conntrack: Replace timeout based expiration lists with rculists.

This patch aims to replace the expiration lists as, due to the way
they are used, besides being a source of contention, they have a known
issue when used with non-default policies for different zones that
could lead to retaining expired connections potentially for a long
time.

This patch replaces them with an array of rculist used to distribute
all the newly created connections in order to, during the sweeping
phase, scan them without locking, and evict the expired connections
only locking during the actual removal.  This allows to reduce the
contention introduced by the pushback performed at every packet
update, also solving the issue related to zones and timeout policies.

Signed-off-by: Gaetan Rivet <grive@u256.net>
Co-authored-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Paolo Valerio <pvalerio@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

4 files changed, 145 insertions, 183 deletions

diff --git a/NEWS b/NEWS
index 433bb1654..d684de6d7 100644
--- a/NEWS
+++ b/NEWS
@@ -46,6 +46,7 @@ Post-v2.17.0
        is started. A failure to create a mempool will now be logged only
        when the VM is started.
    - Userspace datapath:
+     * Improved multi-thread scalability of the userspace connection tracking.
      * 'dpif-netdev/subtable-lookup-prio-get' appctl command renamed to
        'dpif-netdev/subtable-lookup-info-get' to better reflect its purpose.
        The old variant is kept for backward compatibility.
diff --git a/lib/conntrack-private.h b/lib/conntrack-private.h
index 34c688821..67882840f 100644
--- a/lib/conntrack-private.h
+++ b/lib/conntrack-private.h
@@ -29,6 +29,7 @@
 #include "openvswitch/list.h"
 #include "openvswitch/types.h"
 #include "packets.h"
+#include "rculist.h"
 #include "unaligned.h"
 #include "dp-packet.h"
 
@@ -86,6 +87,31 @@ struct alg_exp_node {
     bool nat_rpl_dst;
 };
 
+/* Timeouts: all the possible timeout states passed to update_expiration()
+ * are listed here. The name will be prefix by CT_TM_ and the value is in
+ * milliseconds */
+#define CT_TIMEOUTS \
+    CT_TIMEOUT(TCP_FIRST_PACKET) \
+    CT_TIMEOUT(TCP_OPENING) \
+    CT_TIMEOUT(TCP_ESTABLISHED) \
+    CT_TIMEOUT(TCP_CLOSING) \
+    CT_TIMEOUT(TCP_FIN_WAIT) \
+    CT_TIMEOUT(TCP_CLOSED) \
+    CT_TIMEOUT(OTHER_FIRST) \
+    CT_TIMEOUT(OTHER_MULTIPLE) \
+    CT_TIMEOUT(OTHER_BIDIR) \
+    CT_TIMEOUT(ICMP_FIRST) \
+    CT_TIMEOUT(ICMP_REPLY)
+
+enum ct_timeout {
+#define CT_TIMEOUT(NAME) CT_TM_##NAME,
+    CT_TIMEOUTS
+#undef CT_TIMEOUT
+    N_CT_TM
+};
+
+#define N_EXP_LISTS 100
+
 enum OVS_PACKED_ENUM ct_conn_type {
     CT_CONN_TYPE_DEFAULT,
     CT_CONN_TYPE_UN_NAT,
@@ -96,11 +122,16 @@ struct conn {
     struct conn_key key;
     struct conn_key rev_key;
     struct conn_key parent_key; /* Only used for orig_tuple support. */
-    struct ovs_list exp_node;
     struct cmap_node cm_node;
     uint16_t nat_action;
     char *alg;
     struct conn *nat_conn; /* The NAT 'conn' context, if there is one. */
+    atomic_flag reclaimed; /* False during the lifetime of the connection,
+                            * True as soon as a thread has started freeing
+                            * its memory. */
+
+    /* Inserted once by a PMD, then managed by the 'ct_clean' thread. */
+    struct rculist node;
 
     /* Mutable data. */
     struct ovs_mutex lock; /* Guards all mutable fields. */
@@ -116,7 +147,6 @@ struct conn {
     /* Mutable data. */
     bool seq_skew_dir; /* TCP sequence skew direction due to NATTing of FTP
                         * control messages; true if reply direction. */
-    bool cleaned; /* True if cleaned from expiry lists. */
 
     /* Immutable data. */
     bool alg_related; /* True if alg data connection. */
@@ -132,22 +162,6 @@ enum ct_update_res {
     CT_UPDATE_VALID_NEW,
 };
 
-/* Timeouts: all the possible timeout states passed to update_expiration()
- * are listed here. The name will be prefix by CT_TM_ and the value is in
- * milliseconds */
-#define CT_TIMEOUTS \
-    CT_TIMEOUT(TCP_FIRST_PACKET) \
-    CT_TIMEOUT(TCP_OPENING) \
-    CT_TIMEOUT(TCP_ESTABLISHED) \
-    CT_TIMEOUT(TCP_CLOSING) \
-    CT_TIMEOUT(TCP_FIN_WAIT) \
-    CT_TIMEOUT(TCP_CLOSED) \
-    CT_TIMEOUT(OTHER_FIRST) \
-    CT_TIMEOUT(OTHER_MULTIPLE) \
-    CT_TIMEOUT(OTHER_BIDIR) \
-    CT_TIMEOUT(ICMP_FIRST) \
-    CT_TIMEOUT(ICMP_REPLY)
-
 #define NAT_ACTION_SNAT_ALL (NAT_ACTION_SRC | NAT_ACTION_SRC_PORT)
 #define NAT_ACTION_DNAT_ALL (NAT_ACTION_DST | NAT_ACTION_DST_PORT)
 
@@ -181,22 +195,19 @@ enum ct_ephemeral_range {
 #define FOR_EACH_PORT_IN_RANGE(curr, min, max) \
     FOR_EACH_PORT_IN_RANGE__(curr, min, max, OVS_JOIN(idx, __COUNTER__))
 
-enum ct_timeout {
-#define CT_TIMEOUT(NAME) CT_TM_##NAME,
-    CT_TIMEOUTS
-#undef CT_TIMEOUT
-    N_CT_TM
-};
-
 struct conntrack {
     struct ovs_mutex ct_lock; /* Protects 2 following fields. */
     struct cmap conns OVS_GUARDED;
-    struct ovs_list exp_lists[N_CT_TM] OVS_GUARDED;
+    struct rculist exp_lists[N_EXP_LISTS];
     struct cmap zone_limits OVS_GUARDED;
     struct cmap timeout_policies OVS_GUARDED;
     uint32_t hash_basis; /* Salt for hashing a connection key. */
     pthread_t clean_thread; /* Periodically cleans up connection tracker. */
     struct latch clean_thread_exit; /* To destroy the 'clean_thread'. */
+    unsigned int next_list; /* Next list where the newly created connection
+                             * gets inserted. */
+    unsigned int next_sweep; /* List from which the gc thread will resume
+                              * the sweeping. */
 
     /* Counting connections. */
     atomic_count n_conn; /* Number of connections currently tracked. */
@@ -216,8 +227,8 @@ struct conntrack {
 };
 
 /* Lock acquisition order:
- *    1. 'ct_lock'
- *    2. 'conn->lock'
+ *    1. 'conn->lock'
+ *    2. 'ct_lock'
  *    3. 'resources_lock'
  */
 
diff --git a/lib/conntrack-tp.c b/lib/conntrack-tp.c
index c2245038b..7b8f9007b 100644
--- a/lib/conntrack-tp.c
+++ b/lib/conntrack-tp.c
@@ -236,27 +236,6 @@ tm_to_ct_dpif_tp(enum ct_timeout tm)
     return CT_DPIF_TP_ATTR_MAX;
 }
 
-static void
-conn_update_expiration__(struct conntrack *ct, struct conn *conn,
-                         enum ct_timeout tm, long long now,
-                         uint32_t tp_value)
-    OVS_REQUIRES(conn->lock)
-{
-    ovs_mutex_unlock(&conn->lock);
-
-    ovs_mutex_lock(&ct->ct_lock);
-    ovs_mutex_lock(&conn->lock);
-    if (!conn->cleaned) {
-        conn->expiration = now + tp_value * 1000;
-        ovs_list_remove(&conn->exp_node);
-        ovs_list_push_back(&ct->exp_lists[tm], &conn->exp_node);
-    }
-    ovs_mutex_unlock(&conn->lock);
-    ovs_mutex_unlock(&ct->ct_lock);
-
-    ovs_mutex_lock(&conn->lock);
-}
-
 /* The conn entry lock must be held on entry and exit. */
 void
 conn_update_expiration(struct conntrack *ct, struct conn *conn,
@@ -266,41 +245,22 @@ conn_update_expiration(struct conntrack *ct, struct conn *conn,
     struct timeout_policy *tp;
     uint32_t val;
 
-    ovs_mutex_unlock(&conn->lock);
-
-    ovs_mutex_lock(&ct->ct_lock);
-    ovs_mutex_lock(&conn->lock);
     tp = timeout_policy_lookup(ct, conn->tp_id);
     if (tp) {
         val = tp->policy.attrs[tm_to_ct_dpif_tp(tm)];
     } else {
         val = ct_dpif_netdev_tp_def[tm_to_ct_dpif_tp(tm)];
     }
-    ovs_mutex_unlock(&conn->lock);
-    ovs_mutex_unlock(&ct->ct_lock);
-
-    ovs_mutex_lock(&conn->lock);
     VLOG_DBG_RL(&rl, "Update timeout %s zone=%u with policy id=%d "
                 "val=%u sec.",
                 ct_timeout_str[tm], conn->key.zone, conn->tp_id, val);
 
-    conn_update_expiration__(ct, conn, tm, now, val);
+    conn->expiration = now + val * 1000;
 }
 
-static void
-conn_init_expiration__(struct conntrack *ct, struct conn *conn,
-                       enum ct_timeout tm, long long now,
-                       uint32_t tp_value)
-{
-    conn->expiration = now + tp_value * 1000;
-    ovs_list_push_back(&ct->exp_lists[tm], &conn->exp_node);
-}
-
-/* ct_lock must be held. */
 void
 conn_init_expiration(struct conntrack *ct, struct conn *conn,
                      enum ct_timeout tm, long long now)
-    OVS_REQUIRES(ct->ct_lock)
 {
     struct timeout_policy *tp;
     uint32_t val;
@@ -315,5 +275,5 @@ conn_init_expiration(struct conntrack *ct, struct conn *conn,
     VLOG_DBG_RL(&rl, "Init timeout %s zone=%u with policy id=%d val=%u sec.",
                 ct_timeout_str[tm], conn->key.zone, conn->tp_id, val);
 
-    conn_init_expiration__(ct, conn, tm, now, val);
+    conn->expiration = now + val * 1000;
 }
diff --git a/lib/conntrack.c b/lib/conntrack.c
index 38ddc9b91..0b329bacb 100644
--- a/lib/conntrack.c
+++ b/lib/conntrack.c
@@ -39,12 +39,12 @@
 #include "ovs-thread.h"
 #include "openvswitch/poll-loop.h"
 #include "random.h"
+#include "rculist.h"
 #include "timeval.h"
 
 VLOG_DEFINE_THIS_MODULE(conntrack);
 
 COVERAGE_DEFINE(conntrack_full);
-COVERAGE_DEFINE(conntrack_long_cleanup);
 COVERAGE_DEFINE(conntrack_l3csum_err);
 COVERAGE_DEFINE(conntrack_l4csum_err);
 COVERAGE_DEFINE(conntrack_lookup_natted_miss);
@@ -94,14 +94,14 @@ static bool valid_new(struct dp_packet *pkt, struct conn_key *);
 static struct conn *new_conn(struct conntrack *ct, struct dp_packet *pkt,
                              struct conn_key *, long long now,
                              uint32_t tp_id);
-static void delete_conn_cmn(struct conn *);
+static void delete_conn__(struct conn *);
 static void delete_conn(struct conn *);
-static void delete_conn_one(struct conn *conn);
 static enum ct_update_res conn_update(struct conntrack *ct, struct conn *conn,
                                       struct dp_packet *pkt,
                                       struct conn_lookup_ctx *ctx,
                                       long long now);
 static bool conn_expired(struct conn *, long long now);
+static void conn_expire_push_front(struct conntrack *ct, struct conn *conn);
 static void set_mark(struct dp_packet *, struct conn *,
                      uint32_t val, uint32_t mask);
 static void set_label(struct dp_packet *, struct conn *,
@@ -309,7 +309,7 @@ conntrack_init(void)
     ovs_mutex_lock(&ct->ct_lock);
     cmap_init(&ct->conns);
     for (unsigned i = 0; i < ARRAY_SIZE(ct->exp_lists); i++) {
-        ovs_list_init(&ct->exp_lists[i]);
+        rculist_init(&ct->exp_lists[i]);
     }
     cmap_init(&ct->zone_limits);
     ct->zone_limit_seq = 0;
@@ -467,52 +467,57 @@ zone_limit_delete(struct conntrack *ct, uint16_t zone)
 }
 
 static void
-conn_clean_cmn(struct conntrack *ct, struct conn *conn)
+conn_clean__(struct conntrack *ct, struct conn *conn)
     OVS_REQUIRES(ct->ct_lock)
 {
+    uint32_t hash;
+
     if (conn->alg) {
         expectation_clean(ct, &conn->key);
     }
 
-    uint32_t hash = conn_key_hash(&conn->key, ct->hash_basis);
+    hash = conn_key_hash(&conn->key, ct->hash_basis);
     cmap_remove(&ct->conns, &conn->cm_node, hash);
 
-    struct zone_limit *zl = zone_limit_lookup(ct, conn->admit_zone);
-    if (zl && zl->czl.zone_limit_seq == conn->zone_limit_seq) {
-        atomic_count_dec(&zl->czl.count);
+    if (conn->nat_conn) {
+        hash = conn_key_hash(&conn->nat_conn->key, ct->hash_basis);
+        cmap_remove(&ct->conns, &conn->nat_conn->cm_node, hash);
     }
+
+    rculist_remove(&conn->node);
 }
 
 /* Must be called with 'conn' of 'conn_type' CT_CONN_TYPE_DEFAULT.  Also
  * removes the associated nat 'conn' from the lookup datastructures. */
 static void
 conn_clean(struct conntrack *ct, struct conn *conn)
-    OVS_REQUIRES(ct->ct_lock)
+    OVS_EXCLUDED(conn->lock, ct->ct_lock)
 {
     ovs_assert(conn->conn_type == CT_CONN_TYPE_DEFAULT);
 
-    conn_clean_cmn(ct, conn);
-    if (conn->nat_conn) {
-        uint32_t hash = conn_key_hash(&conn->nat_conn->key, ct->hash_basis);
-        cmap_remove(&ct->conns, &conn->nat_conn->cm_node, hash);
+    if (atomic_flag_test_and_set(&conn->reclaimed)) {
+        return;
     }
-    ovs_list_remove(&conn->exp_node);
-    conn->cleaned = true;
+
+    ovs_mutex_lock(&ct->ct_lock);
+    conn_clean__(ct, conn);
+    ovs_mutex_unlock(&ct->ct_lock);
+
+    struct zone_limit *zl = zone_limit_lookup(ct, conn->admit_zone);
+    if (zl && zl->czl.zone_limit_seq == conn->zone_limit_seq) {
+        atomic_count_dec(&zl->czl.count);
+    }
+
     ovsrcu_postpone(delete_conn, conn);
     atomic_count_dec(&ct->n_conn);
 }
 
 static void
-conn_clean_one(struct conntrack *ct, struct conn *conn)
-    OVS_REQUIRES(ct->ct_lock)
+conn_force_expire(struct conn *conn)
 {
-    conn_clean_cmn(ct, conn);
-    if (conn->conn_type == CT_CONN_TYPE_DEFAULT) {
-        ovs_list_remove(&conn->exp_node);
-        conn->cleaned = true;
-        atomic_count_dec(&ct->n_conn);
-    }
-    ovsrcu_postpone(delete_conn_one, conn);
+    ovs_mutex_lock(&conn->lock);
+    conn->expiration = 0;
+    ovs_mutex_unlock(&conn->lock);
 }
 
 /* Destroys the connection tracker 'ct' and frees all the allocated memory.
@@ -522,15 +527,16 @@ void
 conntrack_destroy(struct conntrack *ct)
 {
     struct conn *conn;
+
     latch_set(&ct->clean_thread_exit);
     pthread_join(ct->clean_thread, NULL);
     latch_destroy(&ct->clean_thread_exit);
 
-    ovs_mutex_lock(&ct->ct_lock);
-    CMAP_FOR_EACH (conn, cm_node, &ct->conns) {
-        conn_clean_one(ct, conn);
+    for (unsigned i = 0; i < N_EXP_LISTS; i++) {
+        RCULIST_FOR_EACH (conn, node, &ct->exp_lists[i]) {
+            conn_clean(ct, conn);
+        }
     }
-    cmap_destroy(&ct->conns);
 
     struct zone_limit *zl;
     CMAP_FOR_EACH (zl, node, &ct->zone_limits) {
@@ -539,7 +545,6 @@ conntrack_destroy(struct conntrack *ct)
         cmap_remove(&ct->zone_limits, &zl->node, hash);
         ovsrcu_postpone(free, zl);
     }
-    cmap_destroy(&ct->zone_limits);
 
     struct timeout_policy *tp;
     CMAP_FOR_EACH (tp, node, &ct->timeout_policies) {
@@ -548,6 +553,11 @@ conntrack_destroy(struct conntrack *ct)
         cmap_remove(&ct->timeout_policies, &tp->node, hash);
         ovsrcu_postpone(free, tp);
     }
+
+    ovs_mutex_lock(&ct->ct_lock);
+
+    cmap_destroy(&ct->conns);
+    cmap_destroy(&ct->zone_limits);
     cmap_destroy(&ct->timeout_policies);
 
     ovs_mutex_unlock(&ct->ct_lock);
@@ -1087,7 +1097,9 @@ conn_not_found(struct conntrack *ct, struct dp_packet *pkt,
         nc->nat_conn = nat_conn;
         ovs_mutex_init_adaptive(&nc->lock);
         nc->conn_type = CT_CONN_TYPE_DEFAULT;
+        atomic_flag_clear(&nc->reclaimed);
         cmap_insert(&ct->conns, &nc->cm_node, ctx->hash);
+        conn_expire_push_front(ct, nc);
         atomic_count_inc(&ct->n_conn);
         ctx->conn = nc; /* For completeness. */
         if (zl) {
@@ -1108,8 +1120,7 @@ conn_not_found(struct conntrack *ct, struct dp_packet *pkt,
      * can limit DoS impact. */
 nat_res_exhaustion:
     free(nat_conn);
-    ovs_list_remove(&nc->exp_node);
-    delete_conn_cmn(nc);
+    delete_conn__(nc);
     static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 5);
     VLOG_WARN_RL(&rl, "Unable to NAT due to tuple space exhaustion - "
                  "if DoS attack, use firewalling and/or zone partitioning.");
@@ -1148,11 +1159,9 @@ conn_update_state(struct conntrack *ct, struct dp_packet *pkt,
             pkt->md.ct_state = CS_INVALID;
             break;
         case CT_UPDATE_NEW:
-            ovs_mutex_lock(&ct->ct_lock);
             if (conn_lookup(ct, &conn->key, now, NULL, NULL)) {
-                conn_clean(ct, conn);
+                conn_force_expire(conn);
             }
-            ovs_mutex_unlock(&ct->ct_lock);
             create_new_conn = true;
             break;
         case CT_UPDATE_VALID_NEW:
@@ -1363,11 +1372,9 @@ process_one(struct conntrack *ct, struct dp_packet *pkt,
 
     /* Delete found entry if in wrong direction. 'force' implies commit. */
     if (OVS_UNLIKELY(force && ctx->reply && conn)) {
-        ovs_mutex_lock(&ct->ct_lock);
         if (conn_lookup(ct, &conn->key, now, NULL, NULL)) {
-            conn_clean(ct, conn);
+            conn_force_expire(conn);
         }
-        ovs_mutex_unlock(&ct->ct_lock);
         conn = NULL;
     }
 
@@ -1548,58 +1555,50 @@ set_label(struct dp_packet *pkt, struct conn *conn,
 }
 
 
-/* Delete the expired connections from 'ctb', up to 'limit'. Returns the
- * earliest expiration time among the remaining connections in 'ctb'.  Returns
- * LLONG_MAX if 'ctb' is empty.  The return value might be smaller than 'now',
- * if 'limit' is reached */
-static long long
-ct_sweep(struct conntrack *ct, long long now, size_t limit)
+static size_t
+ct_sweep(struct conntrack *ct, struct rculist *list, long long now)
+    OVS_NO_THREAD_SAFETY_ANALYSIS
 {
     struct conn *conn;
-    long long min_expiration = LLONG_MAX;
     size_t count = 0;
 
-    ovs_mutex_lock(&ct->ct_lock);
-
-    for (unsigned i = 0; i < N_CT_TM; i++) {
-        LIST_FOR_EACH_SAFE (conn, exp_node, &ct->exp_lists[i]) {
-            ovs_mutex_lock(&conn->lock);
-            if (now < conn->expiration || count >= limit) {
-                min_expiration = MIN(min_expiration, conn->expiration);
-                ovs_mutex_unlock(&conn->lock);
-                if (count >= limit) {
-                    /* Do not check other lists. */
-                    COVERAGE_INC(conntrack_long_cleanup);
-                    goto out;
-                }
-                break;
-            } else {
-                ovs_mutex_unlock(&conn->lock);
-                conn_clean(ct, conn);
-            }
-            count++;
+    RCULIST_FOR_EACH (conn, node, list) {
+        if (conn_expired(conn, now)) {
+            conn_clean(ct, conn);
         }
+
+        count++;
     }
 
-out:
-    VLOG_DBG("conntrack cleanup %"PRIuSIZE" entries in %lld msec", count,
-             time_msec() - now);
-    ovs_mutex_unlock(&ct->ct_lock);
-    return min_expiration;
+    return count;
 }
 
-/* Cleans up old connection entries from 'ct'.  Returns the time when the
- * next expiration might happen.  The return value might be smaller than
- * 'now', meaning that an internal limit has been reached, and some expired
- * connections have not been deleted. */
+/* Cleans up old connection entries from 'ct'.  Returns the time
+ * when the next wake will happen. The return value might be zero,
+ * meaning that an internal limit has been reached. */
 static long long
 conntrack_clean(struct conntrack *ct, long long now)
 {
-    unsigned int n_conn_limit;
+    long long next_wakeup = now + 20 * 1000;
+    unsigned int n_conn_limit, i;
+    size_t clean_end, count = 0;
+
     atomic_read_relaxed(&ct->n_conn_limit, &n_conn_limit);
-    size_t clean_max = n_conn_limit > 10 ? n_conn_limit / 10 : 1;
-    long long min_exp = ct_sweep(ct, now, clean_max);
-    long long next_wakeup = MIN(min_exp, now + CT_DPIF_NETDEV_TP_MIN);
+    clean_end = n_conn_limit / 64;
+
+    for (i = ct->next_sweep; i < N_EXP_LISTS; i++) {
+        count += ct_sweep(ct, &ct->exp_lists[i], now);
+
+        if (count > clean_end) {
+            next_wakeup = 0;
+            break;
+        }
+    }
+
+    ct->next_sweep = (i < N_EXP_LISTS) ? i : 0;
+
+    VLOG_DBG("conntrack cleanup %"PRIuSIZE" entries in %lld msec", count,
+             time_msec() - now);
 
     return next_wakeup;
 }
@@ -1607,27 +1606,12 @@ conntrack_clean(struct conntrack *ct, long long now)
 /* Cleanup:
  *
  * We must call conntrack_clean() periodically.  conntrack_clean() return
- * value gives an hint on when the next cleanup must be done (either because
- * there is an actual connection that expires, or because a new connection
- * might be created with the minimum timeout).
- *
- * The logic below has two goals:
- *
- * - We want to reduce the number of wakeups and batch connection cleanup
- *   when the load is not very high.  CT_CLEAN_INTERVAL ensures that if we
- *   are coping with the current cleanup tasks, then we wait at least
- *   5 seconds to do further cleanup.
- *
- * - We don't want to keep the map locked too long, as we might prevent
- *   traffic from flowing.  CT_CLEAN_MIN_INTERVAL ensures that if cleanup is
- *   behind, there is at least some 200ms blocks of time when the map will be
- *   left alone, so the datapath can operate unhindered.
- */
-#define CT_CLEAN_INTERVAL 5000 /* 5 seconds */
-#define CT_CLEAN_MIN_INTERVAL 200  /* 0.2 seconds */
+ * value gives an hint on when the next cleanup must be done. */
+#define CT_CLEAN_MIN_INTERVAL_MS 200
 
 static void *
 clean_thread_main(void *f_)
+    OVS_NO_THREAD_SAFETY_ANALYSIS
 {
     struct conntrack *ct = f_;
 
@@ -1637,9 +1621,9 @@ clean_thread_main(void *f_)
         next_wake = conntrack_clean(ct, now);
 
         if (next_wake < now) {
-            poll_timer_wait_until(now + CT_CLEAN_MIN_INTERVAL);
+            poll_timer_wait_until(now + CT_CLEAN_MIN_INTERVAL_MS);
         } else {
-            poll_timer_wait_until(MAX(next_wake, now + CT_CLEAN_INTERVAL));
+            poll_timer_wait_until(next_wake);
         }
         latch_wait(&ct->clean_thread_exit);
         poll_block();
@@ -2513,6 +2497,16 @@ conn_update(struct conntrack *ct, struct conn *conn, struct dp_packet *pkt,
     return update_res;
 }
 
+static void
+conn_expire_push_front(struct conntrack *ct, struct conn *conn)
+    OVS_REQUIRES(ct->ct_lock)
+{
+    unsigned int curr = ct->next_list;
+
+    ct->next_list = (ct->next_list + 1) % N_EXP_LISTS;
+    rculist_push_front(&ct->exp_lists[curr], &conn->node);
+}
+
 static bool
 conn_expired(struct conn *conn, long long now)
 {
@@ -2539,7 +2533,7 @@ new_conn(struct conntrack *ct, struct dp_packet *pkt, struct conn_key *key,
 }
 
 static void
-delete_conn_cmn(struct conn *conn)
+delete_conn__(struct conn *conn)
 {
     free(conn->alg);
     free(conn);
@@ -2551,18 +2545,9 @@ delete_conn(struct conn *conn)
     ovs_assert(conn->conn_type == CT_CONN_TYPE_DEFAULT);
     ovs_mutex_destroy(&conn->lock);
     free(conn->nat_conn);
-    delete_conn_cmn(conn);
+    delete_conn__(conn);
 }
 
-/* Only used by conn_clean_one(). */
-static void
-delete_conn_one(struct conn *conn)
-{
-    if (conn->conn_type == CT_CONN_TYPE_DEFAULT) {
-        ovs_mutex_destroy(&conn->lock);
-    }
-    delete_conn_cmn(conn);
-}
 
 /* Convert a conntrack address 'a' into an IP address 'b' based on 'dl_type'.
  *
@@ -2714,6 +2699,11 @@ conntrack_dump_next(struct conntrack_dump *dump, struct ct_dpif_entry *entry)
         }
         struct conn *conn;
         INIT_CONTAINER(conn, cm_node, cm_node);
+
+        if (conn_expired(conn, now)) {
+            continue;
+        }
+
         if ((!dump->filter_zone || conn->key.zone == dump->zone) &&
             (conn->conn_type != CT_CONN_TYPE_UN_NAT)) {
             conn_to_ct_dpif_entry(conn, entry, now);
@@ -2735,13 +2725,15 @@ conntrack_flush(struct conntrack *ct, const uint16_t *zone)
 {
     struct conn *conn;
 
-    ovs_mutex_lock(&ct->ct_lock);
     CMAP_FOR_EACH (conn, cm_node, &ct->conns) {
+        if (conn->conn_type != CT_CONN_TYPE_DEFAULT) {
+            continue;
+        }
+
         if (!zone || *zone == conn->key.zone) {
-            conn_clean_one(ct, conn);
+            conn_clean(ct, conn);
         }
     }
-    ovs_mutex_unlock(&ct->ct_lock);
 
     return 0;
 }
@@ -2756,7 +2748,6 @@ conntrack_flush_tuple(struct conntrack *ct, const struct ct_dpif_tuple *tuple,
 
     memset(&key, 0, sizeof(key));
     tuple_to_conn_key(tuple, zone, &key);
-    ovs_mutex_lock(&ct->ct_lock);
     conn_lookup(ct, &key, time_msec(), &conn, NULL);
 
     if (conn && conn->conn_type == CT_CONN_TYPE_DEFAULT) {
@@ -2766,7 +2757,6 @@ conntrack_flush_tuple(struct conntrack *ct, const struct ct_dpif_tuple *tuple,
         error = ENOENT;
     }
 
-    ovs_mutex_unlock(&ct->ct_lock);
     return error;
 }