Commit 15492700 authored Mar 12, 2025 by Eric Dumazet Committed by Paolo Abeni Mar 18, 2025

tcp: cache RTAX_QUICKACK metric in a hot cache line



tcp_in_quickack_mode() is called from input path for small packets.

It calls __sk_dst_get() which reads sk->sk_dst_cache which has been
put in sock_read_tx group (for good reasons).

Then dst_metric(dst, RTAX_QUICKACK) also needs extra cache line misses.

Cache RTAX_QUICKACK in icsk->icsk_ack.dst_quick_ack to no longer pull
these cache lines for the cases a delayed ACK is scheduled.

After this patch TCP receive path does not longer access sock_read_tx
group.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250312083907.1931644-1-edumazet@google.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>

parent 3c6b97a9

include/net/inet_connection_sock.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -117,7 +117,8 @@ struct inet_connection_sock {
		#define ATO_BITS 8
		__u32 ato:ATO_BITS, /* Predicted tick of soft clock */
		lrcv_flowlabel:20, /* last received ipv6 flowlabel */
		unused:4;
		dst_quick_ack:1, /* cache dst RTAX_QUICKACK */
		unused:3;
		unsigned long timeout; /* Currently scheduled timeout */
		__u32 lrcvtime; /* timestamp of last received data packet */
		__u16 last_seg_size; /* Size of last incoming segment */

net/core/sock.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -2565,8 +2565,12 @@ void sk_setup_caps(struct sock sk, struct dst_entry dst)
		u32 max_segs = 1;

		sk->sk_route_caps = dst->dev->features;
		if (sk_is_tcp(sk))
		if (sk_is_tcp(sk)) {
		struct inet_connection_sock *icsk = inet_csk(sk);

		sk->sk_route_caps \|= NETIF_F_GSO;
		icsk->icsk_ack.dst_quick_ack = dst_metric(dst, RTAX_QUICKACK);
		}
		if (sk->sk_route_caps & NETIF_F_GSO)
		sk->sk_route_caps \|= NETIF_F_GSO_SOFTWARE;
		if (unlikely(sk->sk_gso_disabled))

net/ipv4/tcp_input.c

+1 −2

Original line number	Diff line number	Diff line
		@@ -334,9 +334,8 @@ static void tcp_enter_quickack_mode(struct sock *sk, unsigned int max_quickacks)
		static bool tcp_in_quickack_mode(struct sock *sk)
		{
		const struct inet_connection_sock *icsk = inet_csk(sk);
		const struct dst_entry *dst = __sk_dst_get(sk);

		return (dst && dst_metric(dst, RTAX_QUICKACK)) \|\|
		return icsk->icsk_ack.dst_quick_ack \|\|
		(icsk->icsk_ack.quick && !inet_csk_in_pingpong_mode(sk));
		}