[Codel] [RFC v2] fq_codel : interval servo on hosts

[Yuchung Cheng]

On Fri, Aug 31, 2012 at 6:57 AM, Eric Dumazet <eric.dumazet at gmail.com> wrote:
> 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.
>>
Just curious: tp->srtt is a very rough estimator, e.g., Delayed-ACks
can easily add 40 - 200ms fuzziness. Will this affect short flows?


>
> 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,
>
>