[PATCH net-next] fq_codel: avoid under-utilization with ce_threshold at low link rates

Commit "fq_codel: generalise ce_threshold marking for subset of traffic"
[1] enables ce_threshold to be used in the Internet, not just in data
centres.

Because ce_threshold is in time units, it can cause poor utilization at
low link rates when it represents <1 packet.
E.g., if link rate <12Mb/s ce_threshold=1ms is <1500B packet.

So, suppress ECN marking unless the backlog is also > 1 MTU.

A similar patch to [1] was tested on an earlier kernel, and a similar
one-packet check prevented poor utilization at low link rates [2].

[1] commit dfcb63ce1de6 ("fq_codel: generalise ce_threshold marking for subset of traffic")

[2] See right hand column of plots at the end of:
https://bobbriscoe.net/projects/latency/dctth_journal_draft20190726.pdf

Signed-off-by: Asad Sajjad Ahmed <redacted>
Signed-off-by: Olga Albisser <redacted>
---
 include/net/codel_impl.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/net/codel_impl.h b/include/net/codel_impl.h
index 137d40d8cbeb..4e3e8473e776 100644
--- a/include/net/codel_impl.h
+++ b/include/net/codel_impl.h

@@ -248,7 +248,8 @@ static struct sk_buff *codel_dequeue(void *ctx,
 						    vars->rec_inv_sqrt);
 	}
 end:
-	if (skb && codel_time_after(vars->ldelay, params->ce_threshold)) {
+	if (skb && codel_time_after(vars->ldelay, params->ce_threshold) &&
+	    *backlog > params->mtu) {
 		bool set_ce = true;
 
 		if (params->ce_threshold_mask) {

-- 
2.30.2