[PATCH v2] net/rps: consolidate RPS dispatch into helpers
From: Jemmy Wong
Date: Sat Jul 11 2026 - 08:10:44 EST
The RPS steering logic in netif_rx_internal(), netif_receive_skb_internal()
and netif_receive_skb_list_internal() was open-coded three times, each with
its own #ifdef CONFIG_RPS block and manual rcu_read_lock()/unlock() pairs.
Factor it into two helpers, netif_rps() for the single-skb path and
netif_rps_list() for the list path, sharing a common __netif_rps() inner
helper that runs the get_rps_cpu()/enqueue_to_backlog() step under RCU.
Both wrappers keep the early static_branch_unlikely(&rps_needed) check
and only enter an RCU read-side critical section when RPS is actually
enabled at runtime, so no extra rcu_read_lock()/unlock() is executed on
the hot netif_rx_internal() path when RPS is compiled in but disabled.
A new internal NET_RX_UNHANDLED sentinel lets a helper report "RPS did
not take this skb" so the caller falls back to the local enqueue /
__netif_receive_skb() path; it never escapes to callers.
No functional change intended.
Signed-off-by: Jemmy Wong <jemmywong512@xxxxxxxxx>
---
Changes since v1 [1]:
- Move rcu_read_lock()/unlock() inside the
static_branch_unlikely(&rps_needed) check, so no RCU read-side section
is entered when RPS is compiled in but disabled at runtime. Addresses
Paolo's concern that netif_rx_internal() is performance critical and
rcu_read_lock() is not a no-op.
- Drop the guard(rcu) / scoped_guard(rcu) conversion; the RCU section
is now scoped inside netif_rps() / netif_rps_list() themselves.
- Split __netif_rps() out of netif_rps() so that netif_rps_list() can
reuse the per-skb steering step without re-testing the static branch
or dropping and reacquiring the RCU lock per skb.
- netif_rps_list() now returns int; when RPS drains the entire list,
netif_receive_skb_list_internal() skips the __netif_receive_skb_list()
fall-through instead of walking an empty list. The result is decided
directly from list_empty(&undo_list): drop the splice on the drained
path and only splice back when there is something to fall through.
Verified generated code with objdump on aarch64 with CONFIG_RPS=y,
CONFIG_XPS=y, CONFIG_SMP=y, CONFIG_PREEMPT=y (so rcu_read_lock()
expands to a real preempt_disable(), i.e. not a no-op -- the
specific scenario Paolo asked about [2]):
scripts/bloat-o-meter net/core/dev.o (before -> after):
add/remove: 2/2 grow/shrink: 1/2 up/down: 720/-716 (4)
Function old new delta
__netif_rps - 660 +660
netif_receive_skb_list_internal 640 700 +60
netif_rx_internal 236 200 -36
netif_receive_skb 204 136 -68
get_rps_cpu 608 - -608
Total: Before=335841, After=335849, chg +0.00%
netif_rx_internal fast path (!rps_needed, i.e. static branch not
taken -- the path Paolo asked about [2]):
- identical instruction sequence to base, no rcu_read_lock/unlock
- stack frame 16 bytes smaller (0x50 -> 0x40)
- static_branch NOP is preserved
netif_receive_skb_internal: inlined in both builds; caller
netif_receive_skb shrinks by 68 bytes.
netif_receive_skb_list_internal: !rps_needed fast path unchanged;
rps_needed=true path grows +60 bytes for LIST_HEAD(undo_list)
setup, the explicit list_empty(&undo_list)/list_splice_init()
branch and its early NET_RX_SUCCESS return.
[1] https://lore.kernel.org/all/20260702152830.39065-1-jemmywong512@xxxxxxxxx/
[2] https://lore.kernel.org/all/e53482b8-4e1d-4a04-929c-f5cb443ba59d@xxxxxxxxxx/
include/linux/netdevice.h | 5 +-
net/core/dev.c | 118 +++++++++++++++++++++++---------------
2 files changed, 74 insertions(+), 49 deletions(-)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 9981d637f8b5..c265b78082e3 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -93,8 +93,9 @@ void netdev_set_default_ethtool_ops(struct net_device *dev,
void netdev_sw_irq_coalesce_default_on(struct net_device *dev);
/* Backlog congestion levels */
-#define NET_RX_SUCCESS 0 /* keep 'em coming, baby */
-#define NET_RX_DROP 1 /* packet dropped */
+#define NET_RX_UNHANDLED -1
+#define NET_RX_SUCCESS 0
+#define NET_RX_DROP 1
#define MAX_NEST_DEV 8
diff --git a/net/core/dev.c b/net/core/dev.c
index 4b3d5cfdf6e0..1a05ac8fdd50 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5426,6 +5426,65 @@ static int enqueue_to_backlog(struct sk_buff *skb, int cpu,
return NET_RX_DROP;
}
+#ifdef CONFIG_RPS
+static inline int __netif_rps(struct sk_buff *skb)
+{
+ int cpu;
+ int ret = NET_RX_UNHANDLED;
+ struct rps_dev_flow voidflow, *rflow = &voidflow;
+
+ cpu = get_rps_cpu(skb->dev, skb, &rflow);
+ if (cpu >= 0)
+ ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
+
+ return ret;
+}
+#endif
+
+static inline int netif_rps(struct sk_buff *skb)
+{
+ int ret = NET_RX_UNHANDLED;
+
+#ifdef CONFIG_RPS
+ if (!static_branch_unlikely(&rps_needed))
+ return ret;
+
+ rcu_read_lock();
+ ret = __netif_rps(skb);
+ rcu_read_unlock();
+#endif
+
+ return ret;
+}
+
+static inline int netif_rps_list(struct list_head *head)
+{
+ int ret = NET_RX_UNHANDLED;
+
+#ifdef CONFIG_RPS
+ LIST_HEAD(undo_list);
+ struct sk_buff *skb, *next;
+
+ if (!static_branch_unlikely(&rps_needed))
+ return ret;
+
+ rcu_read_lock();
+ list_for_each_entry_safe(skb, next, head, list) {
+ skb_list_del_init(skb);
+ if (__netif_rps(skb) == NET_RX_UNHANDLED)
+ list_add_tail(&skb->list, &undo_list);
+ }
+ rcu_read_unlock();
+
+ if (list_empty(&undo_list))
+ ret = NET_RX_SUCCESS;
+ else
+ list_splice_init(&undo_list, head);
+#endif
+
+ return ret;
+}
+
static struct netdev_rx_queue *netif_get_rxqueue(struct sk_buff *skb)
{
struct net_device *dev = skb->dev;
@@ -5695,34 +5754,18 @@ EXPORT_SYMBOL_GPL(do_xdp_generic);
static int netif_rx_internal(struct sk_buff *skb)
{
+ unsigned int qtail;
int ret;
net_timestamp_check(READ_ONCE(net_hotdata.tstamp_prequeue), skb);
trace_netif_rx(skb);
-#ifdef CONFIG_RPS
- if (static_branch_unlikely(&rps_needed)) {
- struct rps_dev_flow voidflow, *rflow = &voidflow;
- int cpu;
-
- rcu_read_lock();
-
- cpu = get_rps_cpu(skb->dev, skb, &rflow);
- if (cpu < 0)
- cpu = smp_processor_id();
-
- ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
-
- rcu_read_unlock();
- } else
-#endif
- {
- unsigned int qtail;
+ ret = netif_rps(skb);
+ if (ret != NET_RX_UNHANDLED)
+ return ret;
- ret = enqueue_to_backlog(skb, smp_processor_id(), &qtail);
- }
- return ret;
+ return enqueue_to_backlog(skb, smp_processor_id(), &qtail);
}
/**
@@ -6389,19 +6432,11 @@ static int netif_receive_skb_internal(struct sk_buff *skb)
if (skb_defer_rx_timestamp(skb))
return NET_RX_SUCCESS;
- rcu_read_lock();
-#ifdef CONFIG_RPS
- if (static_branch_unlikely(&rps_needed)) {
- struct rps_dev_flow voidflow, *rflow = &voidflow;
- int cpu = get_rps_cpu(skb->dev, skb, &rflow);
+ ret = netif_rps(skb);
+ if (ret != NET_RX_UNHANDLED)
+ return ret;
- if (cpu >= 0) {
- ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
- rcu_read_unlock();
- return ret;
- }
- }
-#endif
+ rcu_read_lock();
ret = __netif_receive_skb(skb);
rcu_read_unlock();
return ret;
@@ -6421,21 +6456,10 @@ void netif_receive_skb_list_internal(struct list_head *head)
}
list_splice_init(&sublist, head);
- rcu_read_lock();
-#ifdef CONFIG_RPS
- if (static_branch_unlikely(&rps_needed)) {
- list_for_each_entry_safe(skb, next, head, list) {
- struct rps_dev_flow voidflow, *rflow = &voidflow;
- int cpu = get_rps_cpu(skb->dev, skb, &rflow);
+ if (netif_rps_list(head) == NET_RX_SUCCESS)
+ return;
- if (cpu >= 0) {
- /* Will be handled, remove from list */
- skb_list_del_init(skb);
- enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
- }
- }
- }
-#endif
+ rcu_read_lock();
__netif_receive_skb_list(head);
rcu_read_unlock();
}
--
2.54.0 (Apple Git-157)