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From ed5cbbc5847527ed0cfc33f521f7c724975c846b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=8D=C3=B1igo=20Huguet?= <ihuguet@redhat.com>
Date: Tue, 30 Apr 2024 12:45:04 +0200
Subject: [PATCH] platform: avoid routes resync for routes that we don't track
When we recibe a Netlink message with a "route change" event, normally
we just ignore it if it's a route that we don't track (i.e. because of
the route protocol).
However, it's not that easy if it has the NLM_F_REPLACE flag because
that means that it might be replacing another route. If the kernel has
similar routes which are candidates for the replacement, it's hard for
NM to guess which one of those is being replaced (as the kernel doesn't
have a "route ID" or similar field to indicate it). Moreover, the kernel
might choose to replace a route that we don't have on cache, so we know
nothing about it.
It is important to note that we cannot just discard Netlink messages of
routes that we don't track if they has the NLM_F_REPLACE. For example,
if we are tracking a route with proto=static, we might receive a replace
message, changing that route to proto=other_proto_that_we_dont_track. We
need to process that message and remove the route from our cache.
As NM doesn't know what route is being replaced, trying to guess will
lead to errors that will leave the cache in an inconsistent state.
Because of that, it just do a cache resync for the routes.
For IPv4 there was an optimization to this: if we don't have in the
cache any route candidate for the replacement there are only 2 possible
options: either add the new route to the cache or discard it if we are
not interested on it. We don't need a resync for that.
This commit is extending that optimization to IPv6 routes. There is no
reason why it shouldn't work in the same way than with IPv4. This
optimization will only work well as long as we find potential candidate
routes in the same way than the kernel (comparing the same fields). NM
calls to this "comparing by WEAK_ID". But this can also happen with IPv4
routes.
It is worth it to enable this optimization because there are routing
daemons using custom routing protocols that makes tens or hundreds of
updates per second. If they use NLM_F_REPLACE, this caused NM to do a
resync hundreds of times per second leading to a 100% CPU usage:
https://issues.redhat.com/browse/RHEL-26195
An additional but smaller optimization is done in this commit: if we
receive a route message for routes that we don't track AND doesn't have
the NLM_F_REPLACE flag, we can ignore the entire message, thus avoiding
the memory allocation of the nmp_object. That nmp_object was going to be
ignored later, anyway, so better to avoid these allocations that, with
the routing daemon of the above's example, can happen hundreds of times
per second.
With this changes, the CPU usage doing `ip route replace` 300 times/s
drops from 100% to 1%. Doing `ip route replace` as fast as possible,
without any rate limitting, still keeps NM with a 3% CPU usage in the
system that I have used to test.
(cherry picked from commit 4d426f581de402e0aebd2ab273ff6649a0a6fee6)
(cherry picked from commit 15ffa8ec6ff7bf43ed1eb123c0d419d6fab8b268)
---
src/libnm-platform/nm-linux-platform.c | 69 ++++++++++++++++----------
src/libnm-platform/nmp-object.c | 22 +++++---
2 files changed, 57 insertions(+), 34 deletions(-)
diff --git a/src/libnm-platform/nm-linux-platform.c b/src/libnm-platform/nm-linux-platform.c
index 9ecac2d9b3..5b595a9b71 100644
--- a/src/libnm-platform/nm-linux-platform.c
+++ b/src/libnm-platform/nm-linux-platform.c
@@ -3903,6 +3903,34 @@ _new_from_nl_addr(const struct nlmsghdr *nlh, gboolean id_only)
return g_steal_pointer(&obj);
}
+static gboolean
+ip_route_is_tracked(guint8 proto, guint8 type)
+{
+ if (proto > RTPROT_STATIC && !NM_IN_SET(proto, RTPROT_DHCP, RTPROT_RA)) {
+ /* We ignore certain rtm_protocol, because NetworkManager would only ever
+ * configure certain protocols. Other routes are not configured by NetworkManager
+ * and we don't track them in the platform cache.
+ *
+ * This is to help with the performance overhead of a huge number of
+ * routes, for example with the bird BGP software, that adds routes
+ * with RTPROT_BIRD protocol. */
+ return FALSE;
+ }
+
+ if (!NM_IN_SET(type,
+ RTN_UNICAST,
+ RTN_LOCAL,
+ RTN_BLACKHOLE,
+ RTN_UNREACHABLE,
+ RTN_PROHIBIT,
+ RTN_THROW)) {
+ /* Certain route types are ignored and not placed into the cache. */
+ return FALSE;
+ }
+
+ return TRUE;
+}
+
/* Copied and heavily modified from libnl3's rtnl_route_parse() and parse_multipath(). */
static NMPObject *
_new_from_nl_route(const struct nlmsghdr *nlh, gboolean id_only, ParseNlmsgIter *parse_nlmsg_iter)
@@ -3963,6 +3991,16 @@ _new_from_nl_route(const struct nlmsghdr *nlh, gboolean id_only, ParseNlmsgIter
* only handle ~supported~ routes.
*****************************************************************/
+ /* If it's a route that we don't need to track, abort here to avoid unnecessary
+ * memory allocations to create the nmp_object. However, if the message has the
+ * NLM_F_REPLACE flag, it might be replacing a route that we were tracking so we
+ * have to stop tracking it. That means that we have to process all messages with
+ * NLM_F_REPLACE. See nmp_cache_update_netlink_route().
+ */
+ if (!ip_route_is_tracked(rtm->rtm_protocol, rtm->rtm_type)
+ && !(nlh->nlmsg_flags & NLM_F_REPLACE))
+ return NULL;
+
addr_family = rtm->rtm_family;
if (addr_family == AF_INET)
@@ -5519,39 +5557,18 @@ ip_route_get_lock_flag(const NMPlatformIPRoute *route)
static gboolean
ip_route_is_alive(const NMPlatformIPRoute *route)
{
- guint8 prot;
+ guint8 proto, type;
nm_assert(route);
nm_assert(route->rt_source >= NM_IP_CONFIG_SOURCE_RTPROT_UNSPEC
&& route->rt_source <= _NM_IP_CONFIG_SOURCE_RTPROT_LAST);
- prot = route->rt_source - 1;
-
- nm_assert(nmp_utils_ip_config_source_from_rtprot(prot) == route->rt_source);
-
- if (prot > RTPROT_STATIC && !NM_IN_SET(prot, RTPROT_DHCP, RTPROT_RA)) {
- /* We ignore certain rtm_protocol, because NetworkManager would only ever
- * configure certain protocols. Other routes are not configured by NetworkManager
- * and we don't track them in the platform cache.
- *
- * This is to help with the performance overhead of a huge number of
- * routes, for example with the bird BGP software, that adds routes
- * with RTPROT_BIRD protocol. */
- return FALSE;
- }
+ proto = route->rt_source - 1;
+ type = nm_platform_route_type_uncoerce(route->type_coerced);
- if (!NM_IN_SET(nm_platform_route_type_uncoerce(route->type_coerced),
- RTN_UNICAST,
- RTN_LOCAL,
- RTN_BLACKHOLE,
- RTN_UNREACHABLE,
- RTN_PROHIBIT,
- RTN_THROW)) {
- /* Certain route types are ignored and not placed into the cache. */
- return FALSE;
- }
+ nm_assert(nmp_utils_ip_config_source_from_rtprot(proto) == route->rt_source);
- return TRUE;
+ return ip_route_is_tracked(proto, type);
}
/* Copied and modified from libnl3's build_route_msg() and rtnl_route_build_msg(). */
diff --git a/src/libnm-platform/nmp-object.c b/src/libnm-platform/nmp-object.c
index 4090da71a3..cb4e9764d1 100644
--- a/src/libnm-platform/nmp-object.c
+++ b/src/libnm-platform/nmp-object.c
@@ -2988,6 +2988,13 @@ nmp_cache_update_netlink_route(NMPCache *cache,
* Since we don't cache all routes (see "route_is_alive"), we cannot know
* with certainty which route was replaced.
*
+ * For example, the kernel might have 3 similar routes (same WEAK_ID), one
+ * of which is not tracked by us so we don't have it into the cache. If we
+ * receive a route replace message, we don't know to what of the 3 routes
+ * it affects (one of the 3 we don't even know that exists). Moreover, if
+ * we only have one route on cache, we don't know if the replace is for a
+ * different one that we don't track.
+ *
* Even if we would cache *all* routes (which we cannot, if kernel adds new
* routing features that modify the known nmp_object_id_equal()), it would
* be hard to find the right route that was replaced. Well, probably we
@@ -3002,15 +3009,14 @@ nmp_cache_update_netlink_route(NMPCache *cache,
* [2] https://bugzilla.redhat.com/show_bug.cgi?id=1337860
*
* We need to resync.
+ *
+ * However, a resync is expensive. Think of a routing daemon that updates
+ * hundreds of routes per second, the performance penalty is huge. We can
+ * optimize it: if we don't have any matching route on cache (by WEAK_ID),
+ * we don't have anything to replace and we don't need a full resync, but
+ * only to add or discard the new route as usual.
*/
- if (NMP_OBJECT_GET_TYPE(obj_hand_over) == NMP_OBJECT_TYPE_IP4_ROUTE
- && !nmp_cache_lookup_all(cache, NMP_CACHE_ID_TYPE_ROUTES_BY_WEAK_ID, obj_hand_over)) {
- /* For IPv4, we can do a small optimization. We skip the resync, if we have
- * no conflicting routes (by weak-id).
- *
- * This optimization does not work for IPv6 (maybe should be fixed).
- */
- } else {
+ if (nmp_cache_lookup_all(cache, NMP_CACHE_ID_TYPE_ROUTES_BY_WEAK_ID, obj_hand_over)) {
entry_replace = NULL;
resync_required = TRUE;
goto out;
--
2.44.0
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