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>
diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index a5e420b..e7b42b0 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -59,6 +59,7 @@
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 @@
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 @@
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 @@
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 @@
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 @@
[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 @@
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 @@
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 @@
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;