[PATCH net-next 0/3] net: lockless skb_attempt_defer_free()

[PATCH net-next 0/3] net: lockless skb_attempt_defer_free()

[Eric Dumazet]

Platforms with many cpus and relatively slow inter connect show
a significant spinlock contention in skb_attempt_defer_free().

This series refactors this infrastructure to be NUMA aware,
and lockless.

Tested on various platforms, including AMD Zen 2/3/4
and Intel Granite Rapids, showing significant cost reductions
under network stress (more than 20 Mpps).

Eric Dumazet (3):
  net: make softnet_data.defer_count an atomic
  net: use llist for sd->defer_list
  net: add NUMA awareness to skb_attempt_defer_free()

 include/linux/netdevice.h |  6 +-----
 include/net/hotdata.h     |  7 +++++++
 net/core/dev.c            | 43 +++++++++++++++++++++++----------------
 net/core/dev.h            |  2 +-
 net/core/skbuff.c         | 24 ++++++++++------------
 5 files changed, 45 insertions(+), 37 deletions(-)

-- 
2.51.0.536.g15c5d4f767-goog

[PATCH net-next 1/3] net: make softnet_data.defer_count an atomic

[Eric Dumazet]

This is preparation work to remove the softnet_data.defer_lock,
as it is contended on hosts with large number of cores.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/netdevice.h | 2 +-
 net/core/dev.c            | 2 +-
 net/core/skbuff.c         | 6 ++----
 3 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 1b85454116f666ced61a1450d3f899940f499c05..27e3fa69253f694b98d32b6138cf491da5a8b824 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3538,7 +3538,7 @@ struct softnet_data {
 
 	/* Another possibly contended cache line */
 	spinlock_t		defer_lock ____cacheline_aligned_in_smp;
-	int			defer_count;
+	atomic_t		defer_count;
 	int			defer_ipi_scheduled;
 	struct sk_buff		*defer_list;
 	call_single_data_t	defer_csd;
diff --git a/net/core/dev.c b/net/core/dev.c
index 8b54fdf0289ab223fc37d27a078536db37646b55..8566678d83444e8aacbfea4842878279cf28516f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6726,7 +6726,7 @@ static void skb_defer_free_flush(struct softnet_data *sd)
 	spin_lock(&sd->defer_lock);
 	skb = sd->defer_list;
 	sd->defer_list = NULL;
-	sd->defer_count = 0;
+	atomic_set(&sd->defer_count, 0);
 	spin_unlock(&sd->defer_lock);
 
 	while (skb != NULL) {
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index daaf6da43cc9e199389c3afcd6621c177d247884..f91571f51c69ecf8c2fffed5f3a3cd33fd95828b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -7201,14 +7201,12 @@ nodefer:	kfree_skb_napi_cache(skb);
 
 	sd = &per_cpu(softnet_data, cpu);
 	defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
-	if (READ_ONCE(sd->defer_count) >= defer_max)
+	if (atomic_read(&sd->defer_count) >= defer_max)
 		goto nodefer;
 
 	spin_lock_bh(&sd->defer_lock);
 	/* Send an IPI every time queue reaches half capacity. */
-	kick = sd->defer_count == (defer_max >> 1);
-	/* Paired with the READ_ONCE() few lines above */
-	WRITE_ONCE(sd->defer_count, sd->defer_count + 1);
+	kick = (atomic_inc_return(&sd->defer_count) - 1) == (defer_max >> 1);
 
 	skb->next = sd->defer_list;
 	/* Paired with READ_ONCE() in skb_defer_free_flush() */
-- 
2.51.0.536.g15c5d4f767-goog

Re: [PATCH net-next 1/3] net: make softnet_data.defer_count an atomic

[Jason Xing]

On Fri, Sep 26, 2025 at 11:13 PM Eric Dumazet <edumazet@google.com> wrote:
>
> This is preparation work to remove the softnet_data.defer_lock,
> as it is contended on hosts with large number of cores.
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>

Re: [PATCH net-next 1/3] net: make softnet_data.defer_count an atomic

[Kuniyuki Iwashima]

On Fri, Sep 26, 2025 at 8:13 AM Eric Dumazet <edumazet@google.com> wrote:
>
> This is preparation work to remove the softnet_data.defer_lock,
> as it is contended on hosts with large number of cores.
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

[PATCH net-next 2/3] net: use llist for sd->defer_list

[Eric Dumazet]

Get rid of sd->defer_lock and adopt llist operations.

We optimize skb_attempt_defer_free() for the common case,
where the packet is queued. Otherwise sd->defer_count
is increasing, until skb_defer_free_flush() clears it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/netdevice.h |  8 ++++----
 net/core/dev.c            | 18 ++++++------------
 net/core/skbuff.c         | 15 +++++++--------
 3 files changed, 17 insertions(+), 24 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 27e3fa69253f694b98d32b6138cf491da5a8b824..5c9aa16933d197f70746d64e5f44cae052d9971c 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3537,10 +3537,10 @@ struct softnet_data {
 	struct numa_drop_counters drop_counters;
 
 	/* Another possibly contended cache line */
-	spinlock_t		defer_lock ____cacheline_aligned_in_smp;
-	atomic_t		defer_count;
-	int			defer_ipi_scheduled;
-	struct sk_buff		*defer_list;
+	struct llist_head	defer_list ____cacheline_aligned_in_smp;
+	atomic_long_t		defer_count;
+
+	int			defer_ipi_scheduled ____cacheline_aligned_in_smp;
 	call_single_data_t	defer_csd;
 };
 
diff --git a/net/core/dev.c b/net/core/dev.c
index 8566678d83444e8aacbfea4842878279cf28516f..fb67372774de10b0b112ca71c7c7a13819c2325b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6717,22 +6717,16 @@ EXPORT_SYMBOL(napi_complete_done);
 
 static void skb_defer_free_flush(struct softnet_data *sd)
 {
+	struct llist_node *free_list;
 	struct sk_buff *skb, *next;
 
-	/* Paired with WRITE_ONCE() in skb_attempt_defer_free() */
-	if (!READ_ONCE(sd->defer_list))
+	if (llist_empty(&sd->defer_list))
 		return;
+	atomic_long_set(&sd->defer_count, 0);
+	free_list = llist_del_all(&sd->defer_list);
 
-	spin_lock(&sd->defer_lock);
-	skb = sd->defer_list;
-	sd->defer_list = NULL;
-	atomic_set(&sd->defer_count, 0);
-	spin_unlock(&sd->defer_lock);
-
-	while (skb != NULL) {
-		next = skb->next;
+	llist_for_each_entry_safe(skb, next, free_list, ll_node) {
 		napi_consume_skb(skb, 1);
-		skb = next;
 	}
 }
 
@@ -12995,7 +12989,7 @@ static int __init net_dev_init(void)
 		sd->cpu = i;
 #endif
 		INIT_CSD(&sd->defer_csd, trigger_rx_softirq, sd);
-		spin_lock_init(&sd->defer_lock);
+		init_llist_head(&sd->defer_list);
 
 		gro_init(&sd->backlog.gro);
 		sd->backlog.poll = process_backlog;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f91571f51c69ecf8c2fffed5f3a3cd33fd95828b..22d9dba0e433cf67243a5b7dda77e61d146baf50 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -7184,6 +7184,7 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
  */
 void skb_attempt_defer_free(struct sk_buff *skb)
 {
+	unsigned long defer_count;
 	int cpu = skb->alloc_cpu;
 	struct softnet_data *sd;
 	unsigned int defer_max;
@@ -7201,17 +7202,15 @@ nodefer:	kfree_skb_napi_cache(skb);
 
 	sd = &per_cpu(softnet_data, cpu);
 	defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
-	if (atomic_read(&sd->defer_count) >= defer_max)
+	defer_count = atomic_long_inc_return(&sd->defer_count);
+
+	if (defer_count >= defer_max)
 		goto nodefer;
 
-	spin_lock_bh(&sd->defer_lock);
-	/* Send an IPI every time queue reaches half capacity. */
-	kick = (atomic_inc_return(&sd->defer_count) - 1) == (defer_max >> 1);
+	llist_add(&skb->ll_node, &sd->defer_list);
 
-	skb->next = sd->defer_list;
-	/* Paired with READ_ONCE() in skb_defer_free_flush() */
-	WRITE_ONCE(sd->defer_list, skb);
-	spin_unlock_bh(&sd->defer_lock);
+	/* Send an IPI every time queue reaches half capacity. */
+	kick = (defer_count - 1) == (defer_max >> 1);
 
 	/* Make sure to trigger NET_RX_SOFTIRQ on the remote CPU
 	 * if we are unlucky enough (this seems very unlikely).
-- 
2.51.0.536.g15c5d4f767-goog

Re: [PATCH net-next 2/3] net: use llist for sd->defer_list

[Jason Xing]

On Fri, Sep 26, 2025 at 11:13 PM Eric Dumazet <edumazet@google.com> wrote:
>
> Get rid of sd->defer_lock and adopt llist operations.
>
> We optimize skb_attempt_defer_free() for the common case,
> where the packet is queued. Otherwise sd->defer_count
> is increasing, until skb_defer_free_flush() clears it.
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Quite interesting optimization! I like the no lock version. Thanks!

Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>

> ---
>  include/linux/netdevice.h |  8 ++++----
>  net/core/dev.c            | 18 ++++++------------
>  net/core/skbuff.c         | 15 +++++++--------
>  3 files changed, 17 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 27e3fa69253f694b98d32b6138cf491da5a8b824..5c9aa16933d197f70746d64e5f44cae052d9971c 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3537,10 +3537,10 @@ struct softnet_data {
>         struct numa_drop_counters drop_counters;
>
>         /* Another possibly contended cache line */
> -       spinlock_t              defer_lock ____cacheline_aligned_in_smp;
> -       atomic_t                defer_count;
> -       int                     defer_ipi_scheduled;
> -       struct sk_buff          *defer_list;
> +       struct llist_head       defer_list ____cacheline_aligned_in_smp;
> +       atomic_long_t           defer_count;
> +
> +       int                     defer_ipi_scheduled ____cacheline_aligned_in_smp;
>         call_single_data_t      defer_csd;
>  };
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8566678d83444e8aacbfea4842878279cf28516f..fb67372774de10b0b112ca71c7c7a13819c2325b 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6717,22 +6717,16 @@ EXPORT_SYMBOL(napi_complete_done);
>
>  static void skb_defer_free_flush(struct softnet_data *sd)
>  {
> +       struct llist_node *free_list;
>         struct sk_buff *skb, *next;
>
> -       /* Paired with WRITE_ONCE() in skb_attempt_defer_free() */
> -       if (!READ_ONCE(sd->defer_list))
> +       if (llist_empty(&sd->defer_list))
>                 return;
> +       atomic_long_set(&sd->defer_count, 0);
> +       free_list = llist_del_all(&sd->defer_list);
>
> -       spin_lock(&sd->defer_lock);
> -       skb = sd->defer_list;
> -       sd->defer_list = NULL;
> -       atomic_set(&sd->defer_count, 0);
> -       spin_unlock(&sd->defer_lock);
> -
> -       while (skb != NULL) {
> -               next = skb->next;
> +       llist_for_each_entry_safe(skb, next, free_list, ll_node) {

nit: no need to keep brackets

>                 napi_consume_skb(skb, 1);
> -               skb = next;
>         }
>  }
>
> @@ -12995,7 +12989,7 @@ static int __init net_dev_init(void)
>                 sd->cpu = i;
>  #endif
>                 INIT_CSD(&sd->defer_csd, trigger_rx_softirq, sd);
> -               spin_lock_init(&sd->defer_lock);
> +               init_llist_head(&sd->defer_list);
>
>                 gro_init(&sd->backlog.gro);
>                 sd->backlog.poll = process_backlog;
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f91571f51c69ecf8c2fffed5f3a3cd33fd95828b..22d9dba0e433cf67243a5b7dda77e61d146baf50 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -7184,6 +7184,7 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
>   */
>  void skb_attempt_defer_free(struct sk_buff *skb)
>  {
> +       unsigned long defer_count;
>         int cpu = skb->alloc_cpu;
>         struct softnet_data *sd;
>         unsigned int defer_max;
> @@ -7201,17 +7202,15 @@ nodefer:        kfree_skb_napi_cache(skb);
>
>         sd = &per_cpu(softnet_data, cpu);
>         defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
> -       if (atomic_read(&sd->defer_count) >= defer_max)
> +       defer_count = atomic_long_inc_return(&sd->defer_count);
> +
> +       if (defer_count >= defer_max)
>                 goto nodefer;
>
> -       spin_lock_bh(&sd->defer_lock);
> -       /* Send an IPI every time queue reaches half capacity. */
> -       kick = (atomic_inc_return(&sd->defer_count) - 1) == (defer_max >> 1);
> +       llist_add(&skb->ll_node, &sd->defer_list);
>
> -       skb->next = sd->defer_list;
> -       /* Paired with READ_ONCE() in skb_defer_free_flush() */
> -       WRITE_ONCE(sd->defer_list, skb);
> -       spin_unlock_bh(&sd->defer_lock);
> +       /* Send an IPI every time queue reaches half capacity. */
> +       kick = (defer_count - 1) == (defer_max >> 1);
>
>         /* Make sure to trigger NET_RX_SOFTIRQ on the remote CPU
>          * if we are unlucky enough (this seems very unlikely).
> --
> 2.51.0.536.g15c5d4f767-goog
>
>

Re: [PATCH net-next 2/3] net: use llist for sd->defer_list

[Kuniyuki Iwashima]

On Fri, Sep 26, 2025 at 8:13 AM Eric Dumazet <edumazet@google.com> wrote:
>
> Get rid of sd->defer_lock and adopt llist operations.
>
> We optimize skb_attempt_defer_free() for the common case,
> where the packet is queued. Otherwise sd->defer_count
> is increasing, until skb_defer_free_flush() clears it.
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

[PATCH net-next 3/3] net: add NUMA awareness to skb_attempt_defer_free()

[Eric Dumazet]

Instead of sharing sd->defer_list & sd->defer_count with
many cpus, add one pair for each NUMA node.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/netdevice.h |  4 ----
 include/net/hotdata.h     |  7 +++++++
 net/core/dev.c            | 37 +++++++++++++++++++++++++------------
 net/core/dev.h            |  2 +-
 net/core/skbuff.c         | 11 ++++++-----
 5 files changed, 39 insertions(+), 22 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 5c9aa16933d197f70746d64e5f44cae052d9971c..d1a687444b275d45d105e336d2ede264fd310f1b 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3536,10 +3536,6 @@ struct softnet_data {
 
 	struct numa_drop_counters drop_counters;
 
-	/* Another possibly contended cache line */
-	struct llist_head	defer_list ____cacheline_aligned_in_smp;
-	atomic_long_t		defer_count;
-
 	int			defer_ipi_scheduled ____cacheline_aligned_in_smp;
 	call_single_data_t	defer_csd;
 };
diff --git a/include/net/hotdata.h b/include/net/hotdata.h
index fda94b2647ffa242c256c95ae929d9ef25e54f96..4acec191c54ab367ca12fff590d1f8c8aad64651 100644
--- a/include/net/hotdata.h
+++ b/include/net/hotdata.h
@@ -2,10 +2,16 @@
 #ifndef _NET_HOTDATA_H
 #define _NET_HOTDATA_H
 
+#include <linux/llist.h>
 #include <linux/types.h>
 #include <linux/netdevice.h>
 #include <net/protocol.h>
 
+struct skb_defer_node {
+	struct llist_head	defer_list;
+	atomic_long_t		defer_count;
+} ____cacheline_aligned_in_smp;
+
 /* Read mostly data used in network fast paths. */
 struct net_hotdata {
 #if IS_ENABLED(CONFIG_INET)
@@ -30,6 +36,7 @@ struct net_hotdata {
 	struct rps_sock_flow_table __rcu *rps_sock_flow_table;
 	u32			rps_cpu_mask;
 #endif
+	struct skb_defer_node __percpu *skb_defer_nodes;
 	int			gro_normal_batch;
 	int			netdev_budget;
 	int			netdev_budget_usecs;
diff --git a/net/core/dev.c b/net/core/dev.c
index fb67372774de10b0b112ca71c7c7a13819c2325b..afcf07352eaa3b9a563173106c84167ebe1ab387 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5180,8 +5180,9 @@ static void napi_schedule_rps(struct softnet_data *sd)
 	__napi_schedule_irqoff(&mysd->backlog);
 }
 
-void kick_defer_list_purge(struct softnet_data *sd, unsigned int cpu)
+void kick_defer_list_purge(unsigned int cpu)
 {
+	struct softnet_data *sd = &per_cpu(softnet_data, cpu);
 	unsigned long flags;
 
 	if (use_backlog_threads()) {
@@ -6715,18 +6716,26 @@ bool napi_complete_done(struct napi_struct *n, int work_done)
 }
 EXPORT_SYMBOL(napi_complete_done);
 
-static void skb_defer_free_flush(struct softnet_data *sd)
+struct skb_defer_node __percpu *skb_defer_nodes;
+
+static void skb_defer_free_flush(void)
 {
 	struct llist_node *free_list;
 	struct sk_buff *skb, *next;
+	struct skb_defer_node *sdn;
+	int node;
 
-	if (llist_empty(&sd->defer_list))
-		return;
-	atomic_long_set(&sd->defer_count, 0);
-	free_list = llist_del_all(&sd->defer_list);
+	for_each_node(node) {
+		sdn = this_cpu_ptr(net_hotdata.skb_defer_nodes) + node;
 
-	llist_for_each_entry_safe(skb, next, free_list, ll_node) {
-		napi_consume_skb(skb, 1);
+		if (llist_empty(&sdn->defer_list))
+			continue;
+		atomic_long_set(&sdn->defer_count, 0);
+		free_list = llist_del_all(&sdn->defer_list);
+
+		llist_for_each_entry_safe(skb, next, free_list, ll_node) {
+			napi_consume_skb(skb, 1);
+		}
 	}
 }
 
@@ -6854,7 +6863,7 @@ static void __napi_busy_loop(unsigned int napi_id,
 		if (work > 0)
 			__NET_ADD_STATS(dev_net(napi->dev),
 					LINUX_MIB_BUSYPOLLRXPACKETS, work);
-		skb_defer_free_flush(this_cpu_ptr(&softnet_data));
+		skb_defer_free_flush();
 		bpf_net_ctx_clear(bpf_net_ctx);
 		local_bh_enable();
 
@@ -7713,7 +7722,7 @@ static void napi_threaded_poll_loop(struct napi_struct *napi)
 			local_irq_disable();
 			net_rps_action_and_irq_enable(sd);
 		}
-		skb_defer_free_flush(sd);
+		skb_defer_free_flush();
 		bpf_net_ctx_clear(bpf_net_ctx);
 		local_bh_enable();
 
@@ -7755,7 +7764,7 @@ static __latent_entropy void net_rx_action(void)
 	for (;;) {
 		struct napi_struct *n;
 
-		skb_defer_free_flush(sd);
+		skb_defer_free_flush();
 
 		if (list_empty(&list)) {
 			if (list_empty(&repoll)) {
@@ -12989,7 +12998,6 @@ static int __init net_dev_init(void)
 		sd->cpu = i;
 #endif
 		INIT_CSD(&sd->defer_csd, trigger_rx_softirq, sd);
-		init_llist_head(&sd->defer_list);
 
 		gro_init(&sd->backlog.gro);
 		sd->backlog.poll = process_backlog;
@@ -12999,6 +13007,11 @@ static int __init net_dev_init(void)
 		if (net_page_pool_create(i))
 			goto out;
 	}
+	net_hotdata.skb_defer_nodes =
+		 __alloc_percpu(sizeof(struct skb_defer_node) * nr_node_ids,
+				__alignof__(struct skb_defer_node));
+	if (!net_hotdata.skb_defer_nodes)
+		goto out;
 	if (use_backlog_threads())
 		smpboot_register_percpu_thread(&backlog_threads);
 
diff --git a/net/core/dev.h b/net/core/dev.h
index d6b08d435479b2ba476b1ddeeaae1dce6ac875a2..900880e8b5b4b9492eca23a4d9201045e6bf7f74 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -357,7 +357,7 @@ static inline void napi_assert_will_not_race(const struct napi_struct *napi)
 	WARN_ON(READ_ONCE(napi->list_owner) != -1);
 }
 
-void kick_defer_list_purge(struct softnet_data *sd, unsigned int cpu);
+void kick_defer_list_purge(unsigned int cpu);
 
 #define XMIT_RECURSION_LIMIT	8
 
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 22d9dba0e433cf67243a5b7dda77e61d146baf50..03ed51050efe81b582c2bad147afecce3a7115e1 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -7184,9 +7184,9 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
  */
 void skb_attempt_defer_free(struct sk_buff *skb)
 {
+	struct skb_defer_node *sdn;
 	unsigned long defer_count;
 	int cpu = skb->alloc_cpu;
-	struct softnet_data *sd;
 	unsigned int defer_max;
 	bool kick;
 
@@ -7200,14 +7200,15 @@ nodefer:	kfree_skb_napi_cache(skb);
 	DEBUG_NET_WARN_ON_ONCE(skb_dst(skb));
 	DEBUG_NET_WARN_ON_ONCE(skb->destructor);
 
-	sd = &per_cpu(softnet_data, cpu);
+	sdn = per_cpu_ptr(net_hotdata.skb_defer_nodes, cpu) + numa_node_id();
+
 	defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
-	defer_count = atomic_long_inc_return(&sd->defer_count);
+	defer_count = atomic_long_inc_return(&sdn->defer_count);
 
 	if (defer_count >= defer_max)
 		goto nodefer;
 
-	llist_add(&skb->ll_node, &sd->defer_list);
+	llist_add(&skb->ll_node, &sdn->defer_list);
 
 	/* Send an IPI every time queue reaches half capacity. */
 	kick = (defer_count - 1) == (defer_max >> 1);
@@ -7216,7 +7217,7 @@ nodefer:	kfree_skb_napi_cache(skb);
 	 * if we are unlucky enough (this seems very unlikely).
 	 */
 	if (unlikely(kick))
-		kick_defer_list_purge(sd, cpu);
+		kick_defer_list_purge(cpu);
 }
 
 static void skb_splice_csum_page(struct sk_buff *skb, struct page *page,
-- 
2.51.0.536.g15c5d4f767-goog

Re: [PATCH net-next 3/3] net: add NUMA awareness to skb_attempt_defer_free()

[Jason Xing]

On Fri, Sep 26, 2025 at 11:13 PM Eric Dumazet <edumazet@google.com> wrote:
>
> Instead of sharing sd->defer_list & sd->defer_count with
> many cpus, add one pair for each NUMA node.

Great! I think I might borrow this idea to optimize xsk in the xmit
path because previously I saw the performance impact among numa nodes.

Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>

Thanks!

Re: [PATCH net-next 3/3] net: add NUMA awareness to skb_attempt_defer_free()

[Kuniyuki Iwashima]

On Fri, Sep 26, 2025 at 8:13 AM Eric Dumazet <edumazet@google.com> wrote:
>
> Instead of sharing sd->defer_list & sd->defer_count with
> many cpus, add one pair for each NUMA node.
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> ---
>  include/linux/netdevice.h |  4 ----
>  include/net/hotdata.h     |  7 +++++++
>  net/core/dev.c            | 37 +++++++++++++++++++++++++------------
>  net/core/dev.h            |  2 +-
>  net/core/skbuff.c         | 11 ++++++-----
>  5 files changed, 39 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 5c9aa16933d197f70746d64e5f44cae052d9971c..d1a687444b275d45d105e336d2ede264fd310f1b 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3536,10 +3536,6 @@ struct softnet_data {
>
>         struct numa_drop_counters drop_counters;
>
> -       /* Another possibly contended cache line */
> -       struct llist_head       defer_list ____cacheline_aligned_in_smp;
> -       atomic_long_t           defer_count;
> -
>         int                     defer_ipi_scheduled ____cacheline_aligned_in_smp;
>         call_single_data_t      defer_csd;
>  };
> diff --git a/include/net/hotdata.h b/include/net/hotdata.h
> index fda94b2647ffa242c256c95ae929d9ef25e54f96..4acec191c54ab367ca12fff590d1f8c8aad64651 100644
> --- a/include/net/hotdata.h
> +++ b/include/net/hotdata.h
> @@ -2,10 +2,16 @@
>  #ifndef _NET_HOTDATA_H
>  #define _NET_HOTDATA_H
>
> +#include <linux/llist.h>
>  #include <linux/types.h>
>  #include <linux/netdevice.h>
>  #include <net/protocol.h>
>
> +struct skb_defer_node {
> +       struct llist_head       defer_list;
> +       atomic_long_t           defer_count;
> +} ____cacheline_aligned_in_smp;
> +
>  /* Read mostly data used in network fast paths. */
>  struct net_hotdata {
>  #if IS_ENABLED(CONFIG_INET)
> @@ -30,6 +36,7 @@ struct net_hotdata {
>         struct rps_sock_flow_table __rcu *rps_sock_flow_table;
>         u32                     rps_cpu_mask;
>  #endif
> +       struct skb_defer_node __percpu *skb_defer_nodes;
>         int                     gro_normal_batch;
>         int                     netdev_budget;
>         int                     netdev_budget_usecs;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index fb67372774de10b0b112ca71c7c7a13819c2325b..afcf07352eaa3b9a563173106c84167ebe1ab387 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5180,8 +5180,9 @@ static void napi_schedule_rps(struct softnet_data *sd)
>         __napi_schedule_irqoff(&mysd->backlog);
>  }
>
> -void kick_defer_list_purge(struct softnet_data *sd, unsigned int cpu)
> +void kick_defer_list_purge(unsigned int cpu)
>  {
> +       struct softnet_data *sd = &per_cpu(softnet_data, cpu);
>         unsigned long flags;
>
>         if (use_backlog_threads()) {
> @@ -6715,18 +6716,26 @@ bool napi_complete_done(struct napi_struct *n, int work_done)
>  }
>  EXPORT_SYMBOL(napi_complete_done);
>
> -static void skb_defer_free_flush(struct softnet_data *sd)
> +struct skb_defer_node __percpu *skb_defer_nodes;

seems this is unused, given it's close to the end of cycle
maybe this can be fixed up while applying ? :)

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Thanks!

Re: [PATCH net-next 3/3] net: add NUMA awareness to skb_attempt_defer_free()

[Eric Dumazet]

On Sat, Sep 27, 2025 at 1:28 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> On Fri, Sep 26, 2025 at 8:13 AM Eric Dumazet <edumazet@google.com> wrote:
> >
> > Instead of sharing sd->defer_list & sd->defer_count with
> > many cpus, add one pair for each NUMA node.
> >
> > Signed-off-by: Eric Dumazet <edumazet@google.com>

> > -static void skb_defer_free_flush(struct softnet_data *sd)
> > +struct skb_defer_node __percpu *skb_defer_nodes;
>
> seems this is unused, given it's close to the end of cycle
> maybe this can be fixed up while applying ? :)

Good catch, I moved it to net_hotdata but forgot to remove this leftover.

Will send a V2.

Thanks.