On Fri, 2012-08-31 at 06:50 -0700, Eric Dumazet wrote:
> On Thu, 2012-08-30 at 23:55 -0700, Eric Dumazet wrote:
> > On locally generated TCP traffic (host), we can override the 100 ms
> > interval value using the more accurate RTT estimation maintained by TCP
> > stack (tp->srtt)
> >
> > Datacenter workload benefits using shorter feedback (say if RTT is below
> > 1 ms, we can react 100 times faster to a congestion)
> >
> > Idea from Yuchung Cheng.
> >
>
> Linux patch would be the following :
>
> I'll do tests next week, but I am sending a raw patch right now if
> anybody wants to try it.
>
> Presumably we also want to adjust target as well.
>
> To get more precise srtt values in the datacenter, we might avoid the
> 'one jiffie slack' on small values in tcp_rtt_estimator(), as we force
> m to be 1 before the scaling by 8 :
>
> if (m == 0)
> m = 1;
>
> We only need to force the least significant bit of srtt to be set.
>
Hmm, I also need to properly init default_interval after
codel_params_init(&q->cparams) :
net/sched/sch_fq_codel.c | 24 ++++++++++++++++++++++--
1 file changed, 22 insertions(+), 2 deletions(-)
diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index 9fc1c62..f04ff6a 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -25,6 +25,7 @@
#include <net/pkt_sched.h>
#include <net/flow_keys.h>
#include <net/codel.h>
+#include <linux/tcp.h>
/* Fair Queue CoDel.
*
@@ -59,6 +60,7 @@ struct fq_codel_sched_data {
u32 perturbation; /* hash perturbation */
u32 quantum; /* psched_mtu(qdisc_dev(sch)); */
struct codel_params cparams;
+ codel_time_t default_interval;
struct codel_stats cstats;
u32 drop_overlimit;
u32 new_flow_count;
@@ -211,6 +213,14 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct
Qdisc *sch)
return NET_XMIT_SUCCESS;
}
+/* Given TCP srtt evaluation, return codel interval.
+ * srtt is given in jiffies, scaled by 8.
+ */
+static codel_time_t tcp_srtt_to_codel(unsigned int srtt)
+{
+ return srtt * ((NSEC_PER_SEC >> (CODEL_SHIFT + 3)) / HZ);
+}
+
/* This is the specific function called from codel_dequeue()
* to dequeue a packet from queue. Note: backlog is handled in
* codel, we dont need to reduce it here.
@@ -220,12 +230,21 @@ static struct sk_buff *dequeue(struct codel_vars *vars,
struct Qdisc *sch)
struct fq_codel_sched_data *q = qdisc_priv(sch);
struct fq_codel_flow *flow;
struct sk_buff *skb = NULL;
+ struct sock *sk;
flow = container_of(vars, struct fq_codel_flow, cvars);
if (flow->head) {
skb = dequeue_head(flow);
q->backlogs[flow - q->flows] -= qdisc_pkt_len(skb);
sch->q.qlen--;
+ sk = skb->sk;
+ q->cparams.interval = q->default_interval;
+ if (sk && sk->sk_protocol == IPPROTO_TCP) {
+ u32 srtt = tcp_sk(sk)->srtt;
+
+ if (srtt)
+ q->cparams.interval = tcp_srtt_to_codel(srtt);
+ }
}
return skb;
}
@@ -330,7 +349,7 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr
*opt)
if (tb[TCA_FQ_CODEL_INTERVAL]) {
u64 interval = nla_get_u32(tb[TCA_FQ_CODEL_INTERVAL]);
- q->cparams.interval = (interval * NSEC_PER_USEC) >> CODEL_SHIFT;
+ q->default_interval = (interval * NSEC_PER_USEC) >> CODEL_SHIFT;
}
if (tb[TCA_FQ_CODEL_LIMIT])
@@ -395,6 +414,7 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr
*opt)
INIT_LIST_HEAD(&q->new_flows);
INIT_LIST_HEAD(&q->old_flows);
codel_params_init(&q->cparams);
+ q->default_interval = q->cparams.interval;
codel_stats_init(&q->cstats);
q->cparams.ecn = true;
@@ -441,7 +461,7 @@ static int fq_codel_dump(struct Qdisc *sch, struct sk_buff
*skb)
nla_put_u32(skb, TCA_FQ_CODEL_LIMIT,
sch->limit) ||
nla_put_u32(skb, TCA_FQ_CODEL_INTERVAL,
- codel_time_to_us(q->cparams.interval)) ||
+ codel_time_to_us(q->default_interval)) ||
nla_put_u32(skb, TCA_FQ_CODEL_ECN,
q->cparams.ecn) ||
nla_put_u32(skb, TCA_FQ_CODEL_QUANTUM,
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