Re: [PATCH net-next-2.6] sch_sfq: allow big packets and be fair

Jarek Poplawski
2010-12-21T10:15:15Z

On 2010-12-21 00:16, Eric Dumazet wrote:
> SFQ is currently 'limited' to small packets, because it uses a 16bit
> allotment number per flow. Switch it to 18bit, and use appropriate
> handling to make sure this allotment is in [1 .. quantum] range before a
> new packet is dequeued, so that fairness is respected.

Well, such two important changes should be in separate patches.

The change of allotment limit looks OK (but I would try scaling, e.g.
in 16-byte chunks, btw).

The change in fair treatment looks dubious. A flow which uses exactly
it's quantum in one round will be skipped in the next round. A flow
which uses a bit more than its quantum in one round, will be skipped
too, while we should only give it less this time to keep the sum up to
2 quantums. (The usual algorithm is to check if a flow has enough
"tickets" for sending its next packet.)

Jarek P.

>
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Jarek Poplawski <jarkao2@gmail.com>
> Cc: Patrick McHardy <kaber@trash.net>
> ---
> net/sched/sch_sfq.c | 24 ++++++++++++++++--------
> 1 file changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
> index c474b4b..878704a 100644
> --- a/net/sched/sch_sfq.c
> +++ b/net/sched/sch_sfq.c
> @@ -67,7 +67,7 @@
>
> IMPLEMENTATION:
> This implementation limits maximal queue length to 128;
> - maximal mtu to 2^15-1; max 128 flows, number of hash buckets to 1024.
> + maximal mtu to 2^16-1; max 128 flows, number of hash buckets to 1024.
> The only goal of this restrictions was that all data
> fit into one 4K page on 32bit arches.
>
> @@ -99,9 +99,10 @@ struct sfq_slot {
> sfq_index qlen; /* number of skbs in skblist */
> sfq_index next; /* next slot in sfq chain */
> struct sfq_head dep; /* anchor in dep[] chains */
> - unsigned short hash; /* hash value (index in ht[]) */
> - short allot; /* credit for this slot */
> + unsigned int hash:14; /* hash value (index in ht[]) */
> + unsigned int allot:18; /* credit for this slot */
> };
> +#define ALLOT_ZERO (1 << 16)
>
> struct sfq_sched_data
> {
> @@ -394,7 +395,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> q->tail->next = x;
> }
> q->tail = slot;
> - slot->allot = q->quantum;
> + slot->allot = ALLOT_ZERO + q->quantum;
> }
> if (++sch->q.qlen <= q->limit) {
> sch->bstats.bytes += qdisc_pkt_len(skb);
> @@ -430,8 +431,14 @@ sfq_dequeue(struct Qdisc *sch)
> if (q->tail == NULL)
> return NULL;
>
> +next:
> a = q->tail->next;
> slot = &q->slots[a];
> + if (slot->allot <= ALLOT_ZERO) {
> + q->tail = slot;
> + slot->allot += q->quantum;
> + goto next;
> + }
> skb = slot_dequeue_head(slot);
> sfq_dec(q, a);
> sch->q.qlen--;
> @@ -446,9 +453,8 @@ sfq_dequeue(struct Qdisc *sch)
> return skb;
> }
> q->tail->next = next_a;
> - } else if ((slot->allot -= qdisc_pkt_len(skb)) <= 0) {
> - q->tail = slot;
> - slot->allot += q->quantum;
> + } else {
> + slot->allot -= qdisc_pkt_len(skb);
> }
> return skb;
> }
> @@ -610,7 +616,9 @@ static int sfq_dump_class_stats(struct Qdisc *sch, unsigned long cl,
> struct sfq_sched_data *q = qdisc_priv(sch);
> const struct sfq_slot *slot = &q->slots[q->ht[cl - 1]];
> struct gnet_stats_queue qs = { .qlen = slot->qlen };
> - struct tc_sfq_xstats xstats = { .allot = slot->allot };
> + struct tc_sfq_xstats xstats = {
> + .allot = slot->allot - ALLOT_ZERO
> + };
> struct sk_buff *skb;
>
> slot_queue_walk(slot, skb)
>
>
> --