From ad971f616aa98ea2503f1a1064637bfb4ef7b21e Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Sat, 11 Oct 2014 15:17:29 -0700 Subject: [PATCH] tcp: fix tcp_ack() performance problem We worked hard to improve tcp_ack() performance, by not accessing skb_shinfo() in fast path (cd7d8498c9a5 tcp: change tcp_skb_pcount() location) We still have one spurious access because of ACK timestamping, added in commit e1c8a607b281 ("net-timestamp: ACK timestamp for bytestreams") By checking if sk_tsflags has SOF_TIMESTAMPING_TX_ACK set, we can avoid two cache line misses for the common case. While we are at it, add two prefetchw() : One in tcp_ack() to bring skb at the head of write queue. One in tcp_clean_rtx_queue() loop to bring following skb, as we will delete skb from the write queue and dirty skb->next->prev. Add a couple of [un]likely() clauses. After this patch, tcp_ack() is no longer the most consuming function in tcp stack. Signed-off-by: Eric Dumazet Cc: Willem de Bruijn Cc: Neal Cardwell Cc: Yuchung Cheng Cc: Van Jacobson Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 36 +++++++++++++++++++++++++++--------- 1 file changed, 27 insertions(+), 9 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 00a41499d52c..a12b455928e5 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -68,6 +68,7 @@ #include #include #include +#include #include #include #include @@ -3029,6 +3030,21 @@ static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb) return packets_acked; } +static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb, + u32 prior_snd_una) +{ + 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))) + return; + + shinfo = skb_shinfo(skb); + if ((shinfo->tx_flags & SKBTX_ACK_TSTAMP) && + between(shinfo->tskey, prior_snd_una, tcp_sk(sk)->snd_una - 1)) + __skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK); +} + /* Remove acknowledged frames from the retransmission queue. If our packet * is before the ack sequence we can discard it as it's confirmed to have * arrived at the other end. @@ -3052,14 +3068,11 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, first_ackt.v64 = 0; while ((skb = tcp_write_queue_head(sk)) && skb != tcp_send_head(sk)) { - struct skb_shared_info *shinfo = skb_shinfo(skb); struct tcp_skb_cb *scb = TCP_SKB_CB(skb); u8 sacked = scb->sacked; u32 acked_pcount; - if (unlikely(shinfo->tx_flags & SKBTX_ACK_TSTAMP) && - between(shinfo->tskey, prior_snd_una, tp->snd_una - 1)) - __skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK); + tcp_ack_tstamp(sk, skb, prior_snd_una); /* Determine how many packets and what bytes were acked, tso and else */ if (after(scb->end_seq, tp->snd_una)) { @@ -3073,10 +3086,12 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, fully_acked = false; } else { + /* Speedup tcp_unlink_write_queue() and next loop */ + prefetchw(skb->next); acked_pcount = tcp_skb_pcount(skb); } - if (sacked & TCPCB_RETRANS) { + if (unlikely(sacked & TCPCB_RETRANS)) { if (sacked & TCPCB_SACKED_RETRANS) tp->retrans_out -= acked_pcount; flag |= FLAG_RETRANS_DATA_ACKED; @@ -3107,7 +3122,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, * connection startup slow start one packet too * quickly. This is severely frowned upon behavior. */ - if (!(scb->tcp_flags & TCPHDR_SYN)) { + if (likely(!(scb->tcp_flags & TCPHDR_SYN))) { flag |= FLAG_DATA_ACKED; } else { flag |= FLAG_SYN_ACKED; @@ -3119,9 +3134,9 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, tcp_unlink_write_queue(skb, sk); sk_wmem_free_skb(sk, skb); - if (skb == tp->retransmit_skb_hint) + if (unlikely(skb == tp->retransmit_skb_hint)) tp->retransmit_skb_hint = NULL; - if (skb == tp->lost_skb_hint) + if (unlikely(skb == tp->lost_skb_hint)) tp->lost_skb_hint = NULL; } @@ -3132,7 +3147,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, flag |= FLAG_SACK_RENEGING; skb_mstamp_get(&now); - if (first_ackt.v64) { + if (likely(first_ackt.v64)) { seq_rtt_us = skb_mstamp_us_delta(&now, &first_ackt); ca_seq_rtt_us = skb_mstamp_us_delta(&now, &last_ackt); } @@ -3394,6 +3409,9 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) int acked = 0; /* Number of packets newly acked */ long sack_rtt_us = -1L; + /* We very likely will need to access write queue head. */ + prefetchw(sk->sk_write_queue.next); + /* If the ack is older than previous acks * then we can probably ignore it. */ -- 2.30.2