[PATCH net-next] tcp: prepare skbs for better sack shifting
From: Eric Dumazet <hidden>
Date: 2016-09-15 16:33:05
From: Eric Dumazet <redacted> With large BDP TCP flows and lossy networks, it is very important to keep a low number of skbs in the write queue. RACK and SACK processing can perform a linear scan of it. We should avoid putting any payload in skb->head, so that SACK shifting can be done if needed. With this patch, we allow to pack ~0.5 MB per skb instead of the 64KB initially cooked at tcp_sendmsg() time. This gives a reduction of number of skbs in write queue by eight. tcp_rack_detect_loss() likes this. We still allow payload in skb->head for first skb put in the queue, to not impact RPC workloads. Signed-off-by: Eric Dumazet <redacted> Cc: Yuchung Cheng <redacted> --- net/ipv4/tcp.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index a13fcb369f52fe85def7c9d856259bc0509f3453..7dae800092e62cec330544851289d20a68642561 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c@@ -1020,17 +1020,31 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset, } EXPORT_SYMBOL(tcp_sendpage); -static inline int select_size(const struct sock *sk, bool sg) +/* Do not bother using a page frag for very small frames. + * But use this heuristic only for the first skb in write queue. + * + * Having no payload in skb->head allows better SACK shifting + * in tcp_shift_skb_data(), reducing sack/rack overhead, because + * write queue has less skbs. + * Each skb can hold up to MAX_SKB_FRAGS * 32Kbytes, or ~0.5 MB. + * This also speeds up tso_fragment(), since it wont fallback + * to tcp_fragment(). + */ +static int linear_payload_sz(bool first_skb) +{ + if (first_skb) + return SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER); + return 0; +} + +static int select_size(const struct sock *sk, bool sg, bool first_skb) { const struct tcp_sock *tp = tcp_sk(sk); int tmp = tp->mss_cache; if (sg) { if (sk_can_gso(sk)) { - /* Small frames wont use a full page: - * Payload will immediately follow tcp header. - */ - tmp = SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER); + tmp = linear_payload_sz(first_skb); } else { int pgbreak = SKB_MAX_HEAD(MAX_TCP_HEADER);
@@ -1161,6 +1175,8 @@ restart: } if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) { + bool first_skb; + new_segment: /* Allocate new segment. If the interface is SG, * allocate skb fitting to single page.
@@ -1172,10 +1188,11 @@ new_segment: process_backlog = false; goto restart; } + first_skb = skb_queue_empty(&sk->sk_write_queue); skb = sk_stream_alloc_skb(sk, - select_size(sk, sg), + select_size(sk, sg, first_skb), sk->sk_allocation, - skb_queue_empty(&sk->sk_write_queue)); + first_skb); if (!skb) goto wait_for_memory;