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, 2 insertions, 2 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 5424a4077c27..0952d4b772e7 100644
--- 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;