fq_codel: generalise ce_threshold marking for subset of traffic

Commit e72aeb9ee0e3 ("fq_codel: implement L4S style ce_threshold_ect1
marking") expanded the ce_threshold feature of FQ-CoDel so it can
be applied to a subset of the traffic, using the ECT(1) bit of the ECN
field as the classifier. However, hard-coding ECT(1) as the only
classifier for this feature seems limiting, so let's expand it to be more
general.

To this end, change the parameter from a ce_threshold_ect1 boolean, to a
one-byte selector/mask pair (ce_threshold_{selector,mask}) which is applied
to the whole diffserv/ECN field in the IP header. This makes it possible to
classify packets by any value in either the ECN field or the diffserv
field. In particular, setting a selector of INET_ECN_ECT_1 and a mask of
INET_ECN_MASK corresponds to the functionality before this patch, and a
mask of ~INET_ECN_MASK allows using the selector as a straight-forward
match against a diffserv code point:

 # apply ce_threshold to ECT(1) traffic
 tc qdisc replace dev eth0 root fq_codel ce_threshold 1ms ce_threshold_selector 0x1/0x3

 # apply ce_threshold to ECN-capable traffic marked as diffserv AF22
 tc qdisc replace dev eth0 root fq_codel ce_threshold 1ms ce_threshold_selector 0x50/0xfc

Regardless of the selector chosen, the normal rules for ECN-marking of
packets still apply, i.e., the flow must still declare itself ECN-capable
by setting one of the bits in the ECN field to get marked at all.

v2:
- Add tc usage examples to patch description

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211019174709.69081-1-toke@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/include/net/codel_impl.h b/include/net/codel_impl.h
index 7af2c3eb3c43..137d40d8cbeb 100644
--- a/include/net/codel_impl.h
+++ b/include/net/codel_impl.h
@@ -54,7 +54,8 @@ static void codel_params_init(struct codel_params *params)
 	params->interval = MS2TIME(100);
 	params->target = MS2TIME(5);
 	params->ce_threshold = CODEL_DISABLED_THRESHOLD;
-	params->ce_threshold_ect1 = false;
+	params->ce_threshold_mask = 0;
+	params->ce_threshold_selector = 0;
 	params->ecn = false;
 }
 
@@ -250,13 +251,12 @@ end:
 	if (skb && codel_time_after(vars->ldelay, params->ce_threshold)) {
 		bool set_ce = true;
 
-		if (params->ce_threshold_ect1) {
-			/* Note: if skb_get_dsfield() returns -1, following
-			 * gives INET_ECN_MASK, which is != INET_ECN_ECT_1.
-			 */
-			u8 ecn = skb_get_dsfield(skb) & INET_ECN_MASK;
+		if (params->ce_threshold_mask) {
+			int dsfield = skb_get_dsfield(skb);
 
-			set_ce = (ecn == INET_ECN_ECT_1);
+			set_ce = (dsfield >= 0 &&
+				  (((u8)dsfield & params->ce_threshold_mask) ==
+				   params->ce_threshold_selector));
 		}
 		if (set_ce && INET_ECN_set_ce(skb))
 			stats->ce_mark++;