[PATCH net-next] rtnetlink: Support fine-grained netdevice bulk deletion

Currently there are 2 means of deleting a netdevice using Netlink:
1. Deleting a single netdevice (either by ifindex using
ifinfomsg::ifi_index, or by name using IFLA_IFNAME)
2. Delete all netdevice that belong to a group (using IFLA_GROUP)

After all netdevice are handled, netdev_run_todo() is called, which
calls rcu_barrier() to finish any outstanding RCU callbacks that were
registered during the deletion of the netdevice, then wait until the
refcount of all the devices is 0 and perform final cleanups.

However, calling rcu_barrier() is a very costly operation, which takes
in the order of ~10ms.

When deleting a large number of netdevice one-by-one, rcu_barrier()
will be called for each netdevice being deleted, causing the whole
operation taking a long time.

Following results are from benchmarking deleting 10K loopback devices,
all of which are UP and with only IPv6 LLA being configured:

1. Deleting one-by-one using 1 thread : 243 seconds
2. Deleting one-by-one using 10 thread: 70 seconds
3. Deleting one-by-one using 50 thread: 54 seconds
4. Deleting all using "group deletion": 30 seconds

Note that even though the deletion logic takes place under the rtnl
lock, since the call to rcu_barrier() is outside the lock we gain
improvements.

Since "group deletion" calls rcu_barrier() only once, it is indeed the
fastest.
However, "group deletion" is too crude as means of deleting large number
of devices

This patch adds support for passing an arbitrary list of ifindex of
netdevices to delete. This gives a more fine-grained control over
which devices to delete, while still resulting in only one rcu_barrier()
being called.
Indeed, the timings of using this new API to delete 10K netdevices is
the same as using the existing "group" deletion.

The size constraints on the list means the API can delete at most 16382
netdevices in a single request.

Signed-off-by: Lahav Schlesinger <redacted>
---
 include/uapi/linux/if_link.h |  1 +
 net/core/rtnetlink.c         | 46 ++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index eebd3894fe89..f950bf6ed025 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h

@@ -348,6 +348,7 @@ enum {
 	IFLA_PARENT_DEV_NAME,
 	IFLA_PARENT_DEV_BUS_NAME,
 
+	IFLA_IFINDEX_LIST,
 	__IFLA_MAX
 };
 

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index fd030e02f16d..150587b4b1a4 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c

@@ -1880,6 +1880,7 @@ static const struct nla_policy ifla_policy[IFLA_MAX+1] = {
 	[IFLA_PROTO_DOWN_REASON] = { .type = NLA_NESTED },
 	[IFLA_NEW_IFINDEX]	= NLA_POLICY_MIN(NLA_S32, 1),
 	[IFLA_PARENT_DEV_NAME]	= { .type = NLA_NUL_STRING },
+	[IFLA_IFINDEX_LIST] 	= { .type = NLA_BINARY, .len = 65535 },
 };
 
 static const struct nla_policy ifla_info_policy[IFLA_INFO_MAX+1] = {

@@ -3050,6 +3051,49 @@ static int rtnl_group_dellink(const struct net *net, int group)
 	return 0;
 }
 
+static int rtnl_list_dellink(struct net *net, void *dev_list, int size)
+{
+	int i;
+	struct net_device *dev, *aux;
+	LIST_HEAD(list_kill);
+	bool found = false;
+
+	if (size < 0 || size % sizeof(int))
+		return -EINVAL;
+
+	for_each_netdev(net, dev) {
+		for (i = 0; i < size/sizeof(int); ++i) {
+			if (dev->ifindex == ((int*)dev_list)[i]) {
+				const struct rtnl_link_ops *ops;
+
+				found = true;
+				ops = dev->rtnl_link_ops;
+				if (!ops || !ops->dellink)
+					return -EOPNOTSUPP;
+				break;
+			}
+		}
+	}
+
+	if (!found)
+		return -ENODEV;
+
+	for_each_netdev_safe(net, dev, aux) {
+		for (i = 0; i < size/sizeof(int); ++i) {
+			if (dev->ifindex == ((int*)dev_list)[i]) {
+				const struct rtnl_link_ops *ops;
+
+				ops = dev->rtnl_link_ops;
+				ops->dellink(dev, &list_kill);
+				break;
+			}
+		}
+	}
+	unregister_netdevice_many(&list_kill);
+
+	return 0;
+}
+
 int rtnl_delete_link(struct net_device *dev)
 {
 	const struct rtnl_link_ops *ops;

@@ -3102,6 +3146,8 @@ static int rtnl_dellink(struct sk_buff *skb, struct nlmsghdr *nlh,
 				   tb[IFLA_ALT_IFNAME], NULL);
 	else if (tb[IFLA_GROUP])
 		err = rtnl_group_dellink(tgt_net, nla_get_u32(tb[IFLA_GROUP]));
+	else if (tb[IFLA_IFINDEX_LIST])
+		err = rtnl_list_dellink(tgt_net, nla_data(tb[IFLA_IFINDEX_LIST]), nla_len(tb[IFLA_IFINDEX_LIST]));
 	else
 		goto out;
 

-- 
2.25.1