[PATCH net-next v6 2/6] net_sched: sch_fq: clear past skb->tstamp if offloading pacing

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

When hardware offload is enabled, FQ will forward packets to the
netdevice for pacing. The device has to test that skb->tstamp is
in the future.

Avoid this cost for packets whose txtime has already passed, by
clearing skb->tstamp.

Also clear slightly into the future, for EDT timestamps that are
so close to now that they fall within a reasonable normal Tx
latency. This slack is set to 400 nsec.

Also disable timer drift logic when offload is enabled, because
time_next_packet can exceed now causing a negative value.

Signed-off-by: Willem de Bruijn <willemb@google.com>

---

Changes
  v5 -> v6
    - switch to skb_set_delivery_time to also clear SKB_CLOCK_REALTIME
    - add fq_offload_slack_ns
  v3 -> v4
    - also reset tstamp_type
    - minor: initialize time_next_packet for more obvious correctness

Sashiko, ignore pre-existing issues. In particular, effects on
non-EDT packets and when queue or sk rate limit is set.

Sashiko, pacing offload is an optimization. Ignore that some packets
may not get offloaded, e.g., when txtime is a few usec in the future.

Claude suggests to only clear delivery time in one location
in fq_dequeue. Unfortunately the separate fastpath location is needed
as that avoids computing now in fq_dequeue for these fastpath packets.
---
 net/sched/sch_fq.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index e0f2cd99e3b7..e7730fb04264 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -159,6 +159,9 @@ struct fq_sched_data {
 	u64		stat_allocation_errors;
 };
 
+/* EDT timestamps to clear beyond now. */
+static const int fq_offload_slack_ns = 400;
+
 /* return the i-th 2-bit value ("crumb") */
 static u8 fq_prio2band(const u8 *prio2band, unsigned int prio)
 {
@@ -399,6 +402,11 @@ static struct fq_flow *fq_classify(struct Qdisc *sch, struct sk_buff *skb,
 		    READ_ONCE(sk->sk_pacing_status) != SK_PACING_FQ)
 			smp_store_release(&sk->sk_pacing_status,
 					  SK_PACING_FQ);
+
+		if (q->offload_horizon &&
+		    fq_skb_cb(skb)->time_to_send <= now + fq_offload_slack_ns)
+			skb_set_delivery_time(skb, 0, 0);
+
 		return &q->internal;
 	}
 
@@ -707,6 +715,7 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 	struct fq_sched_data *q = qdisc_priv(sch);
 	struct fq_perband_flows *pband;
 	struct fq_flow_head *head;
+	u64 time_next_packet = 0;
 	struct sk_buff *skb;
 	struct fq_flow *f;
 	unsigned long rate;
@@ -721,7 +730,7 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 	if (skb) {
 		q->internal.qlen--;
 		fq_dequeue_skb(sch, &q->internal, skb);
-		goto out;
+		return skb;
 	}
 
 	now = ktime_get_ns();
@@ -758,8 +767,8 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 
 	skb = fq_peek(f);
 	if (skb) {
-		u64 time_next_packet = max_t(u64, fq_skb_cb(skb)->time_to_send,
-					     f->time_next_packet);
+		time_next_packet = max_t(u64, fq_skb_cb(skb)->time_to_send,
+					 f->time_next_packet);
 
 		if (now + q->offload_horizon < time_next_packet) {
 			head->first = f->next;
@@ -828,11 +837,16 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 		 * f->time_next_packet was set when prior packet was sent,
 		 * and current time (@now) can be too late by tens of us.
 		 */
-		if (f->time_next_packet)
+		if (f->time_next_packet && f->time_next_packet < now)
 			len -= min(len/2, now - f->time_next_packet);
 		f->time_next_packet = now + len;
 	}
+
 out:
+	if (q->offload_horizon &&
+	    time_next_packet && time_next_packet <= now + fq_offload_slack_ns)
+		skb_set_delivery_time(skb, 0, 0);
+
 	return skb;
 }
 
-- 
2.55.0.691.gc56d675ccc-goog