sch_cake: Make gso-splitting configurable from userspace

This patch restores cake's deployed behavior at line rate to always
split gso, and makes gso splitting configurable from userspace.

running cake unlimited (unshaped) at 1gigE, local traffic:

no-split-gso bql limit: 131966
split-gso bql limit:   ~42392-45420

On this 4 stream test splitting gso apart results in halving the
observed interpacket latency at no loss in throughput.

Summary of tcp_nup test run 'gso-split' (at 2018-07-26 16:03:51.824728):

 Ping (ms) ICMP :         0.83         0.81 ms              341
 TCP upload avg :       235.43       235.39 Mbits/s         301
 TCP upload sum :       941.71       941.56 Mbits/s         301
 TCP upload::1  :       235.45       235.43 Mbits/s         271
 TCP upload::2  :       235.45       235.41 Mbits/s         289
 TCP upload::3  :       235.40       235.40 Mbits/s         288
 TCP upload::4  :       235.41       235.40 Mbits/s         291

verses

Summary of tcp_nup test run 'no-split-gso' (at 2018-07-26 16:37:23.563960):

                           avg       median          # data pts
 Ping (ms) ICMP :         1.67         1.73 ms              348
 TCP upload avg :       234.56       235.37 Mbits/s         301
 TCP upload sum :       938.24       941.49 Mbits/s         301
 TCP upload::1  :       234.55       235.38 Mbits/s         285
 TCP upload::2  :       234.57       235.37 Mbits/s         286
 TCP upload::3  :       234.58       235.37 Mbits/s         274
 TCP upload::4  :       234.54       235.42 Mbits/s         288
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_cake.c

@@ -80,7 +80,6 @@
 #define CAKE_QUEUES (1024)
 #define CAKE_FLOW_MASK 63
 #define CAKE_FLOW_NAT_FLAG 64
-#define CAKE_SPLIT_GSO_THRESHOLD (125000000) /* 1Gbps */
 
 /* struct cobalt_params - contains codel and blue parameters
  * @interval:	codel initial drop rate
@@ -2569,10 +2568,12 @@ static int cake_change(struct Qdisc *sch, struct nlattr *opt,
 	if (tb[TCA_CAKE_MEMORY])
 		q->buffer_config_limit = nla_get_u32(tb[TCA_CAKE_MEMORY]);
 
-	if (q->rate_bps && q->rate_bps <= CAKE_SPLIT_GSO_THRESHOLD)
-		q->rate_flags |= CAKE_FLAG_SPLIT_GSO;
-	else
-		q->rate_flags &= ~CAKE_FLAG_SPLIT_GSO;
+	if (tb[TCA_CAKE_SPLIT_GSO]) {
+		if (!!nla_get_u32(tb[TCA_CAKE_SPLIT_GSO]))
+			q->rate_flags |= CAKE_FLAG_SPLIT_GSO;
+		else
+			q->rate_flags &= ~CAKE_FLAG_SPLIT_GSO;
+	}
 
 	if (q->tins) {
 		sch_tree_lock(sch);
@@ -2608,7 +2609,7 @@ static int cake_init(struct Qdisc *sch, struct nlattr *opt,
 	q->target   =   5000; /* 5ms: codel RFC argues
 			       * for 5 to 10% of interval
 			       */
-
+	q->rate_flags |= CAKE_FLAG_SPLIT_GSO;
 	q->cur_tin = 0;
 	q->cur_flow  = 0;