[PATCH v2 net-next] net: add sysctl to toggle napi_consume_skb() alien skb defer

[Jason Xing <kerneljasonxing@gmail.com>]

Commit e20dfbad8aab ("net: fix napi_consume_skb() with alien skbs")
defers freeing of alien SKBs (alloc_cpu != current cpu) via
skb_attempt_defer_free() on the TX completion path to reduce cross-NUMA
SLUB spinlock contention to improve multi-queue UDP workloads.

However, this unconditionally impacts the napi_skb_cache fast recycle
path for single-flow / few-flow workloads (e.g. AF_XDP benchmarks[1]):
when the TX completion NAPI CPU differs from the SKB allocation CPU,
SKBs are deferred instead of being returned to the local napi_skb_cache,
forcing RX allocations back to the slow slab path.

The existing net.core.skb_defer_max=0 could disable this, but it is a
global switch that also disables the defer mechanism in TCP/UDP/MPTCP
recvmsg paths, losing its positive SLUB locality benefits there. AF_XDP
can co-exist with other protocols. That's the reason why I gave up
reusing skb_defer_disable_key. Besides, if the defer path is disabled,
that means TCP/UDP/MPTCP in process path will trigger directly freeing
skb with enabling/disabling bottom half(in kfree_skb_napi_cache())
which could affect others. So my thinking is not to touch this path.

Add a dedicated sysctl net.core.napi_consume_skb_defer backed by a
static key to selectively control the alien skb defer feature. Let
users decide which is the best fit for their own requirements.

This patch also avoids touching local_bh* pair(in kfree_skb_napi_cache())
to minimize the overhead.

[1]: taskset -c 0 ./xdpsock -i enp2s0f1 -q 1 -t -S -s 64
1) sysctl -w net.core.napi_consume_skb_defer=1 (as default)
 sock0@enp2s0f1:1 txonly xdp-skb
                   pps            pkts           1.00
rx                 0              0
tx                 1,851,950      20,397,952

2)sysctl -w net.core.napi_consume_skb_defer=0
 sock0@enp2s0f1:1 txonly xdp-skb
                   pps            pkts           1.00
rx                 0              0
tx                 1,985,067      25,530,432

For AF_XDP scenario, it turns out to be around 6.6% improvement.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
V2
Link: https://lore.kernel.org/all/20260326144249.97213-1-kerneljasonxing@gmail.com/
1. reuse proc_do_static_key() (Eric)
2. add doc (Stan)
---
 Documentation/admin-guide/sysctl/net.rst | 13 +++++++++++++
 net/core/net-sysfs.h                     |  1 +
 net/core/skbuff.c                        |  5 ++++-
 net/core/sysctl_net_core.c               |  7 +++++++
 4 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
index 0724a793798f..42e06f93306f 100644
--- a/Documentation/admin-guide/sysctl/net.rst
+++ b/Documentation/admin-guide/sysctl/net.rst
@@ -368,6 +368,19 @@ by the cpu which allocated them.
 
 Default: 128
 
+napi_consume_skb_defer
+----------------------
+When set to 1 (default), napi_consume_skb() defers freeing SKBs whose
+allocation CPU differs from the current CPU via skb_attempt_defer_free().
+This reduces cross-NUMA SLUB spinlock contention for multi-queue workloads.
+
+Setting this to 0 disables the defer path in napi_consume_skb() only,
+allowing SKBs to be returned to the local napi_skb_cache immediately.
+This can benefit single-flow or few-flow workloads (e.g. AF_XDP TX)
+where the defer detour hurts the fast recycle path.
+
+Default: 1
+
 optmem_max
 ----------
 
diff --git a/net/core/net-sysfs.h b/net/core/net-sysfs.h
index 38e2e3ffd0bd..a026f757867e 100644
--- a/net/core/net-sysfs.h
+++ b/net/core/net-sysfs.h
@@ -14,4 +14,5 @@ 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);
+DECLARE_STATIC_KEY_TRUE(napi_consume_skb_defer_key);
 #endif
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 3d6978dd0aa8..3db90a9aa61d 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -94,6 +94,7 @@
 
 #include "dev.h"
 #include "devmem.h"
+#include "net-sysfs.h"
 #include "netmem_priv.h"
 #include "sock_destructor.h"
 
@@ -1519,7 +1520,8 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
 
 	DEBUG_NET_WARN_ON_ONCE(!in_softirq());
 
-	if (skb->alloc_cpu != smp_processor_id() && !skb_shared(skb)) {
+	if (static_branch_likely(&napi_consume_skb_defer_key) &&
+	    skb->alloc_cpu != smp_processor_id() && !skb_shared(skb)) {
 		skb_release_head_state(skb);
 		return skb_attempt_defer_free(skb);
 	}
@@ -7257,6 +7259,7 @@ static void kfree_skb_napi_cache(struct sk_buff *skb)
 }
 
 DEFINE_STATIC_KEY_FALSE(skb_defer_disable_key);
+DEFINE_STATIC_KEY_TRUE(napi_consume_skb_defer_key);
 
 /**
  * skb_attempt_defer_free - queue skb for remote freeing
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index b508618bfc12..e85ce10afa1f 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -676,6 +676,13 @@ static struct ctl_table net_core_table[] = {
 		.proc_handler	= proc_do_skb_defer_max,
 		.extra1		= SYSCTL_ZERO,
 	},
+	{
+		.procname	= "napi_consume_skb_defer",
+		.data		= &napi_consume_skb_defer_key.key,
+		.maxlen		= sizeof(napi_consume_skb_defer_key),
+		.mode		= 0644,
+		.proc_handler	= proc_do_static_key,
+	},
 };
 
 static struct ctl_table netns_core_table[] = {
-- 
2.41.3