Re: [RFC] net_sched: mark packet staying on queue too long

Eric Dumazet
2011-01-04T18:20:21Z

Le mardi 04 janvier 2011 à 16:02 +0100, Eric Dumazet a écrit :

> I'd like to try kind of a SFQRED implementation, ie :
>
> classify flows, then instead of using plain pfifo queues (currently done
> in SFQ), use N pseudo RED queues.
>
> RED is a bit complex because it tries to make the probability estimation
> given queue backlog average. It has to use expensive time services (on
> some machines at least, if TSC not available)
>
> My idea was to take into account the delay packets stay in its queue, so
> that no extra state is needed : Only take a timestamp when packet is
> enqueued, compute delta when dequeued, get
>
> Px = delta * Prob_per_time_unit;
> and drop/mark packet with Px probability.
>
> Ram usage of SFQRED would be the same than SFQ, and cost roughly the
> same (because we could use jiffies based time sampling, (and HZ=1000 for
> a ms unit)).
>
>

Here is the POC patch I am currently testing, with a probability to
"early drop" a packet of one percent per ms (HZ=1000 here), only if
packet stayed at least 4 ms on queue.

Of course, this only apply where SFQ is used, with known SFQ limits :)

The term "early drop" is a lie. RED really early mark/drop a packet at
enqueue() time, while I do it at dequeue() time [since I need to compute
the delay]. But effect is the same on sent packets. This might use a bit
more memory, but no more than current SFQ [and only if flows dont react
to mark/drops]

insmod net/sched/sch_sfq.ko red_delay=4

By the way, I do think we should lower SFQ_DEPTH a bit and increase
SFQ_SLOTS by same amount. Allowing 127 packets per flow seems not
necessary in most situations SFQ might be used.

net/sched/sch_sfq.c | 37 +++++++++++++++++++++++++++++++++----
1 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index d54ac94..4f958e3 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -24,6 +24,8 @@
#include <net/ip.h>
#include <net/netlink.h>
#include <net/pkt_sched.h>
+#include <net/inet_ecn.h>
+#include <linux/moduleparam.h>

/* Stochastic Fairness Queuing algorithm.
@@ -86,6 +88,10 @@
/* This type should contain at least SFQ_DEPTH + SFQ_SLOTS values */
typedef unsigned char sfq_index;

+static int red_delay; /* default : no RED handling */
+module_param(red_delay, int, 0);
+MODULE_PARM_DESC(red_delay, "mark/drop packets if they stay in queue longer than red_delay ticks");
+
/*
* We dont use pointers to save space.
* Small indexes [0 ... SFQ_SLOTS - 1] are 'pointers' to slots[] array
@@ -391,6 +397,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)

sch->qstats.backlog += qdisc_pkt_len(skb);
slot_queue_add(slot, skb);
+ qdisc_skb_cb(skb)->timestamp = jiffies;
sfq_inc(q, x);
if (slot->qlen == 1) { /* The flow is new */
if (q->tail == NULL) { /* It is the first flow */
@@ -402,11 +409,8 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
q->tail = slot;
slot->allot = q->scaled_quantum;
}
- if (++sch->q.qlen <= q->limit) {
- sch->bstats.bytes += qdisc_pkt_len(skb);
- sch->bstats.packets++;
+ if (++sch->q.qlen <= q->limit)
return NET_XMIT_SUCCESS;
- }

sfq_drop(sch);
return NET_XMIT_CN;
@@ -432,6 +436,7 @@ sfq_dequeue(struct Qdisc *sch)
sfq_index a, next_a;
struct sfq_slot *slot;

+restart:
/* No active slots */
if (q->tail == NULL)
return NULL;
@@ -455,12 +460,36 @@ next_slot:
next_a = slot->next;
if (a == next_a) {
q->tail = NULL; /* no more active slots */
+ /* last packet queued, dont even try to apply RED */
return skb;
}
q->tail->next = next_a;
} else {
slot->allot -= SFQ_ALLOT_SIZE(qdisc_pkt_len(skb));
}
+ if (red_delay) {
+ long delay = jiffies - qdisc_skb_cb(skb)->timestamp;
+
+ if (delay >= red_delay) {
+ long Px = delay * (0xFFFFFF / 100); /* 1 percent per jiffy */
+ if ((net_random() & 0xFFFFFF) < Px) {
+ if (INET_ECN_set_ce(skb)) {
+ /* no ecnmark counter yet :) */
+ sch->qstats.overlimits++;
+ } else {
+ /* penalize this flow : we drop the
+ * packet while we changed slot->allot
+ */
+ kfree_skb(skb);
+ /* no early_drop counter yet :) */
+ sch->qstats.drops++;
+ goto restart;
+ }
+ }
+ }
+ }
+ sch->bstats.bytes += qdisc_pkt_len(skb);
+ sch->bstats.packets++;
return skb;
}