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fq_codel: add batch ability to fq_codel_drop() 

In presence of inelastic flows and stress, we can call
fq_codel_drop() for every packet entering fq_codel qdisc.

fq_codel_drop() is quite expensive, as it does a linear scan
of 4 KB of memory to find a fat flow.
Once found, it drops the oldest packet of this flow.

Instead of dropping a single packet, try to drop 50% of the backlog
of this fat flow, with a configurable limit of 64 packets per round.

TCA_FQ_CODEL_DROP_BATCH_SIZE is the new attribute to make this
limit configurable.

With this strategy the 4 KB search is amortized to a single cache line
per drop [1], so fq_codel_drop() no longer appears at the top of kernel
profile in presence of few inelastic flows.

[1] Assuming a 64byte cache line, and 1024 buckets

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dave Taht <dave.taht@gmail.com>
Cc: Jonathan Morton <chromatix99@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Dave Taht
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2016-05-01 16:47:26 -07:00 committed by David S. Miller
parent 094e43d50d
commit 9d18562a22
2 changed files with 47 additions and 20 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/uapi/linux/pkt_sched.h
View File

@ -718,6 +718,7 @@ enum {
TCA_FQ_CODEL_FLOWS,
TCA_FQ_CODEL_QUANTUM,
TCA_FQ_CODEL_CE_THRESHOLD,
TCA_FQ_CODEL_DROP_BATCH_SIZE,
__TCA_FQ_CODEL_MAX
};

66
net/sched/sch_fq_codel.c
View File

@ -59,6 +59,7 @@ struct fq_codel_sched_data {
u32 flows_cnt; /* number of flows */
u32 perturbation; /* hash perturbation */
u32 quantum; /* psched_mtu(qdisc_dev(sch)); */
u32 drop_batch_size;
struct codel_params cparams;
struct codel_stats cstats;
u32 drop_overlimit;
@ -135,17 +136,20 @@ static inline void flow_queue_add(struct fq_codel_flow *flow,
skb->next = NULL;
}
static unsigned int fq_codel_drop(struct Qdisc *sch)
static unsigned int fq_codel_drop(struct Qdisc *sch, unsigned int max_packets)
{
struct fq_codel_sched_data *q = qdisc_priv(sch);
struct sk_buff *skb;
unsigned int maxbacklog = 0, idx = 0, i, len;
struct fq_codel_flow *flow;
unsigned int threshold;
/* Queue is full! Find the fat flow and drop packet from it.
/* Queue is full! Find the fat flow and drop packet(s) from it.
* This might sound expensive, but with 1024 flows, we scan
* 4KB of memory, and we dont need to handle a complex tree
* in fast path (packet queue/enqueue) with many cache misses.
* In stress mode, we'll try to drop 64 packets from the flow,
* amortizing this linear lookup to one cache line per drop.
*/
for (i = 0; i < q->flows_cnt; i++) {
if (q->backlogs[i] > maxbacklog) {
@ -153,15 +157,24 @@ static unsigned int fq_codel_drop(struct Qdisc *sch)
idx = i;
}
}
/* Our goal is to drop half of this fat flow backlog */
threshold = maxbacklog >> 1;
flow = &q->flows[idx];
skb = dequeue_head(flow);
len = qdisc_pkt_len(skb);
len = 0;
i = 0;
do {
skb = dequeue_head(flow);
len += qdisc_pkt_len(skb);
kfree_skb(skb);
} while (++i < max_packets && len < threshold);
flow->dropped += i;
q->backlogs[idx] -= len;
sch->q.qlen--;
qdisc_qstats_drop(sch);
qdisc_qstats_backlog_dec(sch, skb);
kfree_skb(skb);
flow->dropped++;
sch->qstats.drops += i;
sch->qstats.backlog -= len;
sch->q.qlen -= i;
return idx;
}
@ -170,14 +183,14 @@ static unsigned int fq_codel_qdisc_drop(struct Qdisc *sch)
unsigned int prev_backlog;
prev_backlog = sch->qstats.backlog;
fq_codel_drop(sch);
fq_codel_drop(sch, 1U);
return prev_backlog - sch->qstats.backlog;
}
static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)
{
struct fq_codel_sched_data *q = qdisc_priv(sch);
unsigned int idx, prev_backlog;
unsigned int idx, prev_backlog, prev_qlen;
struct fq_codel_flow *flow;
int uninitialized_var(ret);
@ -206,16 +219,22 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)
return NET_XMIT_SUCCESS;
prev_backlog = sch->qstats.backlog;
q->drop_overlimit++;
/* Return Congestion Notification only if we dropped a packet
* from this flow.
*/
if (fq_codel_drop(sch) == idx)
return NET_XMIT_CN;
prev_qlen = sch->q.qlen;
/* As we dropped a packet, better let upper stack know this */
qdisc_tree_reduce_backlog(sch, 1, prev_backlog - sch->qstats.backlog);
return NET_XMIT_SUCCESS;
/* fq_codel_drop() is quite expensive, as it performs a linear search
* in q->backlogs[] to find a fat flow.
* So instead of dropping a single packet, drop half of its backlog
* with a 64 packets limit to not add a too big cpu spike here.
*/
ret = fq_codel_drop(sch, q->drop_batch_size);
q->drop_overlimit += prev_qlen - sch->q.qlen;
/* As we dropped packet(s), better let upper stack know this */
qdisc_tree_reduce_backlog(sch, prev_qlen - sch->q.qlen,
prev_backlog - sch->qstats.backlog);
return ret == idx ? NET_XMIT_CN : NET_XMIT_SUCCESS;
}
/* This is the specific function called from codel_dequeue()
@ -335,6 +354,7 @@ static const struct nla_policy fq_codel_policy[TCA_FQ_CODEL_MAX + 1] = {
[TCA_FQ_CODEL_FLOWS] = { .type = NLA_U32 },
[TCA_FQ_CODEL_QUANTUM] = { .type = NLA_U32 },
[TCA_FQ_CODEL_CE_THRESHOLD] = { .type = NLA_U32 },
[TCA_FQ_CODEL_DROP_BATCH_SIZE] = { .type = NLA_U32 },
};
static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt)
@ -386,6 +406,9 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt)
if (tb[TCA_FQ_CODEL_QUANTUM])
q->quantum = max(256U, nla_get_u32(tb[TCA_FQ_CODEL_QUANTUM]));
if (tb[TCA_FQ_CODEL_DROP_BATCH_SIZE])
q->drop_batch_size = min(1U, nla_get_u32(tb[TCA_FQ_CODEL_DROP_BATCH_SIZE]));
while (sch->q.qlen > sch->limit) {
struct sk_buff *skb = fq_codel_dequeue(sch);
@ -431,6 +454,7 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt)
sch->limit = 10*1024;
q->flows_cnt = 1024;
q->drop_batch_size = 64;
q->quantum = psched_mtu(qdisc_dev(sch));
q->perturbation = prandom_u32();
INIT_LIST_HEAD(&q->new_flows);
@ -489,6 +513,8 @@ static int fq_codel_dump(struct Qdisc *sch, struct sk_buff *skb)
q->cparams.ecn) ||
nla_put_u32(skb, TCA_FQ_CODEL_QUANTUM,
q->quantum) ||
nla_put_u32(skb, TCA_FQ_CODEL_DROP_BATCH_SIZE,
q->drop_batch_size) ||
nla_put_u32(skb, TCA_FQ_CODEL_FLOWS,
q->flows_cnt))
goto nla_put_failure;