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 <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Van Jacobson <vanj@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 00a4149..a12b455 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -68,6 +68,7 @@
#include <linux/module.h>
#include <linux/sysctl.h>
#include <linux/kernel.h>
+#include <linux/prefetch.h>
#include <net/dst.h>
#include <net/tcp.h>
#include <net/inet_common.h>
@@ -3029,6 +3030,21 @@
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 @@
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 @@
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 @@
* 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 @@
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 @@
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 @@
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.
*/
