Re: [PATCH net-next] sch_netem: faster rb tree removal

On 9/24/17 7:57 PM, David Ahern wrote:

On 9/23/17 12:07 PM, Eric Dumazet wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

While running TCP tests involving netem storing millions of packets,
I had the idea to speed up tfifo_reset() and did experiments.

I tried the rbtree_postorder_for_each_entry_safe() method that is
used in skb_rbtree_purge() but discovered it was slower than the
current tfifo_reset() method.

I measured time taken to release skbs with three occupation levels :
10^4, 10^5 and 10^6 skbs with three methods :

1) (current 'naive' method)

	while ((p = rb_first(&q->t_root))) {
		struct sk_buff *skb = netem_rb_to_skb(p);
 
		rb_erase(p, &q->t_root);
		rtnl_kfree_skbs(skb, skb);
	}

2) Use rb_next() instead of rb_first() in the loop :

	p = rb_first(&q->t_root);
	while (p) {
		struct sk_buff *skb = netem_rb_to_skb(p);

		p = rb_next(p);
		rb_erase(&skb->rbnode, &q->t_root);
		rtnl_kfree_skbs(skb, skb);
	}

3) "optimized" method using rbtree_postorder_for_each_entry_safe()

	struct sk_buff *skb, *next;

	rbtree_postorder_for_each_entry_safe(skb, next,
					     &q->t_root, rbnode) {
               rtnl_kfree_skbs(skb, skb);
	}
	q->t_root = RB_ROOT;

Results :

method_1:while (rb_first()) rb_erase() 10000 skbs in 690378 ns (69 ns per skb)
method_2:rb_first; while (p) { p = rb_next(p); ...}  10000 skbs in 541846 ns (54 ns per skb)
method_3:rbtree_postorder_for_each_entry_safe() 10000 skbs in 868307 ns (86 ns per skb)

method_1:while (rb_first()) rb_erase() 99996 skbs in 7804021 ns (78 ns per skb)
method_2:rb_first; while (p) { p = rb_next(p); ...}  100000 skbs in 5942456 ns (59 ns per skb)
method_3:rbtree_postorder_for_each_entry_safe() 100000 skbs in 11584940 ns (115 ns per skb)

method_1:while (rb_first()) rb_erase() 1000000 skbs in 108577838 ns (108 ns per skb)
method_2:rb_first; while (p) { p = rb_next(p); ...}  1000000 skbs in 82619635 ns (82 ns per skb)
method_3:rbtree_postorder_for_each_entry_safe() 1000000 skbs in 127328743 ns (127 ns per skb)

Method 2) is simply faster, probably because it maintains a smaller
working size set.

Note that this is the method we use in tcp_ofo_queue() already.

I will also change skb_rbtree_purge() in a second patch.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/sched/sch_netem.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 063a4bdb9ee6f26b01387959e8f6ccd15ec16191..5a4f1008029068372019a965186e7a3c0a18aac3 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c

@@ -361,12 +361,13 @@ static psched_time_t packet_len_2_sched_time(unsigned int len, struct netem_sche
 static void tfifo_reset(struct Qdisc *sch)
 {
 	struct netem_sched_data *q = qdisc_priv(sch);
-	struct rb_node *p;
+	struct rb_node *p = rb_first(&q->t_root);
 
-	while ((p = rb_first(&q->t_root))) {
+	while (p) {
 		struct sk_buff *skb = netem_rb_to_skb(p);
 
-		rb_erase(p, &q->t_root);
+		p = rb_next(p);
+		rb_erase(&skb->rbnode, &q->t_root);
 		rtnl_kfree_skbs(skb, skb);
 	}
 }

Hi Eric:

I'm guessing the cost is in the rb_first and rb_next computations. Did
you consider something like this:

        struct rb_root *root
        struct rb_node **p = &root->rb_node;

        while (*p != NULL) {
                struct foobar *fb;

                fb = container_of(*p, struct foobar, rb_node);
                // fb processing

		  rb_erase(&nh->rb_node, root);

                p = &root->rb_node;
        }

Oops, dropped the rb_erase in my consolidating the code to this snippet.