tcp/fq: move back to CLOCK_MONOTONIC

In the recent TCP/EDT patch series, I switched TCP and sch_fq
clocks from MONOTONIC to TAI, in order to meet the choice done
earlier for sch_etf packet scheduler.

But sure enough, this broke some setups were the TAI clock
jumps forward (by almost 50 year...), as reported
by Leonard Crestez.

If we want to converge later, we'll probably need to add
an skb field to differentiate the clock bases, or a socket option.

In the meantime, an UDP application will need to use CLOCK_MONOTONIC
base for its SCM_TXTIME timestamps if using fq packet scheduler.

Fixes: 72b0094f91 ("tcp: switch tcp_clock_ns() to CLOCK_TAI base")
Fixes: 142537e419 ("net_sched: sch_fq: switch to CLOCK_TAI")
Fixes: fd2bca2aa7 ("tcp: switch internal pacing timer to CLOCK_TAI")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Tested-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq.c

@@ -412,7 +412,7 @@ static void fq_check_throttled(struct fq_sched_data *q, u64 now)
 static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 {
 	struct fq_sched_data *q = qdisc_priv(sch);
-	u64 now = ktime_get_tai_ns();
+	u64 now = ktime_get_ns();
 	struct fq_flow_head *head;
 	struct sk_buff *skb;
 	struct fq_flow *f;
@@ -776,7 +776,7 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt,
 	q->fq_trees_log		= ilog2(1024);
 	q->orphan_mask		= 1024 - 1;
 	q->low_rate_threshold	= 550000 / 8;
-	qdisc_watchdog_init_clockid(&q->watchdog, sch, CLOCK_TAI);
+	qdisc_watchdog_init_clockid(&q->watchdog, sch, CLOCK_MONOTONIC);
 
 	if (opt)
 		err = fq_change(sch, opt, extack);
@@ -831,7 +831,7 @@ static int fq_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 	st.flows_plimit		  = q->stat_flows_plimit;
 	st.pkts_too_long	  = q->stat_pkts_too_long;
 	st.allocation_errors	  = q->stat_allocation_errors;
-	st.time_next_delayed_flow = q->time_next_delayed_flow - ktime_get_tai_ns();
+	st.time_next_delayed_flow = q->time_next_delayed_flow - ktime_get_ns();
 	st.flows		  = q->flows;
 	st.inactive_flows	  = q->inactive_flows;
 	st.throttled_flows	  = q->throttled_flows;