* [PATCH net-next] net: add sysctl to toggle napi_consume_skb() alien skb defer
@ 2026-03-26 14:42 Jason Xing
2026-03-26 14:55 ` Eric Dumazet
0 siblings, 1 reply; 2+ messages in thread
From: Jason Xing @ 2026-03-26 14:42 UTC (permalink / raw)
To: davem, edumazet, kuba, pabeni, horms; +Cc: netdev, Jason Xing
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>
---
net/core/net-sysfs.h | 1 +
net/core/skbuff.c | 5 ++++-
net/core/sysctl_net_core.c | 35 +++++++++++++++++++++++++++++++++++
3 files changed, 40 insertions(+), 1 deletion(-)
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..33e8217ee1ce 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -372,6 +372,35 @@ static int proc_do_skb_defer_max(const struct ctl_table *table, int write,
return ret;
}
+static int proc_do_napi_consume_skb_defer(const struct ctl_table *table,
+ int write, void *buffer,
+ size_t *lenp, loff_t *ppos)
+{
+ static DEFINE_MUTEX(napi_consume_skb_defer_mutex);
+ int val, ret;
+
+ mutex_lock(&napi_consume_skb_defer_mutex);
+
+ val = static_key_enabled(&napi_consume_skb_defer_key);
+ struct ctl_table tmp = {
+ .data = &val,
+ .maxlen = sizeof(val),
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_ONE,
+ };
+
+ ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+ if (!ret && write) {
+ if (val)
+ static_branch_enable(&napi_consume_skb_defer_key);
+ else
+ static_branch_disable(&napi_consume_skb_defer_key);
+ }
+
+ mutex_unlock(&napi_consume_skb_defer_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,
@@ -676,6 +705,12 @@ static struct ctl_table net_core_table[] = {
.proc_handler = proc_do_skb_defer_max,
.extra1 = SYSCTL_ZERO,
},
+ {
+ .procname = "napi_consume_skb_defer",
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_do_napi_consume_skb_defer,
+ },
};
static struct ctl_table netns_core_table[] = {
--
2.41.3
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH net-next] net: add sysctl to toggle napi_consume_skb() alien skb defer
2026-03-26 14:42 [PATCH net-next] net: add sysctl to toggle napi_consume_skb() alien skb defer Jason Xing
@ 2026-03-26 14:55 ` Eric Dumazet
0 siblings, 0 replies; 2+ messages in thread
From: Eric Dumazet @ 2026-03-26 14:55 UTC (permalink / raw)
To: Jason Xing; +Cc: davem, kuba, pabeni, horms, netdev
On Thu, Mar 26, 2026 at 7:43 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> 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>
> ---
> net/core/net-sysfs.h | 1 +
> net/core/skbuff.c | 5 ++++-
> net/core/sysctl_net_core.c | 35 +++++++++++++++++++++++++++++++++++
> 3 files changed, 40 insertions(+), 1 deletion(-)
>
> 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..33e8217ee1ce 100644
> --- a/net/core/sysctl_net_core.c
> +++ b/net/core/sysctl_net_core.c
> @@ -372,6 +372,35 @@ static int proc_do_skb_defer_max(const struct ctl_table *table, int write,
> return ret;
> }
>
> +static int proc_do_napi_consume_skb_defer(const struct ctl_table *table,
> + int write, void *buffer,
> + size_t *lenp, loff_t *ppos)
> +{
> + static DEFINE_MUTEX(napi_consume_skb_defer_mutex);
> + int val, ret;
> +
> + mutex_lock(&napi_consume_skb_defer_mutex);
> +
> + val = static_key_enabled(&napi_consume_skb_defer_key);
> + struct ctl_table tmp = {
> + .data = &val,
> + .maxlen = sizeof(val),
> + .extra1 = SYSCTL_ZERO,
> + .extra2 = SYSCTL_ONE,
> + };
> +
> + ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
> + if (!ret && write) {
> + if (val)
> + static_branch_enable(&napi_consume_skb_defer_key);
> + else
> + static_branch_disable(&napi_consume_skb_defer_key);
> + }
> +
> + mutex_unlock(&napi_consume_skb_defer_mutex);
> + return ret;
> +}
> +
Seems a copy/paste of proc_do_static_key() ?
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