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>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 75dcf4daca72..d40d4cc53319 100644
--- 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) {