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 --- 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