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net: netem: use a list in addition to rbtree

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When testing high-bandwidth TCP streams with large windows,
high latency, and low jitter, netem consumes a lot of CPU cycles
doing rbtree rebalancing.

This patch uses a linear list/queue in addition to the rbtree:
if an incoming packet is past the tail of the linear queue, it is
added there, otherwise it is inserted into the rbtree.

Without this patch, perf shows netem_enqueue, netem_dequeue,
and rb_* functions among the top offenders. With this patch,
only netem_enqueue is noticeable if jitter is low/absent.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Oskolkov <posk@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Peter Oskolkov 2018-12-04 11:55:56 -08:00 committed by David S. Miller
parent 932c441707
commit d66280b12b
1 changed files with 69 additions and 20 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

89
net/sched/sch_netem.c
View file

@ -77,6 +77,10 @@ struct netem_sched_data {
/* internal t(ime)fifo qdisc uses t_root and sch->limit */
struct rb_root t_root;
/* a linear queue; reduces rbtree rebalancing when jitter is low */
struct sk_buff *t_head;
struct sk_buff *t_tail;
/* optional qdisc for classful handling (NULL at netem init) */
struct Qdisc *qdisc;
@ -369,26 +373,39 @@ static void tfifo_reset(struct Qdisc *sch)
rb_erase(&skb->rbnode, &q->t_root);
rtnl_kfree_skbs(skb, skb);
}
rtnl_kfree_skbs(q->t_head, q->t_tail);
q->t_head = NULL;
q->t_tail = NULL;
}
static void tfifo_enqueue(struct sk_buff *nskb, struct Qdisc *sch)
{
struct netem_sched_data *q = qdisc_priv(sch);
u64 tnext = netem_skb_cb(nskb)->time_to_send;
struct rb_node **p = &q->t_root.rb_node, *parent = NULL;
while (*p) {
struct sk_buff *skb;
parent = *p;
skb = rb_to_skb(parent);
if (tnext >= netem_skb_cb(skb)->time_to_send)
p = &parent->rb_right;
if (!q->t_tail || tnext >= netem_skb_cb(q->t_tail)->time_to_send) {
if (q->t_tail)
q->t_tail->next = nskb;
else
p = &parent->rb_left;
q->t_head = nskb;
q->t_tail = nskb;
} else {
struct rb_node **p = &q->t_root.rb_node, *parent = NULL;
while (*p) {
struct sk_buff *skb;
parent = *p;
skb = rb_to_skb(parent);
if (tnext >= netem_skb_cb(skb)->time_to_send)
p = &parent->rb_right;
else
p = &parent->rb_left;
}
rb_link_node(&nskb->rbnode, parent, p);
rb_insert_color(&nskb->rbnode, &q->t_root);
}
rb_link_node(&nskb->rbnode, parent, p);
rb_insert_color(&nskb->rbnode, &q->t_root);
sch->q.qlen++;
}
@ -530,9 +547,16 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
t_skb = skb_rb_last(&q->t_root);
t_last = netem_skb_cb(t_skb);
if (!last ||
t_last->time_to_send > last->time_to_send) {
t_last->time_to_send > last->time_to_send)
last = t_last;
}
if (q->t_tail) {
struct netem_skb_cb *t_last =
netem_skb_cb(q->t_tail);
if (!last ||
t_last->time_to_send > last->time_to_send)
last = t_last;
}
}
if (last) {
@ -611,11 +635,38 @@ static void get_slot_next(struct netem_sched_data *q, u64 now)
q->slot.bytes_left = q->slot_config.max_bytes;
}
static struct sk_buff *netem_peek(struct netem_sched_data *q)
{
struct sk_buff *skb = skb_rb_first(&q->t_root);
u64 t1, t2;
if (!skb)
return q->t_head;
if (!q->t_head)
return skb;
t1 = netem_skb_cb(skb)->time_to_send;
t2 = netem_skb_cb(q->t_head)->time_to_send;
if (t1 < t2)
return skb;
return q->t_head;
}
static void netem_erase_head(struct netem_sched_data *q, struct sk_buff *skb)
{
if (skb == q->t_head) {
q->t_head = skb->next;
if (!q->t_head)
q->t_tail = NULL;
} else {
rb_erase(&skb->rbnode, &q->t_root);
}
}
static struct sk_buff *netem_dequeue(struct Qdisc *sch)
{
struct netem_sched_data *q = qdisc_priv(sch);
struct sk_buff *skb;
struct rb_node *p;
tfifo_dequeue:
skb = __qdisc_dequeue_head(&sch->q);
@ -625,20 +676,18 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
qdisc_bstats_update(sch, skb);
return skb;
}
p = rb_first(&q->t_root);
if (p) {
skb = netem_peek(q);
if (skb) {
u64 time_to_send;
u64 now = ktime_get_ns();
skb = rb_to_skb(p);
/* if more time remaining? */
time_to_send = netem_skb_cb(skb)->time_to_send;
if (q->slot.slot_next && q->slot.slot_next < time_to_send)
get_slot_next(q, now);
if (time_to_send <= now && q->slot.slot_next <= now) {
rb_erase(p, &q->t_root);
if (time_to_send <= now && q->slot.slot_next <= now) {
netem_erase_head(q, skb);
sch->q.qlen--;
qdisc_qstats_backlog_dec(sch, skb);
skb->next = NULL;