tcp: tsq: no longer use limit_output_bytes for paced flows

FQ pacing guarantees that paced packets queued by one flow do not
add head-of-line blocking for other flows.

After TCP GSO conversion, increasing limit_output_bytes to 1 MB is safe,
since this maps to 16 skbs at most in qdisc or device queues.
(or slightly more if some drivers lower {gso_max_segs|size})

We still can queue at most 1 ms worth of traffic (this can be scaled
by wifi drivers if they need to)

Tested:

# ethtool -c eth0 | egrep "tx-usecs:|tx-frames:" # 40 Gbit mlx4 NIC
tx-usecs: 16
tx-frames: 16
# tc qdisc replace dev eth0 root fq
# for f in {1..10};do netperf -P0 -H lpaa24,6 -o THROUGHPUT;done

Before patch:
27711
26118
27107
27377
27712
27388
27340
27117
27278
27509

After patch:
37434
36949
36658
36998
37711
37291
37605
36659
36544
37349

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index aa9e6a3316791d9dea5a2115530c9da801c01fc2..af2a69439b93be51400909b7cecbed776fb8dce1 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -759,7 +759,7 @@ tcp_limit_output_bytes - INTEGER
        flows, for typical pfifo_fast qdiscs.  tcp_limit_output_bytes
        limits the number of bytes on qdisc or device to reduce artificial
        RTT/cwnd and reduce bufferbloat.
-       Default: 262144
+       Default: 1048576 (16 * 65536)
 
 tcp_challenge_ack_limit - INTEGER
        Limits number of Challenge ACK sent per second, as recommended

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 5424a4077c272170ba961858d933d8ba56c6b8a7..0952d4b772e7aa3cfb936596aa586dbd2dd6efde 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2574,8 +2574,8 @@ static int __net_init tcp_sk_init(struct net *net)
         * which are too large can cause TCP streams to be bursty.
         */
        net->ipv4.sysctl_tcp_tso_win_divisor = 3;
-       /* Default TSQ limit of four TSO segments */
-       net->ipv4.sysctl_tcp_limit_output_bytes = 262144;
+       /* Default TSQ limit of 16 TSO segments */
+       net->ipv4.sysctl_tcp_limit_output_bytes = 16 * 65536;
        /* rfc5961 challenge ack rate limiting */
        net->ipv4.sysctl_tcp_challenge_ack_limit = 1000;
        net->ipv4.sysctl_tcp_min_tso_segs = 2;

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 75dcf4daca724a6819e9ecc9d0f3e6dc6df72e9b..d40d4cc533197f4199b6c0eba4c0e9aea5448474 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2220,8 +2220,9 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
        limit = max_t(unsigned long,
                      2 * skb->truesize,
                      sk->sk_pacing_rate >> sk->sk_pacing_shift);
-       limit = min_t(unsigned long, limit,
-                     sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes);
+       if (sk->sk_pacing_status == SK_PACING_NONE)
+               limit = min_t(unsigned long, limit,
+                             sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes);
        limit <<= factor;
 
        if (refcount_read(&sk->sk_wmem_alloc) > limit) {