vhost: make poll group refcount per-session

Change the way we increase poll group reference counts
for round-robin scheduling.

So far we used to increase them whenever someone called
vhost_get_poll_group() and this worked fine for Vhost-Block
which picks a new poll group for each session. Vhost-SCSI,
however, picks only one poll group for all sessions on
a vhost device. This means that some threads will have
multiple Vhost-SCSI pollers but will still appear to the
vhost scheduler as if they had only one.

To fix it, increase poll group refcnt only when sessions
are really being started - in vhost_session_start_done().

Change-Id: I60f0d2101239e5a91138a5afd30c51dc1ccf7c2e
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/466733
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>

lib/vhost/vhost.c

@@ -890,23 +890,18 @@ vhost_get_poll_group(struct spdk_cpuset *cpumask)
 	}
 
 	assert(selected_pg != NULL);
-	assert(selected_pg->ref < UINT_MAX);
-	selected_pg->ref++;
 	return selected_pg;
 }
 
-void
-vhost_put_poll_group(struct vhost_poll_group *pg)
-{
-	assert(pg->ref > 0);
-	pg->ref--;
-}
-
 void
 vhost_session_start_done(struct spdk_vhost_session *vsession, int response)
 {
 	if (response == 0) {
 		vsession->started = true;
+		assert(vsession->poll_group != NULL);
+		assert(vsession->poll_group->ref < UINT_MAX);
+		vsession->poll_group->ref++;
+
 		assert(vsession->vdev->active_session_num < UINT32_MAX);
 		vsession->vdev->active_session_num++;
 	}
@@ -920,6 +915,11 @@ vhost_session_stop_done(struct spdk_vhost_session *vsession, int response)
 {
 	if (response == 0) {
 		vsession->started = false;
+		assert(vsession->poll_group != NULL);
+		assert(vsession->poll_group->ref > 0);
+		vsession->poll_group->ref--;
+		vsession->poll_group = NULL;
+
 		assert(vsession->vdev->active_session_num > 0);
 		vsession->vdev->active_session_num--;
 	}

lib/vhost/vhost_blk.c

@@ -712,17 +712,10 @@ static int
 vhost_blk_start(struct spdk_vhost_session *vsession)
 {
 	struct vhost_poll_group *pg;
-	int rc;
 
 	pg = vhost_get_poll_group(vsession->vdev->cpumask);
-	rc = vhost_session_send_event(pg, vsession, vhost_blk_start_cb,
+	return vhost_session_send_event(pg, vsession, vhost_blk_start_cb,
 					3, "start session");
-
-	if (rc != 0) {
-		vhost_put_poll_group(pg);
-	}
-
-	return rc;
 }
 
 static int

lib/vhost/vhost_nvme.c

@@ -1114,7 +1114,6 @@ static int
 spdk_vhost_nvme_start(struct spdk_vhost_session *vsession)
 {
 	struct vhost_poll_group *pg;
-	int rc;
 
 	if (vsession->vdev->active_session_num > 0) {
 		/* We're trying to start a second session */
@@ -1123,14 +1122,8 @@ spdk_vhost_nvme_start(struct spdk_vhost_session *vsession)
 	}
 
 	pg = vhost_get_poll_group(vsession->vdev->cpumask);
-	rc = vhost_session_send_event(pg, vsession, spdk_vhost_nvme_start_cb,
+	return vhost_session_send_event(pg, vsession, spdk_vhost_nvme_start_cb,
 					3, "start session");
-
-	if (rc != 0) {
-		vhost_put_poll_group(pg);
-	}
-
-	return rc;
 }
 
 static void

lib/vhost/vhost_scsi.c

@@ -1358,7 +1358,6 @@ vhost_scsi_start(struct spdk_vhost_session *vsession)
 				      3, "start session");
 	if (rc != 0) {
 		if (svdev->vdev.active_session_num == 0) {
-			vhost_put_poll_group(svdev->poll_group);
 			svdev->poll_group = NULL;
 		}
 	}
@@ -1459,7 +1458,6 @@ vhost_scsi_stop(struct spdk_vhost_session *vsession)
 	}
 
 	if (vsession->vdev->active_session_num == 0) {
-		vhost_put_poll_group(svsession->svdev->poll_group);
 		svsession->svdev->poll_group = NULL;
 	}
 	return 0;