sch_sfq: rehash queues in perturb timer

A known Out Of Order (OOO) problem hurts SFQ when timer changes
perturbation value, since all new packets delivered to SFQ enqueue might
end on different slots than previous in-flight packets.

With round robin delivery, we can thus deliver packets in a different
order.

Since SFQ is limited to small amount of in-flight packets, we can rehash
packets so that this OOO problem is fixed.

This rehashing is performed only if internal flow classifier is in use.

We now store in skb->cb[] the "struct flow_keys" so that we dont call
skb_flow_dissect() again while rehashing.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 81 insertions, 6 deletions

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 30cda707e400..d329a8a72357 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -136,16 +136,30 @@ static inline struct sfq_head *sfq_dep_head(struct sfq_sched_data *q, sfq_index
         return &q->dep[val - SFQ_SLOTS];
 }
 
+/*
+ * In order to be able to quickly rehash our queue when timer changes
+ * q->perturbation, we store flow_keys in skb->cb[]
+ */
+struct sfq_skb_cb {
+       struct flow_keys        keys;
+};
+
+static inline struct sfq_skb_cb *sfq_skb_cb(const struct sk_buff *skb)
+{
+       BUILD_BUG_ON(sizeof(skb->cb) <
+               sizeof(struct qdisc_skb_cb) + sizeof(struct sfq_skb_cb));
+       return (struct sfq_skb_cb *)qdisc_skb_cb(skb)->data;
+}
+
 static unsigned int sfq_hash(const struct sfq_sched_data *q,
                              const struct sk_buff *skb)
 {
-        struct flow_keys keys;
+        const struct flow_keys *keys = &sfq_skb_cb(skb)->keys;
         unsigned int hash;
 
-        skb_flow_dissect(skb, &keys);
-        hash = jhash_3words((__force u32)keys.dst,
-                            (__force u32)keys.src ^ keys.ip_proto,
-                            (__force u32)keys.ports, q->perturbation);
+        hash = jhash_3words((__force u32)keys->dst,
+                            (__force u32)keys->src ^ keys->ip_proto,
+                            (__force u32)keys->ports, q->perturbation);
         return hash & (q->divisor - 1);
 }
 
@@ -161,8 +175,10 @@ static unsigned int sfq_classify(struct sk_buff *skb, struct Qdisc *sch,
             TC_H_MIN(skb->priority) <= q->divisor)
                 return TC_H_MIN(skb->priority);
 
-        if (!q->filter_list)
+        if (!q->filter_list) {
+                skb_flow_dissect(skb, &sfq_skb_cb(skb)->keys);
                 return sfq_hash(q, skb) + 1;
+        }
 
         *qerr = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
         result = tc_classify(skb, q->filter_list, &res);
@@ -423,12 +439,71 @@ sfq_reset(struct Qdisc *sch)
                 kfree_skb(skb);
 }
 
+/*
+ * When q->perturbation is changed, we rehash all queued skbs
+ * to avoid OOO (Out Of Order) effects.
+ * We dont use sfq_dequeue()/sfq_enqueue() because we dont want to change
+ * counters.
+ */
+static void sfq_rehash(struct sfq_sched_data *q)
+{
+        struct sk_buff *skb;
+        int i;
+        struct sfq_slot *slot;
+        struct sk_buff_head list;
+
+        __skb_queue_head_init(&list);
+
+        for (i = 0; i < SFQ_SLOTS; i++) {
+                slot = &q->slots[i];
+                if (!slot->qlen)
+                        continue;
+                while (slot->qlen) {
+                        skb = slot_dequeue_head(slot);
+                        sfq_dec(q, i);
+                        __skb_queue_tail(&list, skb);
+                }
+                q->ht[slot->hash] = SFQ_EMPTY_SLOT;
+        }
+        q->tail = NULL;
+
+        while ((skb = __skb_dequeue(&list)) != NULL) {
+                unsigned int hash = sfq_hash(q, skb);
+                sfq_index x = q->ht[hash];
+
+                slot = &q->slots[x];
+                if (x == SFQ_EMPTY_SLOT) {
+                        x = q->dep[0].next; /* get a free slot */
+                        q->ht[hash] = x;
+                        slot = &q->slots[x];
+                        slot->hash = hash;
+                }
+                slot_queue_add(slot, skb);
+                sfq_inc(q, x);
+                if (slot->qlen == 1) {          /* The flow is new */
+                        if (q->tail == NULL) {  /* It is the first flow */
+                                slot->next = x;
+                        } else {
+                                slot->next = q->tail->next;
+                                q->tail->next = x;
+                        }
+                        q->tail = slot;
+                        slot->allot = q->scaled_quantum;
+                }
+        }
+}
+
 static void sfq_perturbation(unsigned long arg)
 {
         struct Qdisc *sch = (struct Qdisc *)arg;
         struct sfq_sched_data *q = qdisc_priv(sch);
+        spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));
 
+        spin_lock(root_lock);
         q->perturbation = net_random();
+        if (!q->filter_list && q->tail)
+                sfq_rehash(q);
+        spin_unlock(root_lock);
 
         if (q->perturb_period)
                 mod_timer(&q->perturb_timer, jiffies + q->perturb_period);