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tcp: use one bit in TCP_SKB_CB to mark ACK timestamps

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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>
This commit is contained in:
Soheil Hassas Yeganeh 2016-04-02 23:08:08 -04:00 committed by David S. Miller
parent 6db8b963a7
commit 6b084928ba
3 changed files with 5 additions and 2 deletions
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3
include/net/tcp.h
View file

@ -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;

2
net/ipv4/tcp.c
View file

@ -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);
}
}

2
net/ipv4/tcp_input.c
View file

@ -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);