Note: Tom Herbert posted almost same patch 3 months back, but for
different reasons.

The reasons we want to get rid of this spin_trylock() are :

1) Under high qdisc pressure, the spin_trylock() has almost no
chance to succeed.

2) We loop multiple times in softirq handler, eventually reaching
the max retry count (10), and we schedule ksoftirqd.

Since we want to adhere more strictly to ksoftirqd being waked up in
the future (https://lwn.net/Articles/687617/), better avoid spurious
wakeups.

3) calls to __netif_reschedule() dirty the cache line containing
q->next_sched, slowing down the owner of qdisc.

4) RT kernels can not use the spin_trylock() here.

With help of busylock, we get the qdisc spinlock fast enough, and
the trylock trick brings only performance penalty.

Depending on qdisc setup, I observed a gain of up to 19 % in qdisc
performance (1016600 pps instead of 853400 pps, using prio+tbf+fq_codel)

("mpstat -I SCPU 1" is much happier now)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>