[PATCH net-next v9 3/7] 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.

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

---

Changes
  v8 -> v9
    - (minor) use SKB_CLOCK_REALTIME instead of 0
    - (minor) remove comments to bot below the --- separator
  v6 -> v7
    - timer drift logic fix moved to separate commit
  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
---
 net/sched/sch_fq.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index fdd9380e445c..60ad87cb1275 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)
 {
@@ -421,6 +424,10 @@ static struct fq_flow *fq_classify(struct Qdisc *sch, struct sk_buff *skb,
 			smp_store_release(&sk->sk_pacing_status,
 					  SK_PACING_FQ);
 
+		if (offload_horizon &&
+		    fq_skb_cb(skb)->time_to_send <= now + fq_offload_slack_ns)
+			skb_set_delivery_time(skb, 0, SKB_CLOCK_REALTIME);
+
 		return &q->internal;
 	}
 
@@ -731,6 +738,7 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 	u64 offload_horizon = fq_offload_horizon(sch, q);
 	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;
@@ -745,7 +753,7 @@ 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();
@@ -782,8 +790,8 @@ 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 + offload_horizon < time_next_packet) {
 			head->first = f->next;
@@ -862,6 +870,10 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 	}
 
 out:
+	if (offload_horizon &&
+	    time_next_packet && time_next_packet <= now + fq_offload_slack_ns)
+		skb_set_delivery_time(skb, 0, SKB_CLOCK_REALTIME);
+
 	return skb;
 }
 
-- 
2.55.0.1007.g17ff1f9808-goog