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Commit 6b084928 authored by Soheil Hassas Yeganeh's avatar Soheil Hassas Yeganeh Committed by David S. Miller
Browse files

tcp: use one bit in TCP_SKB_CB to mark ACK timestamps 



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: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: default avatarWillem de Bruijn <willemb@google.com>
Acked-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 6db8b963

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);