From: Felix Fietkau <nbd@nbd.name>
Date: Thu, 16 Feb 2023 18:39:04 +0100
Subject: [PATCH] net/core: add optional threading for backlog processing

When dealing with few flows or an imbalance on CPU utilization, static RPS
CPU assignment can be too inflexible. Add support for enabling threaded NAPI
for backlog processing in order to allow the scheduler to better balance
processing. This helps better spread the load across idle CPUs.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---

--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -520,6 +520,7 @@ static inline bool napi_complete(struct
 }
 
 int dev_set_threaded(struct net_device *dev, bool threaded);
+int backlog_set_threaded(bool threaded);
 
 /**
  *	napi_disable - prevent NAPI from scheduling
@@ -3126,6 +3127,7 @@ struct softnet_data {
 	unsigned int		processed;
 	unsigned int		time_squeeze;
 	unsigned int		received_rps;
+	unsigned int		process_queue_empty;
 #ifdef CONFIG_RPS
 	struct softnet_data	*rps_ipi_list;
 #endif
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4604,7 +4604,7 @@ static int napi_schedule_rps(struct soft
 	struct softnet_data *mysd = this_cpu_ptr(&softnet_data);
 
 #ifdef CONFIG_RPS
-	if (sd != mysd) {
+	if (sd != mysd && !test_bit(NAPI_STATE_THREADED, &sd->backlog.state)) {
 		sd->rps_ipi_next = mysd->rps_ipi_list;
 		mysd->rps_ipi_list = sd;
 
@@ -5785,6 +5785,8 @@ static DEFINE_PER_CPU(struct work_struct
 /* Network device is going away, flush any packets still pending */
 static void flush_backlog(struct work_struct *work)
 {
+	unsigned int process_queue_empty;
+	bool threaded, flush_processq;
 	struct sk_buff *skb, *tmp;
 	struct softnet_data *sd;
 
@@ -5799,8 +5801,17 @@ static void flush_backlog(struct work_st
 			input_queue_head_incr(sd);
 		}
 	}
+
+	threaded = test_bit(NAPI_STATE_THREADED, &sd->backlog.state);
+	flush_processq = threaded &&
+			 !skb_queue_empty_lockless(&sd->process_queue);
+	if (flush_processq)
+		process_queue_empty = sd->process_queue_empty;
 	rps_unlock_irq_enable(sd);
 
+	if (threaded)
+		goto out;
+
 	skb_queue_walk_safe(&sd->process_queue, skb, tmp) {
 		if (skb->dev->reg_state == NETREG_UNREGISTERING) {
 			__skb_unlink(skb, &sd->process_queue);
@@ -5808,7 +5819,16 @@ static void flush_backlog(struct work_st
 			input_queue_head_incr(sd);
 		}
 	}
+
+out:
 	local_bh_enable();
+
+	while (flush_processq) {
+		msleep(1);
+		rps_lock_irq_disable(sd);
+		flush_processq = process_queue_empty == sd->process_queue_empty;
+		rps_unlock_irq_enable(sd);
+	}
 }
 
 static bool flush_required(int cpu)
@@ -5940,6 +5960,7 @@ static int process_backlog(struct napi_s
 		}
 
 		rps_lock_irq_disable(sd);
+		sd->process_queue_empty++;
 		if (skb_queue_empty(&sd->input_pkt_queue)) {
 			/*
 			 * Inline a custom version of __napi_complete().
@@ -5949,7 +5970,8 @@ static int process_backlog(struct napi_s
 			 * We can use a plain write instead of clear_bit(),
 			 * and we dont need an smp_mb() memory barrier.
 			 */
-			napi->state = 0;
+			napi->state &= ~(NAPIF_STATE_SCHED |
+					 NAPIF_STATE_SCHED_THREADED);
 			again = false;
 		} else {
 			skb_queue_splice_tail_init(&sd->input_pkt_queue,
@@ -6365,6 +6387,55 @@ int dev_set_threaded(struct net_device *
 }
 EXPORT_SYMBOL(dev_set_threaded);
 
+int backlog_set_threaded(bool threaded)
+{
+	static bool backlog_threaded;
+	int err = 0;
+	int i;
+
+	if (backlog_threaded == threaded)
+		return 0;
+
+	for_each_possible_cpu(i) {
+		struct softnet_data *sd = &per_cpu(softnet_data, i);
+		struct napi_struct *n = &sd->backlog;
+
+		if (n->thread)
+			continue;
+		n->thread = kthread_run(napi_threaded_poll, n, "napi/backlog-%d", i);
+		if (IS_ERR(n->thread)) {
+			err = PTR_ERR(n->thread);
+			pr_err("kthread_run failed with err %d\n", err);
+			n->thread = NULL;
+			threaded = false;
+			break;
+		}
+
+	}
+
+	backlog_threaded = threaded;
+
+	/* Make sure kthread is created before THREADED bit
+	 * is set.
+	 */
+	smp_mb__before_atomic();
+
+	for_each_possible_cpu(i) {
+		struct softnet_data *sd = &per_cpu(softnet_data, i);
+		struct napi_struct *n = &sd->backlog;
+		unsigned long flags;
+
+		rps_lock_irqsave(sd, &flags);
+		if (threaded)
+			n->state |= NAPIF_STATE_THREADED;
+		else
+			n->state &= ~NAPIF_STATE_THREADED;
+		rps_unlock_irq_restore(sd, &flags);
+	}
+
+	return err;
+}
+
 void netif_napi_add_weight(struct net_device *dev, struct napi_struct *napi,
 			   int (*poll)(struct napi_struct *, int), int weight)
 {
@@ -11137,6 +11208,9 @@ static int dev_cpu_dead(unsigned int old
 	raise_softirq_irqoff(NET_TX_SOFTIRQ);
 	local_irq_enable();
 
+	if (test_bit(NAPI_STATE_THREADED, &oldsd->backlog.state))
+		return 0;
+
 #ifdef CONFIG_RPS
 	remsd = oldsd->rps_ipi_list;
 	oldsd->rps_ipi_list = NULL;
@@ -11440,6 +11514,7 @@ static int __init net_dev_init(void)
 		INIT_CSD(&sd->defer_csd, trigger_rx_softirq, sd);
 		spin_lock_init(&sd->defer_lock);
 
+		INIT_LIST_HEAD(&sd->backlog.poll_list);
 		init_gro_hash(&sd->backlog);
 		sd->backlog.poll = process_backlog;
 		sd->backlog.weight = weight_p;
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -29,6 +29,7 @@ static int int_3600 = 3600;
 static int min_sndbuf = SOCK_MIN_SNDBUF;
 static int min_rcvbuf = SOCK_MIN_RCVBUF;
 static int max_skb_frags = MAX_SKB_FRAGS;
+static int backlog_threaded;
 
 static int net_msg_warn;	/* Unused, but still a sysctl */
 
@@ -112,6 +113,23 @@ static int rps_sock_flow_sysctl(struct c
 }
 #endif /* CONFIG_RPS */
 
+static int backlog_threaded_sysctl(struct ctl_table *table, int write,
+			       void *buffer, size_t *lenp, loff_t *ppos)
+{
+	static DEFINE_MUTEX(backlog_threaded_mutex);
+	int ret;
+
+	mutex_lock(&backlog_threaded_mutex);
+
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	if (write && !ret)
+		ret = backlog_set_threaded(backlog_threaded);
+
+	mutex_unlock(&backlog_threaded_mutex);
+
+	return ret;
+}
+
 #ifdef CONFIG_NET_FLOW_LIMIT
 static DEFINE_MUTEX(flow_limit_update_mutex);
 
@@ -473,6 +491,15 @@ static struct ctl_table net_core_table[]
 		.proc_handler	= rps_sock_flow_sysctl
 	},
 #endif
+	{
+		.procname	= "backlog_threaded",
+		.data		= &backlog_threaded,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= backlog_threaded_sysctl,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE
+	},
 #ifdef CONFIG_NET_FLOW_LIMIT
 	{
 		.procname	= "flow_limit_cpu_bitmap",
