[PATCH net-next v2] net: add SO_BYPASS_RFS to exclude a socket from RFS and aRFS

From: Vitaly Zuevsky <hidden>
Date: 2026-11-09 01:45:54
Also in: linux-alpha, linux-arch, linux-doc, linux-mips, lkml, sparclinux

RFS and accelerated RFS are enabled per host and per rx queue, but
whether steering helps is a property of the flow. A host that carries
a few long-lived flows which benefit from steering alongside a large
churn of short-lived flows which arrive on the right CPU already has
no way to express that today.

Add a boolean socket option, SO_BYPASS_RFS. While set,
sock_rps_record_flow() leaves the socket out of rps_sock_flow_table,
so get_rps_cpu() finds no match and neither software RFS nor
ndo_rx_flow_steer() acts on the flow; it stays on the CPU chosen by
RSS or RPS.

The bit lives in sock_common next to skc_state, in the space reclaimed
by commit 13df9cc6fc0b ("net: Shrink skc_reuse:4 to 2 bits."), so the
test shares the cache line the record path already reads and
sock_copy() carries it to accepted children: setting it on a listening
socket excludes every connection accepted from it, while sockets the
same process connects itself remain eligible.

Setting the option clears the socket's entry, so a flow recorded
earlier stops being steered with the next packet. Closing a bypassing
socket leaves the table alone: a close clears its slot without checking
who last wrote it, and a churn of bypassing sockets could evict
the hints of the steered flows.

Both tests sit behind the rfs_needed static branch, so hosts without
RFS pay nothing.

Signed-off-by: Vitaly Zuevsky <redacted>
---
v2:
 - fold the documentation into this patch
 - rename SO_NO_RFS to SO_BYPASS_RFS (Kuniyuki Iwashima)
 - keep the state in a sock_common bit instead of sk_flags (Kuniyuki)
 - test it in the rps.h wrappers, next to rfs_is_needed()
 - clear the table entry when the option is set on a recorded socket
 - drop the comments and the unlikely()
 - guard the sockopt with CONFIG_RPS
v1: https://lore.kernel.org/netdev/20261031113330.45704-1-vitaly.zuevsky@gcore.com/ (local)

 Documentation/networking/scaling.rst  | 15 +++++++++++++++
 arch/alpha/include/uapi/asm/socket.h  |  2 ++
 arch/mips/include/uapi/asm/socket.h   |  2 ++
 arch/parisc/include/uapi/asm/socket.h |  2 ++
 arch/sparc/include/uapi/asm/socket.h  |  2 ++
 include/net/rps.h                     |  4 ++--
 include/net/sock.h                    |  5 ++++-
 include/uapi/asm-generic/socket.h     |  2 ++
 net/core/sock.c                       | 15 +++++++++++++++
 9 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/Documentation/networking/scaling.rst b/Documentation/networking/scaling.rst
index 6c9836000a8d..b23c87405622 100644
--- a/Documentation/networking/scaling.rst
+++ b/Documentation/networking/scaling.rst

@@ -428,6 +428,21 @@ queues. So for instance, if rps_sock_flow_entries is set to 131072 and there
 are 16 configured receive queues, rps_flow_cnt for each queue might be
 configured as 8192.
 
+Excluding individual sockets
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+RFS is configured per host and per queue, but whether it pays off
+depends on the traffic pattern. SO_BYPASS_RFS keeps a socket's flow
+out of rps_sock_flow_table, so neither RFS nor accelerated RFS acts on
+it: packets stay on the CPU chosen by RSS or RPS, and no filter is
+programmed into the NIC. The option is inherited through accept(), so
+a listening socket that sets it excludes every connection it accepts
+while the sockets the process connects itself remain steered. It may
+be set at any time; an entry already recorded is removed. This suits
+a host carrying a few long-lived flows worth steering alongside a
+churn of short-lived ones for which the cross-CPU enqueue, the wakeup
+and the filter update cost more than they save.
+
 
 Accelerated RFS
 ===============

diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
index 5d3524c26b2b..4df4e1c520fb 100644
--- a/arch/alpha/include/uapi/asm/socket.h
+++ b/arch/alpha/include/uapi/asm/socket.h

@@ -161,6 +161,8 @@
 
 #define SO_PEERPIDFD_THREAD    87
 
+#define SO_BYPASS_RFS          88
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64

diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
index 245a43f52fb1..e1e9d34afdb3 100644
--- a/arch/mips/include/uapi/asm/socket.h
+++ b/arch/mips/include/uapi/asm/socket.h

@@ -172,6 +172,8 @@
 
 #define SO_PEERPIDFD_THREAD    87
 
+#define SO_BYPASS_RFS          88
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64

diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
index f23710e1c671..c38445422129 100644
--- a/arch/parisc/include/uapi/asm/socket.h
+++ b/arch/parisc/include/uapi/asm/socket.h

@@ -153,6 +153,8 @@
 
 #define SO_PEERPIDFD_THREAD	0x4055
 
+#define SO_BYPASS_RFS		0x4056
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64

diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
index b35b25bdefc2..f0cf0a3d2916 100644
--- a/arch/sparc/include/uapi/asm/socket.h
+++ b/arch/sparc/include/uapi/asm/socket.h

@@ -154,6 +154,8 @@
 
 #define SO_PEERPIDFD_THREAD      0x0060
 
+#define SO_BYPASS_RFS            0x0061
+
 #if !defined(__KERNEL__)

diff --git a/include/net/rps.h b/include/net/rps.h
index e33c6a2fa8bb..7c62ae607d18 100644
--- a/include/net/rps.h
+++ b/include/net/rps.h

@@ -149,7 +149,7 @@ static inline void sock_rps_record_flow_hash(__u32 hash)
 static inline void sock_rps_record_flow(const struct sock *sk)
 {
 #ifdef CONFIG_RPS
-	if (!rfs_is_needed())
+	if (!rfs_is_needed() || sk->sk_bypass_rfs)
 		return;
 
 	_sock_rps_record_flow(sk);

@@ -159,7 +159,7 @@ static inline void sock_rps_record_flow(const struct sock *sk)
 static inline void sock_rps_delete_flow(const struct sock *sk)
 {
 #ifdef CONFIG_RPS
-	if (!rfs_is_needed())
+	if (!rfs_is_needed() || sk->sk_bypass_rfs)
 		return;
 
 	_sock_rps_delete_flow(sk);

diff --git a/include/net/sock.h b/include/net/sock.h
index 9bba1e793f1e..a52db61775d8 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h

@@ -124,6 +124,7 @@ typedef __u64 __bitwise __addrpair;
  *	@skc_ipv6only: socket is IPV6 only
  *	@skc_net_refcnt: socket is using net ref counting
  *	@skc_bypass_prot_mem: bypass the per-protocol memory accounting for skb
+ *	@skc_bypass_rfs: %SO_BYPASS_RFS setting
  *	@skc_unused: unused bits
  *	@skc_bound_dev_if: bound device index if != 0
  *	@skc_bind_node: bind hash linkage for various protocol lookup tables

@@ -182,7 +183,8 @@ struct sock_common {
 	unsigned char		skc_ipv6only:1;
 	unsigned char		skc_net_refcnt:1;
 	unsigned char		skc_bypass_prot_mem:1;
-	unsigned char		skc_unused:2;
+	unsigned char		skc_bypass_rfs:1;
+	unsigned char		skc_unused:1;
 	int			skc_bound_dev_if;
 	union {
 		struct hlist_node	skc_bind_node;

@@ -395,6 +397,7 @@ struct sock {
 #define sk_ipv6only		__sk_common.skc_ipv6only
 #define sk_net_refcnt		__sk_common.skc_net_refcnt
 #define sk_bypass_prot_mem	__sk_common.skc_bypass_prot_mem
+#define sk_bypass_rfs		__sk_common.skc_bypass_rfs
 #define sk_bound_dev_if		__sk_common.skc_bound_dev_if
 #define sk_bind_node		__sk_common.skc_bind_node
 #define sk_prot			__sk_common.skc_prot

diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index 56fed7ab27ab..6f6987cb8a53 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h

@@ -156,6 +156,8 @@
 
 #define SO_PEERPIDFD_THREAD	87
 
+#define SO_BYPASS_RFS		88
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))

diff --git a/net/core/sock.c b/net/core/sock.c
index e2401d735c5a..00808217aebf 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -144,6 +144,7 @@
 #include <trace/events/sock.h>
 
 #include <net/psp.h>
+#include <net/rps.h>
 #include <net/tcp.h>
 #include <net/busy_poll.h>
 #include <net/phonet/phonet.h>

@@ -1597,6 +1598,14 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
 		reuseport_update_incoming_cpu(sk, val);
 		break;
 
+#ifdef CONFIG_RPS
+	case SO_BYPASS_RFS:
+		if (valbool)
+			sock_rps_delete_flow(sk);
+		sk->sk_bypass_rfs = valbool;
+		break;
+#endif
+
 	case SO_CNX_ADVICE:
 		if (val == 1)
 			dst_negative_advice(sk);

@@ -2118,6 +2127,12 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		v.val = READ_ONCE(sk->sk_incoming_cpu);
 		break;
 
+#ifdef CONFIG_RPS
+	case SO_BYPASS_RFS:
+		v.val = sk->sk_bypass_rfs;
+		break;
+#endif
+
 	case SO_MEMINFO:
 	{
 		u32 meminfo[SK_MEMINFO_VARS];

base-commit: fa5196392691051963726787944db872420dfa75
-- 
2.34.1

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