[PATCH v3] net_sched: sch_sfq: fix allot handling

Eric Dumazet
2010-12-15T17:30:32Z

Le mercredi 15 décembre 2010 à 18:21 +0100, Patrick McHardy a écrit :
> On 15.12.2010 18:09, Eric Dumazet wrote:
> > Le mercredi 15 décembre 2010 à 18:03 +0100, Patrick McHardy a écrit :
> >
> >> Right, that's odd. It shouldn't be necessary anymore though since
> >> now we initialize allot in sfq_enqueue() for all new flows and
> >> increase allotment for all active flows once per round in sfq_dequeue().
> >> The above code causes a second increase for the flow following a flow
> >> which went inactive.
> >
> > Well, we do this in three places. Each time we 'select' a flow as the
> > next packet provider, we increase its allot by quantum.
> >
> > We could change this, adding quantum to the current slot when its allot
> > becomes negative (and we select the next slot for next round)
>
> Right, I again missed that 'a' refers to the next flow in the second
> condition in sfq_dequeue().
>
> I have to run now, will have another look at this tommorrow.

Here is v3 : Now we add a quantum only when current flow consumed all
its allot, not "in advance for next slot". this should use less cpu too.

Thanks

[PATCH v3] net_sched: sch_sfq: fix allot handling

When deploying SFQ/IFB here at work, I found the allot management was
pretty wrong in sfq, even changing allot from short to int...

We should init allot for each new flow, not using a previous value found
in slot.

Before patch, I saw bursts of several packets per flow, apparently
denying the default "quantum 1514" limit I had on my SFQ class.

class sfq 11:1 parent 11:
(dropped 0, overlimits 0 requeues 0)
backlog 0b 7p requeues 0
allot 11546

class sfq 11:46 parent 11:
(dropped 0, overlimits 0 requeues 0)
backlog 0b 1p requeues 0
allot -23873

class sfq 11:78 parent 11:
(dropped 0, overlimits 0 requeues 0)
backlog 0b 5p requeues 0
allot 11393

After patch, better fairness among each flow, allot limit being
respected, allot is positive :

class sfq 11:e parent 11:
(dropped 0, overlimits 0 requeues 86)
backlog 0b 3p requeues 86
allot 596

class sfq 11:94 parent 11:
(dropped 0, overlimits 0 requeues 0)
backlog 0b 3p requeues 0
allot 1468

class sfq 11:a4 parent 11:
(dropped 0, overlimits 0 requeues 0)
backlog 0b 4p requeues 0
allot 650

class sfq 11:bb parent 11:
(dropped 0, overlimits 0 requeues 0)
backlog 0b 3p requeues 0
allot 596

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
net/sched/sch_sfq.c | 20 ++++++++------------
1 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 3cf478d..7150705 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -270,7 +270,6 @@ static unsigned int sfq_drop(struct Qdisc *sch)
/* It is difficult to believe, but ALL THE SLOTS HAVE LENGTH 1. */
d = q->next[q->tail];
q->next[q->tail] = q->next[d];
- q->allot[q->next[d]] += q->quantum;
skb = q->qs[d].prev;
len = qdisc_pkt_len(skb);
__skb_unlink(skb, &q->qs[d]);
@@ -321,14 +320,13 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
sfq_inc(q, x);
if (q->qs[x].qlen == 1) { /* The flow is new */
if (q->tail == SFQ_DEPTH) { /* It is the first flow */
- q->tail = x;
q->next[x] = x;
- q->allot[x] = q->quantum;
} else {
q->next[x] = q->next[q->tail];
q->next[q->tail] = x;
- q->tail = x;
}
+ q->tail = x;
+ q->allot[x] = q->quantum;
}
if (++sch->q.qlen <= q->limit) {
sch->bstats.bytes += qdisc_pkt_len(skb);
@@ -359,13 +357,13 @@ sfq_dequeue(struct Qdisc *sch)
{
struct sfq_sched_data *q = qdisc_priv(sch);
struct sk_buff *skb;
- sfq_index a, old_a;
+ sfq_index a, next_a;

/* No active slots */
if (q->tail == SFQ_DEPTH)
return NULL;

- a = old_a = q->next[q->tail];
+ a = q->next[q->tail];

/* Grab packet */
skb = __skb_dequeue(&q->qs[a]);
@@ -376,17 +374,15 @@ sfq_dequeue(struct Qdisc *sch)
/* Is the slot empty? */
if (q->qs[a].qlen == 0) {
q->ht[q->hash[a]] = SFQ_DEPTH;
- a = q->next[a];
- if (a == old_a) {
+ next_a = q->next[a];
+ if (a == next_a) {
q->tail = SFQ_DEPTH;
return skb;
}
- q->next[q->tail] = a;
- q->allot[a] += q->quantum;
+ q->next[q->tail] = next_a;
} else if ((q->allot[a] -= qdisc_pkt_len(skb)) <= 0) {
- q->tail = a;
- a = q->next[a];
q->allot[a] += q->quantum;
+ q->tail = a;
}
return skb;
}