Subject: Re: [RFC] [PATCH] Avoid enqueuing skb for default qdiscs

From: Krishna Kumar <[email-masked]>
Date: Tue, 04 Aug 2009 21:21:35 +0530

> Jarek Poplawski <[email-masked]> wrote on 08/04/2009 12:52:05 PM:

Hi Jarek,

(Sorry that you are getting this mail twice since the mail didn't go out to
others, but this time I have fixed my mailer)

> Actually I meant:
> -      qdisc->q.qlen--;
> +      qdisc->q.qlen = 0;
> because you can do it after another qdisc->q.qlen = 0.

You are right - I saw some of the sched resets setting it to zero (while others
dont touch it). I will reset to zero.

> > > I wonder if we shouldn't update bstats yet. Btw, try checkpatch.
> >
> > Rate estimator isn't used for default qdisc, so is anything
> > required?
>
> I meant "Sent" stats from "tc -s qdisc sh" could be misleading.

You are right - for a single netperf run, the stats magically show 0
packets/bytes sent!

> > Also, checkpatch suggested to change the original code:
> >         "if (unlikely((skb = dequeue_skb(q)) == NULL))"
> > since it appears as "new code", hope that change is fine.
>
> I mainly meant a whitespace warning.

Yes, I had fixed that also.

A compile and single-netperf-run tested patch is inlined below - the only
two changes from last patch is setting qlen=0 and updating bstats. Now
the stats show:

qdisc pfifo_fast 0: root bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 10170288228 bytes 157522 pkt (dropped 0, overlimits 0 requeues 0) 
 rate 0bit 0pps backlog 0b 0p requeues 0 

On the original kernel, I got:
qdisc pfifo_fast 0: root bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 10157266798 bytes 177539 pkt (dropped 0, overlimits 0 requeues 0) 
 rate 0bit 0pps backlog 0b 0p requeues 0 

Thanks,

- KK

Signed-off-by: Krishna Kumar <[email-masked]>
---
 include/net/pkt_sched.h   |    3 +
 include/net/sch_generic.h |   15 +++++
 net/core/dev.c            |   51 ++++++++++++++-----
 net/sched/sch_generic.c   |   93 ++++++++++++++++++++++--------------
 4 files changed, 111 insertions(+), 51 deletions(-)

diff -ruNp org2/include/net/pkt_sched.h new5/include/net/pkt_sched.h
--- org2/include/net/pkt_sched.h	2009-08-04 11:41:21.000000000 +0530
+++ new5/include/net/pkt_sched.h	2009-08-04 18:47:16.000000000 +0530
@@ -87,6 +87,9 @@ extern struct qdisc_rate_table *qdisc_ge
 extern void qdisc_put_rtab(struct qdisc_rate_table *tab);
 extern void qdisc_put_stab(struct qdisc_size_table *tab);
 extern void qdisc_warn_nonwc(char *txt, struct Qdisc *qdisc);
+extern int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
+			   struct net_device *dev, struct netdev_queue *txq,
+			   spinlock_t *root_lock);
 
 extern void __qdisc_run(struct Qdisc *q);
 
diff -ruNp org2/include/net/sch_generic.h new5/include/net/sch_generic.h
--- org2/include/net/sch_generic.h	2009-03-24 08:54:16.000000000 +0530
+++ new5/include/net/sch_generic.h	2009-08-04 19:58:26.000000000 +0530
@@ -45,6 +45,7 @@ struct Qdisc
 #define TCQ_F_BUILTIN		1
 #define TCQ_F_THROTTLED		2
 #define TCQ_F_INGRESS		4
+#define TCQ_F_CAN_BYPASS	8
 #define TCQ_F_WARN_NONWC	(1 << 16)
 	int			padded;
 	struct Qdisc_ops	*ops;
@@ -182,6 +183,11 @@ struct qdisc_skb_cb {
 	char			data[];
 };
 
+static inline int qdisc_qlen(struct Qdisc *q)
+{
+	return q->q.qlen;
+}
+
 static inline struct qdisc_skb_cb *qdisc_skb_cb(struct sk_buff *skb)
 {
 	return (struct qdisc_skb_cb *)skb->cb;
@@ -387,13 +393,18 @@ static inline int qdisc_enqueue_root(str
 	return qdisc_enqueue(skb, sch) & NET_XMIT_MASK;
 }
 
+static inline int __qdisc_update_bstats(struct Qdisc *sch, unsigned int len)
+{
+	sch->bstats.bytes += len;
+	sch->bstats.packets++;
+}
+
 static inline int __qdisc_enqueue_tail(struct sk_buff *skb, struct Qdisc *sch,
 				       struct sk_buff_head *list)
 {
 	__skb_queue_tail(list, skb);
 	sch->qstats.backlog += qdisc_pkt_len(skb);
-	sch->bstats.bytes += qdisc_pkt_len(skb);
-	sch->bstats.packets++;
+	__qdisc_update_bstats(sch, qdisc_pkt_len(skb));
 
 	return NET_XMIT_SUCCESS;
 }
diff -ruNp org2/net/core/dev.c new5/net/core/dev.c
--- org2/net/core/dev.c	2009-07-27 09:08:24.000000000 +0530
+++ new5/net/core/dev.c	2009-08-04 20:26:57.000000000 +0530
@@ -1786,6 +1786,43 @@ static struct netdev_queue *dev_pick_tx(
 	return netdev_get_tx_queue(dev, queue_index);
 }
 
+static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
+				 struct net_device *dev,
+				 struct netdev_queue *txq)
+{
+	spinlock_t *root_lock = qdisc_lock(q);
+	int rc;
+
+	spin_lock(root_lock);
+	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
+		kfree_skb(skb);
+		rc = NET_XMIT_DROP;
+	} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
+		   !test_and_set_bit(__QDISC_STATE_RUNNING, &q->state)) {
+		unsigned int len = skb->len;
+
+		/*
+		 * This is a work-conserving queue; there are no old skbs
+		 * waiting to be sent out; and the qdisc is not running -
+		 * xmit the skb directly.
+		 */
+		if (sch_direct_xmit(skb, q, dev, txq, root_lock))
+			__qdisc_run(q);
+		else
+			clear_bit(__QDISC_STATE_RUNNING, &q->state);
+
+		__qdisc_update_bstats(q, len);
+
+		rc = NET_XMIT_SUCCESS;
+	} else {
+		rc = qdisc_enqueue_root(skb, q);
+		qdisc_run(q);
+	}
+	spin_unlock(root_lock);
+
+	return rc;
+}
+
 /**
  *	dev_queue_xmit - transmit a buffer
  *	@skb: buffer to transmit
@@ -1859,19 +1896,7 @@ gso:
 	skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
 #endif
 	if (q->enqueue) {
-		spinlock_t *root_lock = qdisc_lock(q);
-
-		spin_lock(root_lock);
-
-		if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
-			kfree_skb(skb);
-			rc = NET_XMIT_DROP;
-		} else {
-			rc = qdisc_enqueue_root(skb, q);
-			qdisc_run(q);
-		}
-		spin_unlock(root_lock);
-
+		rc = __dev_xmit_skb(skb, q, dev, txq);
 		goto out;
 	}
 
diff -ruNp org2/net/sched/sch_generic.c new5/net/sched/sch_generic.c
--- org2/net/sched/sch_generic.c	2009-05-25 07:48:07.000000000 +0530
+++ new5/net/sched/sch_generic.c	2009-08-04 19:38:27.000000000 +0530
@@ -37,15 +37,11 @@
  * - updates to tree and tree walking are only done under the rtnl mutex.
  */
 
-static inline int qdisc_qlen(struct Qdisc *q)
-{
-	return q->q.qlen;
-}
-
 static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
 {
 	q->gso_skb = skb;
 	q->qstats.requeues++;
+	q->q.qlen++;	/* it's still part of the queue */
 	__netif_schedule(q);
 
 	return 0;
@@ -61,9 +57,11 @@ static inline struct sk_buff *dequeue_sk
 
 		/* check the reason of requeuing without tx lock first */
 		txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
-		if (!netif_tx_queue_stopped(txq) && !netif_tx_queue_frozen(txq))
+		if (!netif_tx_queue_stopped(txq) &&
+		    !netif_tx_queue_frozen(txq)) {
 			q->gso_skb = NULL;
-		else
+			q->q.qlen--;
+		} else
 			skb = NULL;
 	} else {
 		skb = q->dequeue(q);
@@ -103,44 +101,23 @@ static inline int handle_dev_cpu_collisi
 }
 
 /*
- * NOTE: Called under qdisc_lock(q) with locally disabled BH.
- *
- * __QDISC_STATE_RUNNING guarantees only one CPU can process
- * this qdisc at a time. qdisc_lock(q) serializes queue accesses for
- * this queue.
- *
- *  netif_tx_lock serializes accesses to device driver.
- *
- *  qdisc_lock(q) and netif_tx_lock are mutually exclusive,
- *  if one is grabbed, another must be free.
- *
- * Note, that this procedure can be called by a watchdog timer
+ * Transmit one skb, and handle the return status as required. Holding the
+ * __QDISC_STATE_RUNNING bit guarantees that only one CPU can execute this
+ * function.
  *
  * Returns to the caller:
  *				0  - queue is empty or throttled.
  *				>0 - queue is not empty.
- *
  */
-static inline int qdisc_restart(struct Qdisc *q)
+int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
+		    struct net_device *dev, struct netdev_queue *txq,
+		    spinlock_t *root_lock)
 {
-	struct netdev_queue *txq;
 	int ret = NETDEV_TX_BUSY;
-	struct net_device *dev;
-	spinlock_t *root_lock;
-	struct sk_buff *skb;
-
-	/* Dequeue packet */
-	if (unlikely((skb = dequeue_skb(q)) == NULL))
-		return 0;
-
-	root_lock = qdisc_lock(q);
 
 	/* And release qdisc */
 	spin_unlock(root_lock);
 
-	dev = qdisc_dev(q);
-	txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
-
 	HARD_TX_LOCK(dev, txq, smp_processor_id());
 	if (!netif_tx_queue_stopped(txq) &&
 	    !netif_tx_queue_frozen(txq))
@@ -177,6 +154,44 @@ static inline int qdisc_restart(struct Q
 	return ret;
 }
 
+/*
+ * NOTE: Called under qdisc_lock(q) with locally disabled BH.
+ *
+ * __QDISC_STATE_RUNNING guarantees only one CPU can process
+ * this qdisc at a time. qdisc_lock(q) serializes queue accesses for
+ * this queue.
+ *
+ *  netif_tx_lock serializes accesses to device driver.
+ *
+ *  qdisc_lock(q) and netif_tx_lock are mutually exclusive,
+ *  if one is grabbed, another must be free.
+ *
+ * Note, that this procedure can be called by a watchdog timer
+ *
+ * Returns to the caller:
+ *				0  - queue is empty or throttled.
+ *				>0 - queue is not empty.
+ *
+ */
+static inline int qdisc_restart(struct Qdisc *q)
+{
+	struct netdev_queue *txq;
+	struct net_device *dev;
+	spinlock_t *root_lock;
+	struct sk_buff *skb;
+
+	/* Dequeue packet */
+	skb = dequeue_skb(q);
+	if (unlikely(!skb))
+		return 0;
+
+	root_lock = qdisc_lock(q);
+	dev = qdisc_dev(q);
+	txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
+
+	return sch_direct_xmit(skb, q, dev, txq, root_lock);
+}
+
 void __qdisc_run(struct Qdisc *q)
 {
 	unsigned long start_time = jiffies;
@@ -547,8 +562,11 @@ void qdisc_reset(struct Qdisc *qdisc)
 	if (ops->reset)
 		ops->reset(qdisc);
 
-	kfree_skb(qdisc->gso_skb);
-	qdisc->gso_skb = NULL;
+	if (qdisc->gso_skb) {
+		kfree_skb(qdisc->gso_skb);
+		qdisc->gso_skb = NULL;
+		qdisc->q.qlen = 0;
+	}
 }
 EXPORT_SYMBOL(qdisc_reset);
 
@@ -605,6 +623,9 @@ static void attach_one_default_qdisc(str
 			printk(KERN_INFO "%s: activation failed\n", dev->name);
 			return;
 		}
+
+		/* Can by-pass the queue discipline for default qdisc */
+		qdisc->flags |= TCQ_F_CAN_BYPASS;
 	} else {
 		qdisc =  &noqueue_qdisc;
 	}