tcp: use one bit in TCP_SKB_CB to mark ACK timestamps (6b084928) · Commits · 戴 / test

Currently, to avoid a cache line miss for accessing skb_shinfo, tcp_ack_tstamp skips socket that do not have SOF_TIMESTAMPING_TX_ACK bit set in sk_tsflags. This is implemented based on an implicit assumption that the SOF_TIMESTAMPING_TX_ACK is set via socket options for the duration that ACK timestamps are needed. To implement per-write timestamps, this check should be removed and replaced with a per-packet alternative that quickly skips packets missing ACK timestamps marks without a cache-line miss. To enable per-packet marking without a cache line miss, use one bit in TCP_SKB_CB to mark a whether a SKB might need a ack tx timestamp or not. Further checks in tcp_ack_tstamp are not modified and work as before. Signed-off-by:

Soheil Hassas Yeganeh <soheil@google.com> Acked-by:

Willem de Bruijn <willemb@google.com> Acked-by:

Eric Dumazet <edumazet@google.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

include/net/tcp.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -754,7 +754,8 @@ struct tcp_skb_cb {
		TCPCB_REPAIRED)

		__u8 ip_dsfield; /* IPv4 tos or IPv6 dsfield */
		/* 1 byte hole */
		__u8 txstamp_ack:1, /* Record TX timestamp for ack? */
		unused:7;
		__u32 ack_seq; /* Sequence number ACK'd */
		union {
		struct inet_skb_parm h4;

net/ipv4/tcp.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -432,10 +432,12 @@ static void tcp_tx_timestamp(struct sock sk, struct sk_buff skb)
		{
		if (sk->sk_tsflags) {
		struct skb_shared_info *shinfo = skb_shinfo(skb);
		struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);

		sock_tx_timestamp(sk, &shinfo->tx_flags);
		if (shinfo->tx_flags & SKBTX_ANY_TSTAMP)
		shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
		tcb->txstamp_ack = !!(shinfo->tx_flags & SKBTX_ACK_TSTAMP);
		}
		}

net/ipv4/tcp_input.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -3082,7 +3082,7 @@ static void tcp_ack_tstamp(struct sock sk, struct sk_buff skb,
		const struct skb_shared_info *shinfo;

		/* Avoid cache line misses to get skb_shinfo() and shinfo->tx_flags */
		if (likely(!(sk->sk_tsflags & SOF_TIMESTAMPING_TX_ACK)))
		if (likely(!TCP_SKB_CB(skb)->txstamp_ack))
		return;

		shinfo = skb_shinfo(skb);

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