net: sched: refine software bypass handling in tc_run

JIRA: https://issues.redhat.com/browse/RHEL-60271
JIRA: https://issues.redhat.com/browse/RHEL-61181
Upstream Status: net-next.git
Tested: compile only

commit a12c76a
Author: Xin Long <lucien.xin@gmail.com>
Date:   Wed Jan 15 09:27:54 2025 -0500

    net: sched: refine software bypass handling in tc_run

    This patch addresses issues with filter counting in block (tcf_block),
    particularly for software bypass scenarios, by introducing a more
    accurate mechanism using useswcnt.

    Previously, filtercnt and skipswcnt were introduced by:

      Commit 2081fd3 ("net: sched: cls_api: add filter counter") and
      Commit f631ef3 ("net: sched: cls_api: add skip_sw counter")

      filtercnt tracked all tp (tcf_proto) objects added to a block, and
      skipswcnt counted tp objects with the skipsw attribute set.

    The problem is: a single tp can contain multiple filters, some with skipsw
    and others without. The current implementation fails in the case:

      When the first filter in a tp has skipsw, both skipswcnt and filtercnt
      are incremented, then adding a second filter without skipsw to the same
      tp does not modify these counters because tp->counted is already set.

      This results in bypass software behavior based solely on skipswcnt
      equaling filtercnt, even when the block includes filters without
      skipsw. Consequently, filters without skipsw are inadvertently bypassed.

    To address this, the patch introduces useswcnt in block to explicitly count
    tp objects containing at least one filter without skipsw. Key changes
    include:

      Whenever a filter without skipsw is added, its tp is marked with usesw
      and counted in useswcnt. tc_run() now uses useswcnt to determine software
      bypass, eliminating reliance on filtercnt and skipswcnt.

      This refined approach prevents software bypass for blocks containing
      mixed filters, ensuring correct behavior in tc_run().

    Additionally, as atomic operations on useswcnt ensure thread safety and
    tp->lock guards access to tp->usesw and tp->counted, the broader lock
    down_write(&block->cb_lock) is no longer required in tc_new_tfilter(),
    and this resolves a performance regression caused by the filter counting
    mechanism during parallel filter insertions.

      The improvement can be demonstrated using the following script:

      # cat insert_tc_rules.sh

        tc qdisc add dev ens1f0np0 ingress
        for i in $(seq 16); do
            taskset -c $i tc -b rules_$i.txt &
        done
        wait

      Each of rules_$i.txt files above includes 100000 tc filter rules to a
      mlx5 driver NIC ens1f0np0.

      Without this patch:

      # time sh insert_tc_rules.sh

        real    0m50.780s
        user    0m23.556s
        sys     4m13.032s

      With this patch:

      # time sh insert_tc_rules.sh

        real    0m17.718s
        user    0m7.807s
        sys     3m45.050s

    Fixes: 047f340 ("net: sched: make skip_sw actually skip software")
    Reported-by: Shuang Li <shuali@redhat.com>
    Signed-off-by: Xin Long <lucien.xin@gmail.com>
    Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
    Reviewed-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
    Tested-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: Xin Long <lxin@redhat.com>

Author: Xin Long

include/net/pkt_cls.h

@@ -75,11 +75,11 @@ static inline bool tcf_block_non_null_shared(struct tcf_block *block)
 }
 
 #ifdef CONFIG_NET_CLS_ACT
-DECLARE_STATIC_KEY_FALSE(tcf_bypass_check_needed_key);
+DECLARE_STATIC_KEY_FALSE(tcf_sw_enabled_key);
 
 static inline bool tcf_block_bypass_sw(struct tcf_block *block)
 {
-	return block && block->bypass_wanted;
+	return block && !atomic_read(&block->useswcnt);
 }
 #endif
 
@@ -759,6 +759,15 @@ tc_cls_common_offload_init(struct flow_cls_common_offload *cls_common,
 		cls_common->extack = extack;
 }
 
+static inline void tcf_proto_update_usesw(struct tcf_proto *tp, u32 flags)
+{
+	if (tp->usesw)
+		return;
+	if (tc_skip_sw(flags) && tc_in_hw(flags))
+		return;
+	tp->usesw = true;
+}
+
 #if IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
 static inline struct tc_skb_ext *tc_skb_ext_alloc(struct sk_buff *skb)
 {

include/net/sch_generic.h

@@ -425,6 +425,7 @@ struct tcf_proto {
 	spinlock_t		lock;
 	bool			deleting;
 	bool			counted;
+	bool			usesw;
 	refcount_t		refcnt;
 	struct rcu_head		rcu;
 	struct hlist_node	destroy_ht_node;
@@ -474,9 +475,7 @@ struct tcf_block {
 	struct flow_block flow_block;
 	struct list_head owner_list;
 	bool keep_dst;
-	bool bypass_wanted;
-	atomic_t filtercnt; /* Number of filters */
-	atomic_t skipswcnt; /* Number of skip_sw filters */
+	atomic_t useswcnt;
 	atomic_t offloadcnt; /* Number of oddloaded filters */
 	unsigned int nooffloaddevcnt; /* Number of devs unable to do offload */
 	unsigned int lockeddevcnt; /* Number of devs that require rtnl lock. */

net/core/dev.c

@@ -2126,8 +2126,8 @@ EXPORT_SYMBOL_GPL(net_dec_egress_queue);
 #endif
 
 #ifdef CONFIG_NET_CLS_ACT
-DEFINE_STATIC_KEY_FALSE(tcf_bypass_check_needed_key);
-EXPORT_SYMBOL(tcf_bypass_check_needed_key);
+DEFINE_STATIC_KEY_FALSE(tcf_sw_enabled_key);
+EXPORT_SYMBOL(tcf_sw_enabled_key);
 #endif
 
 DEFINE_STATIC_KEY_FALSE(netstamp_needed_key);
@@ -3973,10 +3973,13 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb,
 	if (!miniq)
 		return ret;
 
-	if (static_branch_unlikely(&tcf_bypass_check_needed_key)) {
-		if (tcf_block_bypass_sw(miniq->block))
-			return ret;
-	}
+	/* Global bypass */
+	if (!static_branch_likely(&tcf_sw_enabled_key))
+		return ret;
+
+	/* Block-wise bypass */
+	if (tcf_block_bypass_sw(miniq->block))
+		return ret;
 
 	tc_skb_cb(skb)->mru = 0;
 	tc_skb_cb(skb)->post_ct = false;

net/sched/cls_api.c

@@ -390,6 +390,7 @@ static struct tcf_proto *tcf_proto_create(const char *kind, u32 protocol,
 	tp->protocol = protocol;
 	tp->prio = prio;
 	tp->chain = chain;
+	tp->usesw = !tp->ops->reoffload;
 	spin_lock_init(&tp->lock);
 	refcount_set(&tp->refcnt, 1);
 
@@ -410,39 +411,31 @@ static void tcf_proto_get(struct tcf_proto *tp)
 	refcount_inc(&tp->refcnt);
 }
 
-static void tcf_maintain_bypass(struct tcf_block *block)
+static void tcf_proto_count_usesw(struct tcf_proto *tp, bool add)
 {
-	int filtercnt = atomic_read(&block->filtercnt);
-	int skipswcnt = atomic_read(&block->skipswcnt);
-	bool bypass_wanted = filtercnt > 0 && filtercnt == skipswcnt;
-
-	if (bypass_wanted != block->bypass_wanted) {
 #ifdef CONFIG_NET_CLS_ACT
-		if (bypass_wanted)
-			static_branch_inc(&tcf_bypass_check_needed_key);
-		else
-			static_branch_dec(&tcf_bypass_check_needed_key);
-#endif
-		block->bypass_wanted = bypass_wanted;
+	struct tcf_block *block = tp->chain->block;
+	bool counted = false;
+
+	if (!add) {
+		if (tp->usesw && tp->counted) {
+			if (!atomic_dec_return(&block->useswcnt))
+				static_branch_dec(&tcf_sw_enabled_key);
+			tp->counted = false;
+		}
+		return;
 	}
-}
-
-static void tcf_block_filter_cnt_update(struct tcf_block *block, bool *counted, bool add)
-{
-	lockdep_assert_not_held(&block->cb_lock);
 
-	down_write(&block->cb_lock);
-	if (*counted != add) {
-		if (add) {
-			atomic_inc(&block->filtercnt);
-			*counted = true;
-		} else {
-			atomic_dec(&block->filtercnt);
-			*counted = false;
-		}
+	spin_lock(&tp->lock);
+	if (tp->usesw && !tp->counted) {
+		counted = true;
+		tp->counted = true;
 	}
-	tcf_maintain_bypass(block);
-	up_write(&block->cb_lock);
+	spin_unlock(&tp->lock);
+
+	if (counted && atomic_inc_return(&block->useswcnt) == 1)
+		static_branch_inc(&tcf_sw_enabled_key);
+#endif
 }
 
 static void tcf_chain_put(struct tcf_chain *chain);
@@ -451,7 +444,7 @@ static void tcf_proto_destroy(struct tcf_proto *tp, bool rtnl_held,
 			      bool sig_destroy, struct netlink_ext_ack *extack)
 {
 	tp->ops->destroy(tp, rtnl_held, extack);
-	tcf_block_filter_cnt_update(tp->chain->block, &tp->counted, false);
+	tcf_proto_count_usesw(tp, false);
 	if (sig_destroy)
 		tcf_proto_signal_destroyed(tp->chain, tp);
 	tcf_chain_put(tp->chain);
@@ -2404,7 +2397,7 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 		tfilter_notify(net, skb, n, tp, block, q, parent, fh,
 			       RTM_NEWTFILTER, false, rtnl_held, extack);
 		tfilter_put(tp, fh);
-		tcf_block_filter_cnt_update(block, &tp->counted, true);
+		tcf_proto_count_usesw(tp, true);
 		/* q pointer is NULL for shared blocks */
 		if (q)
 			q->flags &= ~TCQ_F_CAN_BYPASS;
@@ -3521,8 +3514,6 @@ static void tcf_block_offload_inc(struct tcf_block *block, u32 *flags)
 	if (*flags & TCA_CLS_FLAGS_IN_HW)
 		return;
 	*flags |= TCA_CLS_FLAGS_IN_HW;
-	if (tc_skip_sw(*flags))
-		atomic_inc(&block->skipswcnt);
 	atomic_inc(&block->offloadcnt);
 }
 
@@ -3531,8 +3522,6 @@ static void tcf_block_offload_dec(struct tcf_block *block, u32 *flags)
 	if (!(*flags & TCA_CLS_FLAGS_IN_HW))
 		return;
 	*flags &= ~TCA_CLS_FLAGS_IN_HW;
-	if (tc_skip_sw(*flags))
-		atomic_dec(&block->skipswcnt);
 	atomic_dec(&block->offloadcnt);
 }
 

net/sched/cls_bpf.c

@@ -509,6 +509,8 @@ static int cls_bpf_change(struct net *net, struct sk_buff *in_skb,
 	if (!tc_in_hw(prog->gen_flags))
 		prog->gen_flags |= TCA_CLS_FLAGS_NOT_IN_HW;
 
+	tcf_proto_update_usesw(tp, prog->gen_flags);
+
 	if (oldprog) {
 		idr_replace(&head->handle_idr, prog, handle);
 		list_replace_rcu(&oldprog->link, &prog->link);

net/sched/cls_flower.c

@@ -2427,6 +2427,8 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
 	if (!tc_in_hw(fnew->flags))
 		fnew->flags |= TCA_CLS_FLAGS_NOT_IN_HW;
 
+	tcf_proto_update_usesw(tp, fnew->flags);
+
 	spin_lock(&tp->lock);
 
 	/* tp was deleted concurrently. -EAGAIN will cause caller to lookup

net/sched/cls_matchall.c

@@ -228,6 +228,8 @@ static int mall_change(struct net *net, struct sk_buff *in_skb,
 	if (!tc_in_hw(new->flags))
 		new->flags |= TCA_CLS_FLAGS_NOT_IN_HW;
 
+	tcf_proto_update_usesw(tp, new->flags);
+
 	*arg = head;
 	rcu_assign_pointer(tp->root, new);
 	return 0;

net/sched/cls_u32.c

@@ -941,6 +941,8 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		if (!tc_in_hw(new->flags))
 			new->flags |= TCA_CLS_FLAGS_NOT_IN_HW;
 
+		tcf_proto_update_usesw(tp, new->flags);
+
 		u32_replace_knode(tp, tp_c, new);
 		tcf_unbind_filter(tp, &n->res);
 		tcf_exts_get_net(&n->exts);
@@ -1154,6 +1156,8 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		if (!tc_in_hw(n->flags))
 			n->flags |= TCA_CLS_FLAGS_NOT_IN_HW;
 
+		tcf_proto_update_usesw(tp, n->flags);
+
 		ins = &ht->ht[TC_U32_HASH(handle)];
 		for (pins = rtnl_dereference(*ins); pins;
 		     ins = &pins->next, pins = rtnl_dereference(*ins))