From b49d252216e4f9e3030865b79d5ca16f050e4a19 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 11 Sep 2023 17:05:28 +0000 Subject: [PATCH 1/4] tcp: no longer release socket ownership in tcp_release_cb() This partially reverts c3f9b01849ef ("tcp: tcp_release_cb() should release socket ownership"). prequeue has been removed by Florian in commit e7942d0633c4 ("tcp: remove prequeue support") __tcp_checksum_complete_user() being gone, we no longer have to release socket ownership in tcp_release_cb(). This is a prereq for third patch in the series ("net: call prot->release_cb() when processing backlog"). Signed-off-by: Eric Dumazet Signed-off-by: Paolo Abeni --- net/core/sock.c | 3 --- net/ipv4/tcp_output.c | 10 ---------- 2 files changed, 13 deletions(-) diff --git a/net/core/sock.c b/net/core/sock.c index 16584e2dd648..21610e3845a5 100644 --- a/net/core/sock.c +++ b/net/core/sock.c @@ -3519,9 +3519,6 @@ void release_sock(struct sock *sk) if (sk->sk_backlog.tail) __release_sock(sk); - /* Warning : release_cb() might need to release sk ownership, - * ie call sock_release_ownership(sk) before us. - */ if (sk->sk_prot->release_cb) sk->sk_prot->release_cb(sk); diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index ccfc8bbf7455..b4cac12d0e63 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1101,16 +1101,6 @@ void tcp_release_cb(struct sock *sk) tcp_tsq_write(sk); __sock_put(sk); } - /* Here begins the tricky part : - * We are called from release_sock() with : - * 1) BH disabled - * 2) sk_lock.slock spinlock held - * 3) socket owned by us (sk->sk_lock.owned == 1) - * - * But following code is meant to be called from BH handlers, - * so we should keep BH disabled, but early release socket ownership - */ - sock_release_ownership(sk); if (flags & TCPF_WRITE_TIMER_DEFERRED) { tcp_write_timer_handler(sk); From 11445469dec803730ac7f78a5ad2a5ed131fa941 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 11 Sep 2023 17:05:29 +0000 Subject: [PATCH 2/4] net: sock_release_ownership() cleanup sock_release_ownership() should only be called by user owning the socket lock. After prior commit, we can remove one condition. Signed-off-by: Eric Dumazet Signed-off-by: Paolo Abeni --- include/net/sock.h | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) diff --git a/include/net/sock.h b/include/net/sock.h index b770261fbdaf..676146e9d181 100644 --- a/include/net/sock.h +++ b/include/net/sock.h @@ -1823,12 +1823,11 @@ static inline bool sock_owned_by_user_nocheck(const struct sock *sk) static inline void sock_release_ownership(struct sock *sk) { - if (sock_owned_by_user_nocheck(sk)) { - sk->sk_lock.owned = 0; + DEBUG_NET_WARN_ON_ONCE(!sock_owned_by_user_nocheck(sk)); + sk->sk_lock.owned = 0; - /* The sk_lock has mutex_unlock() semantics: */ - mutex_release(&sk->sk_lock.dep_map, _RET_IP_); - } + /* The sk_lock has mutex_unlock() semantics: */ + mutex_release(&sk->sk_lock.dep_map, _RET_IP_); } /* no reclassification while locks are held */ From 4505dc2a522826975167823f64f0896bac1323fb Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 11 Sep 2023 17:05:30 +0000 Subject: [PATCH 3/4] net: call prot->release_cb() when processing backlog __sk_flush_backlog() / sk_flush_backlog() are used when TCP recvmsg()/sendmsg() process large chunks, to not let packets in the backlog too long. It makes sense to call tcp_release_cb() to also process actions held in sk->sk_tsq_flags for smoother scheduling. Signed-off-by: Eric Dumazet Signed-off-by: Paolo Abeni --- net/core/sock.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/core/sock.c b/net/core/sock.c index 21610e3845a5..bb89b88bc1e8 100644 --- a/net/core/sock.c +++ b/net/core/sock.c @@ -3001,6 +3001,9 @@ void __sk_flush_backlog(struct sock *sk) { spin_lock_bh(&sk->sk_lock.slock); __release_sock(sk); + + if (sk->sk_prot->release_cb) + sk->sk_prot->release_cb(sk); spin_unlock_bh(&sk->sk_lock.slock); } EXPORT_SYMBOL_GPL(__sk_flush_backlog); From 133c4c0d37175f510a10fa9bed51e223936073fc Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 11 Sep 2023 17:05:31 +0000 Subject: [PATCH 4/4] tcp: defer regular ACK while processing socket backlog This idea came after a particular workload requested the quickack attribute set on routes, and a performance drop was noticed for large bulk transfers. For high throughput flows, it is best to use one cpu running the user thread issuing socket system calls, and a separate cpu to process incoming packets from BH context. (With TSO/GRO, bottleneck is usually the 'user' cpu) Problem is the user thread can spend a lot of time while holding the socket lock, forcing BH handler to queue most of incoming packets in the socket backlog. Whenever the user thread releases the socket lock, it must first process all accumulated packets in the backlog, potentially adding latency spikes. Due to flood mitigation, having too many packets in the backlog increases chance of unexpected drops. Backlog processing unfortunately shifts a fair amount of cpu cycles from the BH cpu to the 'user' cpu, thus reducing max throughput. This patch takes advantage of the backlog processing, and the fact that ACK are mostly cumulative. The idea is to detect we are in the backlog processing and defer all eligible ACK into a single one, sent from tcp_release_cb(). This saves cpu cycles on both sides, and network resources. Performance of a single TCP flow on a 200Gbit NIC: - Throughput is increased by 20% (100Gbit -> 120Gbit). - Number of generated ACK per second shrinks from 240,000 to 40,000. - Number of backlog drops per second shrinks from 230 to 0. Benchmark context: - Regular netperf TCP_STREAM (no zerocopy) - Intel(R) Xeon(R) Platinum 8481C (Saphire Rapids) - MAX_SKB_FRAGS = 17 (~60KB per GRO packet) This feature is guarded by a new sysctl, and enabled by default: /proc/sys/net/ipv4/tcp_backlog_ack_defer Signed-off-by: Eric Dumazet Acked-by: Yuchung Cheng Acked-by: Neal Cardwell Acked-by: Soheil Hassas Yeganeh Acked-by: Dave Taht Signed-off-by: Paolo Abeni --- Documentation/networking/ip-sysctl.rst | 7 +++++++ include/linux/tcp.h | 14 ++++++++------ include/net/netns/ipv4.h | 1 + net/ipv4/sysctl_net_ipv4.c | 9 +++++++++ net/ipv4/tcp_input.c | 8 ++++++++ net/ipv4/tcp_ipv4.c | 1 + net/ipv4/tcp_output.c | 5 ++++- 7 files changed, 38 insertions(+), 7 deletions(-) diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index a66054d0763a..5bfa1837968c 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -745,6 +745,13 @@ tcp_comp_sack_nr - INTEGER Default : 44 +tcp_backlog_ack_defer - BOOLEAN + If set, user thread processing socket backlog tries sending + one ACK for the whole queue. This helps to avoid potential + long latencies at end of a TCP socket syscall. + + Default : true + tcp_slow_start_after_idle - BOOLEAN If set, provide RFC2861 behavior and time out the congestion window after an idle period. An idle period is defined at diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 3c5efeeb024f..44d946161d4a 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -463,15 +463,17 @@ enum tsq_enum { TCP_MTU_REDUCED_DEFERRED, /* tcp_v{4|6}_err() could not call * tcp_v{4|6}_mtu_reduced() */ + TCP_ACK_DEFERRED, /* TX pure ack is deferred */ }; enum tsq_flags { - TSQF_THROTTLED = (1UL << TSQ_THROTTLED), - TSQF_QUEUED = (1UL << TSQ_QUEUED), - TCPF_TSQ_DEFERRED = (1UL << TCP_TSQ_DEFERRED), - TCPF_WRITE_TIMER_DEFERRED = (1UL << TCP_WRITE_TIMER_DEFERRED), - TCPF_DELACK_TIMER_DEFERRED = (1UL << TCP_DELACK_TIMER_DEFERRED), - TCPF_MTU_REDUCED_DEFERRED = (1UL << TCP_MTU_REDUCED_DEFERRED), + TSQF_THROTTLED = BIT(TSQ_THROTTLED), + TSQF_QUEUED = BIT(TSQ_QUEUED), + TCPF_TSQ_DEFERRED = BIT(TCP_TSQ_DEFERRED), + TCPF_WRITE_TIMER_DEFERRED = BIT(TCP_WRITE_TIMER_DEFERRED), + TCPF_DELACK_TIMER_DEFERRED = BIT(TCP_DELACK_TIMER_DEFERRED), + TCPF_MTU_REDUCED_DEFERRED = BIT(TCP_MTU_REDUCED_DEFERRED), + TCPF_ACK_DEFERRED = BIT(TCP_ACK_DEFERRED), }; #define tcp_sk(ptr) container_of_const(ptr, struct tcp_sock, inet_conn.icsk_inet.sk) diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h index 7a41c4791536..d96d05b08819 100644 --- a/include/net/netns/ipv4.h +++ b/include/net/netns/ipv4.h @@ -132,6 +132,7 @@ struct netns_ipv4 { u8 sysctl_tcp_syncookies; u8 sysctl_tcp_migrate_req; u8 sysctl_tcp_comp_sack_nr; + u8 sysctl_tcp_backlog_ack_defer; int sysctl_tcp_reordering; u8 sysctl_tcp_retries1; u8 sysctl_tcp_retries2; diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index 6ac890b4073f..e7f024d93572 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -1366,6 +1366,15 @@ static struct ctl_table ipv4_net_table[] = { .proc_handler = proc_dou8vec_minmax, .extra1 = SYSCTL_ZERO, }, + { + .procname = "tcp_backlog_ack_defer", + .data = &init_net.ipv4.sysctl_tcp_backlog_ack_defer, + .maxlen = sizeof(u8), + .mode = 0644, + .proc_handler = proc_dou8vec_minmax, + .extra1 = SYSCTL_ZERO, + .extra2 = SYSCTL_ONE, + }, { .procname = "tcp_reflect_tos", .data = &init_net.ipv4.sysctl_tcp_reflect_tos, diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 06fe1cf645d5..41b471748437 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5553,6 +5553,14 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible) tcp_in_quickack_mode(sk) || /* Protocol state mandates a one-time immediate ACK */ inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOW) { + /* If we are running from __release_sock() in user context, + * Defer the ack until tcp_release_cb(). + */ + if (sock_owned_by_user_nocheck(sk) && + READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_backlog_ack_defer)) { + set_bit(TCP_ACK_DEFERRED, &sk->sk_tsq_flags); + return; + } send_now: tcp_send_ack(sk); return; diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 27140e5cdc06..f13eb7e23d03 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -3263,6 +3263,7 @@ static int __net_init tcp_sk_init(struct net *net) net->ipv4.sysctl_tcp_comp_sack_delay_ns = NSEC_PER_MSEC; net->ipv4.sysctl_tcp_comp_sack_slack_ns = 100 * NSEC_PER_USEC; net->ipv4.sysctl_tcp_comp_sack_nr = 44; + net->ipv4.sysctl_tcp_backlog_ack_defer = 1; net->ipv4.sysctl_tcp_fastopen = TFO_CLIENT_ENABLE; net->ipv4.sysctl_tcp_fastopen_blackhole_timeout = 0; atomic_set(&net->ipv4.tfo_active_disable_times, 0); diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index b4cac12d0e63..1fc1f879cfd6 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1077,7 +1077,8 @@ static void tcp_tasklet_func(struct tasklet_struct *t) #define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED | \ TCPF_WRITE_TIMER_DEFERRED | \ TCPF_DELACK_TIMER_DEFERRED | \ - TCPF_MTU_REDUCED_DEFERRED) + TCPF_MTU_REDUCED_DEFERRED | \ + TCPF_ACK_DEFERRED) /** * tcp_release_cb - tcp release_sock() callback * @sk: socket @@ -1114,6 +1115,8 @@ void tcp_release_cb(struct sock *sk) inet_csk(sk)->icsk_af_ops->mtu_reduced(sk); __sock_put(sk); } + if ((flags & TCPF_ACK_DEFERRED) && inet_csk_ack_scheduled(sk)) + tcp_send_ack(sk); } EXPORT_SYMBOL(tcp_release_cb);