[PATCH 5.4 016/155] net: cbs: Fix software cbs to consider packet sending time

[Greg Kroah-Hartman]

From: Zh-yuan Ye <ye.zh-yuan@xxxxxxxxxxxxx>

[ Upstream commit 961d0e5b32946703125964f9f5b6321d60f4d706 ]

Currently the software CBS does not consider the packet sending time
when depleting the credits. It caused the throughput to be
Idleslope[kbps] * (Port transmit rate[kbps] / |Sendslope[kbps]|) where
Idleslope * (Port transmit rate / (Idleslope + |Sendslope|)) = Idleslope
is expected. In order to fix the issue above, this patch takes the time
when the packet sending completes into account by moving the anchor time
variable "last" ahead to the send completion time upon transmission and
adding wait when the next dequeue request comes before the send
completion time of the previous packet.

changelog:
V2->V3:
 - remove unnecessary whitespace cleanup
 - add the checks if port_rate is 0 before division

V1->V2:
 - combine variable "send_completed" into "last"
 - add the comment for estimate of the packet sending

Fixes: 585d763af09c ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
Signed-off-by: Zh-yuan Ye <ye.zh-yuan@xxxxxxxxxxxxx>
Reviewed-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/sched/sch_cbs.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/net/sched/sch_cbs.c
+++ b/net/sched/sch_cbs.c
@@ -181,6 +181,11 @@ static struct sk_buff *cbs_dequeue_soft(
 	s64 credits;
 	int len;
 
+	/* The previous packet is still being sent */
+	if (now < q->last) {
+		qdisc_watchdog_schedule_ns(&q->watchdog, q->last);
+		return NULL;
+	}
 	if (q->credits < 0) {
 		credits = timediff_to_credits(now - q->last, q->idleslope);
 
@@ -212,7 +217,12 @@ static struct sk_buff *cbs_dequeue_soft(
 	credits += q->credits;
 
 	q->credits = max_t(s64, credits, q->locredit);
-	q->last = now;
+	/* Estimate of the transmission of the last byte of the packet in ns */
+	if (unlikely(atomic64_read(&q->port_rate) == 0))
+		q->last = now;
+	else
+		q->last = now + div64_s64(len * NSEC_PER_SEC,
+					  atomic64_read(&q->port_rate));
 
 	return skb;
 }