net: add skb_defer_disable_key static key

Add a static key to bypass skb_attempt_defer_free() steps
if net.core.skb_defer_max is set to zero.

Main benefit is the atomic_long_inc_return() avoidance.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260311191340.1996888-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/net-sysfs.h

@@ -13,4 +13,5 @@ int netdev_change_owner(struct net_device *, const struct net *net_old,
 
 extern struct mutex rps_default_mask_mutex;
 
+DECLARE_STATIC_KEY_FALSE(skb_defer_disable_key);
 #endif

net/core/skbuff.c

@@ -7256,6 +7256,8 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
 	local_bh_enable();
 }
 
+DEFINE_STATIC_KEY_FALSE(skb_defer_disable_key);
+
 /**
  * skb_attempt_defer_free - queue skb for remote freeing
  * @skb: buffer
@@ -7272,6 +7274,9 @@ void skb_attempt_defer_free(struct sk_buff *skb)
 	bool kick;
 	int cpu;
 
+	if (static_branch_unlikely(&skb_defer_disable_key))
+		goto nodefer;
+
 	/* zero copy notifications should not be delayed. */
 	if (skb_zcopy(skb))
 		goto nodefer;

net/core/sysctl_net_core.c

@@ -349,6 +349,29 @@ static int proc_do_rss_key(const struct ctl_table *table, int write,
 	return proc_dostring(&fake_table, write, buffer, lenp, ppos);
 }
 
+static int proc_do_skb_defer_max(const struct ctl_table *table, int write,
+		 void *buffer, size_t *lenp, loff_t *ppos)
+{
+	static DEFINE_MUTEX(skb_defer_max_mutex);
+	int ret, oval, nval;
+
+	mutex_lock(&skb_defer_max_mutex);
+
+	oval = !net_hotdata.sysctl_skb_defer_max;
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	nval = !net_hotdata.sysctl_skb_defer_max;
+
+	if (nval != oval) {
+		if (nval)
+			static_branch_enable(&skb_defer_disable_key);
+		else
+			static_branch_disable(&skb_defer_disable_key);
+	}
+
+	mutex_unlock(&skb_defer_max_mutex);
+	return ret;
+}
+
 #ifdef CONFIG_BPF_JIT
 static int proc_dointvec_minmax_bpf_enable(const struct ctl_table *table, int write,
 					   void *buffer, size_t *lenp,
@@ -650,7 +673,7 @@ static struct ctl_table net_core_table[] = {
 		.data		= &net_hotdata.sysctl_skb_defer_max,
 		.maxlen		= sizeof(unsigned int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec_minmax,
+		.proc_handler	= proc_do_skb_defer_max,
 		.extra1		= SYSCTL_ZERO,
 	},
 };