From afb83012cc7236c8f5cefbd0fd4ba628ec34ce02 Mon Sep 17 00:00:00 2001 From: Soheil Hassas Yeganeh Date: Mon, 14 Sep 2020 17:52:10 -0400 Subject: tcp: schedule EPOLLOUT after a partial sendmsg For EPOLLET, applications must call sendmsg until they get EAGAIN. Otherwise, there is no guarantee that EPOLLOUT is sent if there was a failure upon memory allocation. As a result on high-speed NICs, userspace observes multiple small sendmsgs after a partial sendmsg until EAGAIN, since TCP can send 1-2 TSOs in between two sendmsg syscalls: // One large partial send due to memory allocation failure. sendmsg(20MB) = 2MB // Many small sends until EAGAIN. sendmsg(18MB) = 64KB sendmsg(17.9MB) = 128KB sendmsg(17.8MB) = 64KB ... sendmsg(...) = EAGAIN // At this point, userspace can assume an EPOLLOUT. To fix this, set the SOCK_NOSPACE on all partial sendmsg scenarios to guarantee that we send EPOLLOUT after partial sendmsg. After this commit userspace can assume that it will receive an EPOLLOUT after the first partial sendmsg. This EPOLLOUT will benefit from sk_stream_write_space() logic delaying the EPOLLOUT until significant space is available in write queue. Signed-off-by: Eric Dumazet Signed-off-by: Soheil Hassas Yeganeh Signed-off-by: David S. Miller --- net/ipv4/tcp.c | 20 +++++++++----------- 1 file changed, 9 insertions(+), 11 deletions(-) diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 48c351804efc..65057744fac8 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -1004,12 +1004,12 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset, !tcp_skb_can_collapse_to(skb)) { new_segment: if (!sk_stream_memory_free(sk)) - goto wait_for_sndbuf; + goto wait_for_space; skb = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, tcp_rtx_and_write_queues_empty(sk)); if (!skb) - goto wait_for_memory; + goto wait_for_space; #ifdef CONFIG_TLS_DEVICE skb->decrypted = !!(flags & MSG_SENDPAGE_DECRYPTED); @@ -1028,7 +1028,7 @@ new_segment: goto new_segment; } if (!sk_wmem_schedule(sk, copy)) - goto wait_for_memory; + goto wait_for_space; if (can_coalesce) { skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy); @@ -1069,9 +1069,8 @@ new_segment: tcp_push_one(sk, mss_now); continue; -wait_for_sndbuf: +wait_for_space: set_bit(SOCK_NOSPACE, &sk->sk_socket->flags); -wait_for_memory: tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH, size_goal); @@ -1282,7 +1281,7 @@ restart: new_segment: if (!sk_stream_memory_free(sk)) - goto wait_for_sndbuf; + goto wait_for_space; if (unlikely(process_backlog >= 16)) { process_backlog = 0; @@ -1293,7 +1292,7 @@ new_segment: skb = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, first_skb); if (!skb) - goto wait_for_memory; + goto wait_for_space; process_backlog++; skb->ip_summed = CHECKSUM_PARTIAL; @@ -1326,7 +1325,7 @@ new_segment: struct page_frag *pfrag = sk_page_frag(sk); if (!sk_page_frag_refill(sk, pfrag)) - goto wait_for_memory; + goto wait_for_space; if (!skb_can_coalesce(skb, i, pfrag->page, pfrag->offset)) { @@ -1340,7 +1339,7 @@ new_segment: copy = min_t(int, copy, pfrag->size - pfrag->offset); if (!sk_wmem_schedule(sk, copy)) - goto wait_for_memory; + goto wait_for_space; err = skb_copy_to_page_nocache(sk, &msg->msg_iter, skb, pfrag->page, @@ -1393,9 +1392,8 @@ new_segment: tcp_push_one(sk, mss_now); continue; -wait_for_sndbuf: +wait_for_space: set_bit(SOCK_NOSPACE, &sk->sk_socket->flags); -wait_for_memory: if (copied) tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH, size_goal); -- cgit v1.2.3