Subject: Re: [RFC net-next 2/2] net: sched: fq_pie: Flow Queue PIE AQM

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 02 Apr 2019 19:25:57 +0200

Gautam Ramakrishnan <[email-masked]> writes:

> Hello, thanks for the feedback
>
> On Tue, Apr 2, 2019 at 4:19 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> Some suggestions below to make fq_pie and fq_codel more similar (ref. my
>> previous email).
>>
>> Also, a few unrelated nits.
>>
>> > From: Mohit P. Tahiliani <[email-masked]>
>> >
>> > FQ-PIE incorporates fair/flow queuing in which every queue
>> > is managed by an instance of PIE queueing discipline.
>> > The algorithm provides good control over the queueing delay
>> > while at the same time, ensures fairness across various
>> > flows sharing the same link.
>> >
>> > Principles:
>> >   - Packets are classified stochastically on flows.
>> >   - Each flow has a PIE managed queue.
>> >   - Flows are linked onto two (Round Robin) lists,
>> >     so that new flows have priority on old ones.
>> >   - For a given flow, packets are dropped on arrival according
>> >     to a drop probability.
>> >   - Tail drops only.
>>
>> Why tail drops only?
>
> I had mentioned this because packets are dropped only at enqueue.

Yup, realise that; was just wondering why you went with this design
instead of doing the head drop that fq_codel does?

>>
>> > Usage:
>> > tc qdisc ... fq_pie [ limit PACKETS ] [ flows NUMBER ]
>> >                     [ alpha NUMBER ] [ beta NUMBER ]
>> >                     [ target TIME us ] [ tupdate TIME us ]
>> >                   [ bytemode ] [ quantum BYTES ]
>> >                   [ ecn | ecn_prob PERCENTAGE ]
>> >
>> > defaults: 1024 flows, 10240 packets limit, quantum : device MTU
>> >            target: 15ms
>> >            tupdate: 15ms
>> >            alpha: 2 (on a scale of 0 to 16)
>> >            beta: 20 (on a scale of 0 to 16)
>> >            ecn: false
>> >            ecn_prob: 10%
>> >
>> > Signed-off-by: Mohit P. Tahiliani <[email-masked]>
>> > Signed-off-by: Sachin D. Patil <[email-masked]>
>> > Signed-off-by: Mohit Bhasi <[email-masked]>
>> > Signed-off-by: V. Saicharan <[email-masked]>
>> > Signed-off-by: Leslie Monis <[email-masked]>
>> > Signed-off-by: Gautam Ramakrishnan <[email-masked]>
>> > Cc: Dave Taht <[email-masked]>
>> > ---
>> >  include/uapi/linux/pkt_sched.h |  28 ++
>> >  net/sched/Kconfig              |  14 +-
>> >  net/sched/Makefile             |   1 +
>> >  net/sched/sch_fq_pie.c         | 485 +++++++++++++++++++++++++++++++++
>> >  4 files changed, 527 insertions(+), 1 deletion(-)
>> >  create mode 100644 net/sched/sch_fq_pie.c
>> >
>> > diff --git a/include/uapi/linux/pkt_sched.h b/include/uapi/linux/pkt_sched.h
>> > index 7ee74c3474bf..005413bd09ee 100644
>> > --- a/include/uapi/linux/pkt_sched.h
>> > +++ b/include/uapi/linux/pkt_sched.h
>> > @@ -964,6 +964,34 @@ struct tc_pie_xstats {
>> >       __u32 ecn_mark;         /* packets marked with ecn*/
>> >  };
>> >
>> > +/* FQ PIE */
>> > +enum {
>> > +     TCA_FQ_PIE_UNSPEC,
>> > +     TCA_FQ_PIE_TARGET,
>> > +     TCA_FQ_PIE_LIMIT,
>> > +     TCA_FQ_PIE_TUPDATE,
>> > +     TCA_FQ_PIE_ALPHA,
>> > +     TCA_FQ_PIE_BETA,
>> > +     TCA_FQ_PIE_ECN,
>> > +     TCA_FQ_PIE_QUANTUM,
>> > +     TCA_FQ_PIE_BYTEMODE,
>> > +     TCA_FQ_PIE_FLOWS,
>> > +     TCA_FQ_PIE_ECN_PROB,
>> > +     __TCA_FQ_PIE_MAX
>> > +};
>> > +#define TCA_FQ_PIE_MAX   (__TCA_FQ_PIE_MAX - 1)
>> > +
>> > +struct tc_fq_pie_xstats {
>> > +     __u32 packets_in;       /* total number of packets enqueued */
>> > +     __u32 dropped;          /* packets dropped due to fq_pie_action */
>> > +     __u32 overlimit;        /* dropped due to lack of space in queue */
>> > +     __u32 ecn_mark;         /* packets marked with ecn*/
>> > +     __u32 new_flow_count;   /* number of time packets
>> > +                                created a 'new flow' */
>> > +     __u32 new_flows_len;    /* count of flows in new list */
>> > +     __u32 old_flows_len;    /* count of flows in old list */
>> > +};
>> > +
>> >  /* CBS */
>> >  struct tc_cbs_qopt {
>> >       __u8 offload;
>> > diff --git a/net/sched/Kconfig b/net/sched/Kconfig
>> > index 5c02ad97ef23..49f4dd9894a0 100644
>> > --- a/net/sched/Kconfig
>> > +++ b/net/sched/Kconfig
>> > @@ -358,13 +358,25 @@ config NET_SCH_PIE
>> >       help
>> >         Say Y here if you want to use the Proportional Integral controller
>> >         Enhanced scheduler packet scheduling algorithm.
>> > -       For more information, please see https://tools.ietf.org/html/rfc8033
>> > +       For more information, please see
>> > +       http://tools.ietf.org/html/draft-pan-tsvwg-pie-00
>> >
>> >         To compile this driver as a module, choose M here: the module
>> >         will be called sch_pie.
>> >
>> >         If unsure, say N.
>> >
>> > +config NET_SCH_FQ_PIE
>> > +     tristate "Flow Queue Proportional Integral controller Enhanced (FQ-PIE) scheduler"
>> > +     help
>> > +       Say Y here if you want to use the Flow Queue Proportional Integral controller
>> > +       Enhanced scheduler packet scheduling algorithm.
>> > +
>> > +       To compile this driver as a module, choose M here: the module
>> > +       will be called sch_fq_pie.
>> > +
>> > +       If unsure, say N.
>> > +
>> >  config NET_SCH_INGRESS
>> >       tristate "Ingress/classifier-action Qdisc"
>> >       depends on NET_CLS_ACT
>> > diff --git a/net/sched/Makefile b/net/sched/Makefile
>> > index 8a40431d7b5c..fdcd3f7b2fb2 100644
>> > --- a/net/sched/Makefile
>> > +++ b/net/sched/Makefile
>> > @@ -55,6 +55,7 @@ obj-$(CONFIG_NET_SCH_CAKE)  += sch_cake.o
>> >  obj-$(CONFIG_NET_SCH_FQ)     += sch_fq.o
>> >  obj-$(CONFIG_NET_SCH_HHF)    += sch_hhf.o
>> >  obj-$(CONFIG_NET_SCH_PIE)    += sch_pie.o
>> > +obj-$(CONFIG_NET_SCH_FQ_PIE)    += sch_fq_pie.o
>> >  obj-$(CONFIG_NET_SCH_CBS)    += sch_cbs.o
>> >  obj-$(CONFIG_NET_SCH_ETF)    += sch_etf.o
>> >  obj-$(CONFIG_NET_SCH_TAPRIO) += sch_taprio.o
>> > diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c
>> > new file mode 100644
>> > index 000000000000..4ccefa0bc7f0
>> > --- /dev/null
>> > +++ b/net/sched/sch_fq_pie.c
>> > @@ -0,0 +1,485 @@
>> > +/*
>> > + * net/sched/sch_fq_pie.c
>> > + *
>> > + * This program is free software; you can redistribute it and/or
>> > + * modify it under the terms of the GNU General Public License
>> > + * as published by the Free Software Foundation; either version 2
>> > + * of the License.
>> > + *
>> > + * This program is distributed in the hope that it will be useful,
>> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> > + * GNU General Public License for more details.
>>
>> Lose the license boilerplate and replace it with a SPDX header line.
>
> I shall do that.
>
>>
>> > + * Author: Mohit P. Tahiliani <[email-masked]>
>> > + * Author: Sachin D. Patil <[email-masked]>
>> > + * Author: Mohit Bhasi <[email-masked]>
>> > + * Author: V Saicharan <[email-masked]>
>> > + * Author: Leslie Monis <[email-masked]>
>> > + * Author: Gautam Ramakrishnan <[email-masked]>
>> > + *
>> > + * References:
>> > + * RFC 8033: https://tools.ietf.org/html/rfc8033
>> > + */
>> > +
>> > +#include <linux/jhash.h>
>> > +#include <linux/vmalloc.h>
>> > +#include <net/pie.h>
>> > +
>> > +struct fq_pie_params {
>> > +     struct pie_params p_params;
>> > +     u32 ecn_prob;
>> > +     u32 flows_cnt;
>> > +};
>> > +
>> > +struct fq_pie_stats {
>> > +     u32 packets_in;         /* total number of packets enqueued */
>> > +     u32 dropped;            /* packets dropped due to fq_pie action */
>> > +     u32 overlimit;          /* dropped due to lack of space in queue */
>> > +     u32 ecn_mark;           /* packets marked with ECN */
>> > +     u32 new_flow_count;     /* number of time packets created a new flow */
>> > +};
>> > +
>> > +struct fq_pie_flow {
>> > +     s32 deficit;            /* number of credits remaining for the flow */
>> > +     u32 backlog;            /* size of data in the flow */
>> > +     u32 qlen;               /* number of packets in the flow */
>> > +     struct sk_buff *head;
>> > +     struct sk_buff *tail;
>> > +     struct list_head flowchain;
>> > +     struct pie_vars vars;   /* pie vars for the flow */
>> > +     struct pie_stats stats; /* pie stats for the flow */
>> > +};
>> > +
>> > +struct fq_pie_sched_data {
>> > +     u32 quantum;            /* number of credits in deficit round robin */
>> > +     struct fq_pie_flow *flows;
>> > +     struct Qdisc *sch;
>> > +     struct fq_pie_params params;
>> > +     struct fq_pie_stats stats;
>> > +     struct list_head old_flows;
>> > +     struct list_head new_flows;
>> > +     struct timer_list adapt_timer;
>> > +};
>>
>> The flow and sched_data structs have quite a bit in common with those in
>> fq_codel; but the members are in a different order.
>>
>> > +static void fq_pie_params_init(struct fq_pie_params *params)
>> > +{
>> > +     pie_params_init(&params->p_params);
>> > +     params->ecn_prob = 10;
>> > +     params->flows_cnt = 1024;
>> > +}
>> > +
>> > +static inline void flow_queue_add(struct fq_pie_flow *flow,
>> > +                               struct sk_buff *skb)
>> > +{
>> > +     if (!flow->head)
>> > +             flow->head = skb;
>> > +     else
>> > +             flow->tail->next = skb;
>> > +     flow->tail = skb;
>> > +     skb->next = NULL;
>> > +}
>> > +
>> > +static int fq_pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>> > +                             struct sk_buff **to_free)
>> > +{
>> > +     struct fq_pie_sched_data *q = qdisc_priv(sch);
>> > +     struct fq_pie_flow *sel_flow;
>> > +     u32 pkt_len;
>> > +     u32 idx;
>> > +     u8 enqueue = false;
>> > +
>> > +     /* Classifies packet into corresponding flow */
>> > +     idx = reciprocal_scale(skb_get_hash(skb), q->params.flows_cnt);
>> > +     sel_flow = &q->flows[idx];
>>
>> This is missing the ability to override the classification from tc
>> filters. See fq_codel_classify().
>
> I wanted to keep it simple initially. Shall add that.
>
>>
>> > +
>> > +     /* Checks if the qdisc is full */
>> > +     if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
>> > +             q->stats.overlimit++;
>> > +             sel_flow->stats.overlimit++;
>> > +             goto out;
>> > +     }
>>
>> The memory_limit checks in fq_codel have turned out to be quite useful
>> on constrained systems. I'd suggest adding them here as well.
>
> I shall add that too.
>
>>
>> > +
>> > +     if (!drop_early(sch, sel_flow->backlog, skb->len, &sel_flow->vars,
>> > +                     &q->params.p_params)) {
>> > +             enqueue = true;
>> > +     } else if (q->params.p_params.ecn &&
>> > +                sel_flow->vars.prob <=
>> > +                (MAX_PROB / 100) * q->params.ecn_prob &&
>> > +                INET_ECN_set_ce(skb)) {
>> > +             /* If packet is ecn capable, mark it if drop probability
>> > +              * is lower than the parameter ecn_prob, else drop it.
>> > +              */
>> > +             q->stats.ecn_mark++;
>> > +             sel_flow->stats.ecn_mark++;
>> > +             enqueue = true;
>> > +     }
>> > +     if (enqueue) {
>> > +             pkt_len = qdisc_pkt_len(skb);
>> > +             q->stats.packets_in++;
>> > +             sch->qstats.backlog += pkt_len;
>> > +             sch->q.qlen++;
>> > +             flow_queue_add(sel_flow, skb);
>> > +             if (list_empty(&sel_flow->flowchain)) {
>> > +                     list_add_tail(&sel_flow->flowchain, &q->new_flows);
>> > +                     q->stats.new_flow_count++;
>> > +                     sel_flow->deficit = q->quantum;
>> > +                     sel_flow->stats.dropped = 0;
>> > +                     sel_flow->qlen = 0;
>> > +                     sel_flow->backlog = 0;
>> > +             }
>> > +             sel_flow->qlen++;
>> > +             sel_flow->stats.packets_in++;
>> > +             sel_flow->backlog += pkt_len;
>> > +             return NET_XMIT_SUCCESS;
>> > +     }
>> > +out:
>> > +     q->stats.dropped++;
>> > +     sel_flow->stats.dropped++;
>> > +     return qdisc_drop(skb, sch, to_free);
>>
>> You probably want to return NET_XMIT_CN instead of NET_XMIT_DROP here.
>>
>
> Should I replace qdisc_drop with __qdisc_drop and return NET_XMIT_CN?

Yeah, I think that would be more appropriate here, as that would
directly throttle sockets on the same host.

>> > +}
>> > +
>> > +static const struct nla_policy fq_pie_policy[TCA_FQ_PIE_MAX + 1] = {
>> > +     [TCA_FQ_PIE_TARGET] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_LIMIT] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_TUPDATE] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_ALPHA] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_BETA] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_ECN] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_QUANTUM] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_BYTEMODE] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_FLOWS] = {.type = NLA_U32},
>> > +     [TCA_FQ_PIE_ECN_PROB] = {.type = NLA_U32}
>> > +};
>> > +
>> > +static inline struct sk_buff *dequeue_head(struct fq_pie_flow *flow)
>> > +{
>> > +     struct sk_buff *skb = flow->head;
>> > +
>> > +     flow->head = skb->next;
>> > +     skb->next = NULL;
>> > +     return skb;
>> > +}
>> > +
>> > +static struct sk_buff *fq_pie_qdisc_dequeue(struct Qdisc *sch)
>> > +{
>> > +     struct fq_pie_sched_data *q = qdisc_priv(sch);
>> > +     struct sk_buff *skb = NULL;
>> > +     struct fq_pie_flow *flow;
>> > +     struct list_head *head;
>> > +     u32 pkt_len;
>> > +
>> > +begin:
>> > +     head = &q->new_flows;
>> > +     if (list_empty(head)) {
>> > +             head = &q->old_flows;
>> > +             if (list_empty(head))
>> > +                     return NULL;
>> > +     }
>> > +
>> > +     flow = list_first_entry(head, struct fq_pie_flow, flowchain);
>> > +     /* Flow has exhausted all its credits */
>> > +     if (flow->deficit <= 0) {
>> > +             flow->deficit += q->quantum;
>> > +             list_move_tail(&flow->flowchain, &q->old_flows);
>> > +             goto begin;
>> > +     }
>> > +
>> > +     if (flow->head) {
>> > +             skb = dequeue_head(flow);
>> > +             pkt_len = qdisc_pkt_len(skb);
>> > +             sch->qstats.backlog -= pkt_len;
>> > +             sch->q.qlen--;
>> > +             qdisc_bstats_update(sch, skb);
>> > +     }
>>
>> If you factor this out into a dequeue_func(), this dequeue() function is
>> very close to identical to the one in fq_codel().
>
> Do you suggest I put the if(flow->head) block in a different function?

Yeah, that was what I meant. However, without doing the full refactoring
it's only borderline useful, so feel free to just keep it the way it
is...

-Toke