rtnetlink: ignore IFLA_WIRELESS events.

Some older wireless drivers - ones relying on the
old and long deprecated wireless extension ioctl
system - can generate quite a bit of IFLA_WIRELESS
events depending on their configuration and
runtime conditions. These are delivered as
RTNLGRP_LINK via RTM_NEWLINK messages.

These tend to be relatively easily identifiable
because they report the change mask being 0. This
isn't guaranteed but in practice it shouldn't be a
problem. None of the wireless events that I ever
observed actually carry any unique information
about netdev states that ovs-vswitchd is
interested in. Hence ignoring these shouldn't
cause any problems.

These events can be responsible for a significant
CPU churn as ovs-vswitchd attempts to do plenty of
work for each and every netlink message regardless
of what that message carries. On low-end devices
such as consumer-grade routers these can lead to a
lot of CPU cycles being wasted, adding up to heat
output and reducing performance.

It could be argued that wireless drivers in
question should be fixed, but that isn't exactly a
trivial thing to do. Patching ovs seems far more
viable while still making sense.

Signed-off-by: Michal Kazior <michal@plume.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/if-notifier.c

@@ -26,9 +26,14 @@ struct if_notifier {
 };
 
 static void
-if_notifier_cb(const struct rtnetlink_change *change OVS_UNUSED, void *aux)
+if_notifier_cb(const struct rtnetlink_change *change, void *aux)
 {
     struct if_notifier *notifier;
+
+    if (change && change->irrelevant) {
+        return;
+    }
+
     notifier = aux;
     notifier->cb(notifier->aux);
 }

lib/netdev-linux.c

@@ -742,7 +742,7 @@ netdev_linux_run(const struct netdev_class *netdev_class OVS_UNUSED)
         if (!error) {
             struct rtnetlink_change change;
 
-            if (rtnetlink_parse(&buf, &change)) {
+            if (rtnetlink_parse(&buf, &change) && !change.irrelevant) {
                 struct netdev *netdev_ = NULL;
                 char dev_name[IFNAMSIZ];
 
@@ -6344,6 +6344,7 @@ netdev_linux_update_via_netlink(struct netdev_linux *netdev)
     }
 
     if (rtnetlink_parse(reply, change)
+        && !change->irrelevant
         && change->nlmsg_type == RTM_NEWLINK) {
         bool changed = false;
         error = 0;

lib/route-table.c

@@ -342,6 +342,10 @@ static void
 name_table_change(const struct rtnetlink_change *change,
                   void *aux OVS_UNUSED)
 {
+    if (change && change->irrelevant) {
+        return;
+    }
+
     /* Changes to interface status can cause routing table changes that some
      * versions of the linux kernel do not advertise for some reason. */
     route_table_valid = false;

lib/rtnetlink.c

@@ -88,6 +88,8 @@ rtnetlink_parse(struct ofpbuf *buf, struct rtnetlink_change *change)
     const struct nlmsghdr *nlmsg = buf->data;
     bool parsed = false;
 
+    change->irrelevant = false;
+
     if (rtnetlink_type_is_rtnlgrp_link(nlmsg->nlmsg_type)) {
         /* Policy for RTNLGRP_LINK messages.
          *
@@ -99,6 +101,7 @@ rtnetlink_parse(struct ofpbuf *buf, struct rtnetlink_change *change)
             [IFLA_MTU]    = { .type = NL_A_U32,    .optional = true },
             [IFLA_ADDRESS] = { .type = NL_A_UNSPEC, .optional = true },
             [IFLA_LINKINFO] = { .type = NL_A_NESTED, .optional = true },
+            [IFLA_WIRELESS] = { .type = NL_A_UNSPEC, .optional = true },
         };
 
         struct nlattr *attrs[ARRAY_SIZE(policy)];
@@ -111,6 +114,23 @@ rtnetlink_parse(struct ofpbuf *buf, struct rtnetlink_change *change)
 
             ifinfo = ofpbuf_at(buf, NLMSG_HDRLEN, sizeof *ifinfo);
 
+            /* Wireless events can be spammy and cause a
+             * lot of unnecessary churn and CPU load in
+             * ovs-vswitchd. The best way to filter them out
+             * is to rely on the IFLA_WIRELESS and
+             * ifi_change. As per rtnetlink_ifinfo_prep() in
+             * the kernel, the ifi_change = 0. That combined
+             * with the fact wireless events never really
+             * change interface state (as far as core
+             * networking is concerned) they can be ignored
+             * by ovs-vswitchd. It doesn't understand
+             * wireless extensions anyway and has no way of
+             * presenting these bits into ovsdb.
+             */
+            if (attrs[IFLA_WIRELESS] && ifinfo->ifi_change == 0) {
+                change->irrelevant = true;
+            }
+
             change->nlmsg_type     = nlmsg->nlmsg_type;
             change->if_index       = ifinfo->ifi_index;
             change->ifname         = nl_attr_get_string(attrs[IFLA_IFNAME]);

lib/rtnetlink.h

@@ -45,6 +45,9 @@ struct rtnetlink_change {
     int mtu;                    /* Current MTU. */
     struct eth_addr mac;
     unsigned int ifi_flags;     /* Flags of network device. */
+    bool irrelevant;            /* Some events, notably wireless extensions,
+                                   don't really indicate real netdev change
+                                   that OVS should care about. */
 
     /* Network device address status. */
     /* xxx To be added when needed. */