
-    while (bdrv_requests_pending(bs)) {
-        /* Keep iterating */
-         aio_poll(bdrv_get_aio_context(bs), true);
-    }
+    bdrv_drain_io_recurse(bs);
     bdrv_io_unplugged_end(bs);
     bdrv_no_throttling_end(bs);
 }
@@ -319,10 +333,7 @@ void bdrv_drain_all(void)
             aio_context_acquire(aio_context);
             while ((bs = bdrv_next(bs))) {
                 if (aio_context == bdrv_get_aio_context(bs)) {
-                    if (bdrv_requests_pending(bs)) {
-                        aio_poll(aio_context, true);
-                        waited = true;
-                    }
+                    waited |= bdrv_drain_io_recurse(bs);
                 }
             }
             aio_context_release(aio_context);
-- 
2.5.0