net: Fix high overhead of vlan sub-device teardown.

When a networking device is taken down that has a non-trivial number
of VLAN devices configured under it, we eat a full synchronize_net()
for every such VLAN device.

This is because of the call chain:

	NETDEV_DOWN notifier
	--> vlan_device_event()
		--> dev_change_flags()
		--> __dev_change_flags()
		--> __dev_close()
		--> __dev_close_many()
		--> dev_deactivate_many()
			--> synchronize_net()

This is kind of rediculous because we already have infrastructure for
batching doing operation X to a list of net devices so that we only
incur one sync.

So make use of that by exporting dev_close_many() and adjusting it's
interfaace so that the caller can fully manage the batch list.  Use
this in vlan_device_event() and all the overhead goes away.

Reported-by: Salam Noureddine <noureddine@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 3 deletions

diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index 64c6bed4a3d3..98a30a5b8664 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -413,7 +413,10 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
                         vlan_transfer_features(dev, vlandev);
                 break;
 
-        case NETDEV_DOWN:
+        case NETDEV_DOWN: {
+                struct net_device *tmp;
+                LIST_HEAD(close_list);
+
                 if (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER)
                         vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
 
@@ -425,11 +428,18 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
 
                         vlan = vlan_dev_priv(vlandev);
                         if (!(vlan->flags & VLAN_FLAG_LOOSE_BINDING))
-                                dev_change_flags(vlandev, flgs & ~IFF_UP);
+                                list_add(&vlandev->close_list, &close_list);
+                }
+
+                dev_close_many(&close_list, false);
+
+                list_for_each_entry_safe(vlandev, tmp, &close_list, close_list) {
                         netif_stacked_transfer_operstate(dev, vlandev);
+                        list_del_init(&vlandev->close_list);
                 }
+                list_del(&close_list);
                 break;
-
+        }
         case NETDEV_UP:
                 /* Put all VLANs for this dev in the up state too.  */
                 vlan_group_for_each_dev(grp, i, vlandev) {