tcp: implement coalescing on backlog queue

In case GRO is not as efficient as it should be or disabled,
we might have a user thread trapped in __release_sock() while
softirq handler flood packets up to the point we have to drop.

This patch balances work done from user thread and softirq,
to give more chances to __release_sock() to complete its work
before new packets are added the the backlog.

This also helps if we receive many ACK packets, since GRO
does not aggregate them.

This patch brings ~60% throughput increase on a receiver
without GRO, but the spectacular gain is really on
1000x release_sock() latency reduction I have measured.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 86 insertions, 6 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 795605a23275..4904250a9aac 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1619,12 +1619,14 @@ int tcp_v4_early_demux(struct sk_buff *skb)
 bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
         u32 limit = sk->sk_rcvbuf + sk->sk_sndbuf;
-
-        /* Only socket owner can try to collapse/prune rx queues
-         * to reduce memory overhead, so add a little headroom here.
-         * Few sockets backlog are possibly concurrently non empty.
-         */
-        limit += 64*1024;
+        struct skb_shared_info *shinfo;
+        const struct tcphdr *th;
+        struct tcphdr *thtail;
+        struct sk_buff *tail;
+        unsigned int hdrlen;
+        bool fragstolen;
+        u32 gso_segs;
+        int delta;
 
         /* In case all data was pulled from skb frags (in __pskb_pull_tail()),
          * we can fix skb->truesize to its real value to avoid future drops.
@@ -1636,6 +1638,84 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
 
         skb_dst_drop(skb);
 
+        if (unlikely(tcp_checksum_complete(skb))) {
+                bh_unlock_sock(sk);
+                __TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
+                __TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
+                return true;
+        }
+
+        /* Attempt coalescing to last skb in backlog, even if we are
+         * above the limits.
+         * This is okay because skb capacity is limited to MAX_SKB_FRAGS.
+         */
+        th = (const struct tcphdr *)skb->data;
+        hdrlen = th->doff * 4;
+        shinfo = skb_shinfo(skb);
+
+        if (!shinfo->gso_size)
+                shinfo->gso_size = skb->len - hdrlen;
+
+        if (!shinfo->gso_segs)
+                shinfo->gso_segs = 1;
+
+        tail = sk->sk_backlog.tail;
+        if (!tail)
+                goto no_coalesce;
+        thtail = (struct tcphdr *)tail->data;
+
+        if (TCP_SKB_CB(tail)->end_seq != TCP_SKB_CB(skb)->seq ||
+            TCP_SKB_CB(tail)->ip_dsfield != TCP_SKB_CB(skb)->ip_dsfield ||
+            ((TCP_SKB_CB(tail)->tcp_flags |
+              TCP_SKB_CB(skb)->tcp_flags) & TCPHDR_URG) ||
+            ((TCP_SKB_CB(tail)->tcp_flags ^
+              TCP_SKB_CB(skb)->tcp_flags) & (TCPHDR_ECE | TCPHDR_CWR)) ||
+#ifdef CONFIG_TLS_DEVICE
+            tail->decrypted != skb->decrypted ||
+#endif
+            thtail->doff != th->doff ||
+            memcmp(thtail + 1, th + 1, hdrlen - sizeof(*th)))
+                goto no_coalesce;
+
+        __skb_pull(skb, hdrlen);
+        if (skb_try_coalesce(tail, skb, &fragstolen, &delta)) {
+                thtail->window = th->window;
+
+                TCP_SKB_CB(tail)->end_seq = TCP_SKB_CB(skb)->end_seq;
+
+                if (after(TCP_SKB_CB(skb)->ack_seq, TCP_SKB_CB(tail)->ack_seq))
+                        TCP_SKB_CB(tail)->ack_seq = TCP_SKB_CB(skb)->ack_seq;
+
+                TCP_SKB_CB(tail)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
+
+                if (TCP_SKB_CB(skb)->has_rxtstamp) {
+                        TCP_SKB_CB(tail)->has_rxtstamp = true;
+                        tail->tstamp = skb->tstamp;
+                        skb_hwtstamps(tail)->hwtstamp = skb_hwtstamps(skb)->hwtstamp;
+                }
+
+                /* Not as strict as GRO. We only need to carry mss max value */
+                skb_shinfo(tail)->gso_size = max(shinfo->gso_size,
+                                                 skb_shinfo(tail)->gso_size);
+
+                gso_segs = skb_shinfo(tail)->gso_segs + shinfo->gso_segs;
+                skb_shinfo(tail)->gso_segs = min_t(u32, gso_segs, 0xFFFF);
+
+                sk->sk_backlog.len += delta;
+                __NET_INC_STATS(sock_net(sk),
+                                LINUX_MIB_TCPBACKLOGCOALESCE);
+                kfree_skb_partial(skb, fragstolen);
+                return false;
+        }
+        __skb_push(skb, hdrlen);
+
+no_coalesce:
+        /* Only socket owner can try to collapse/prune rx queues
+         * to reduce memory overhead, so add a little headroom here.
+         * Few sockets backlog are possibly concurrently non empty.
+         */
+        limit += 64*1024;
+
         if (unlikely(sk_add_backlog(sk, skb, limit))) {
                 bh_unlock_sock(sk);
                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPBACKLOGDROP);