net: sched: allow qdiscs to handle locking

This patch adds a flag for queueing disciplines to indicate the stack
does not need to use the qdisc lock to protect operations. This can
be used to build lockless scheduling algorithms and improving
performance.

The flag is checked in the tx path and the qdisc lock is only taken
if it is not set. For now use a conditional if statement. Later we
could be more aggressive if it proves worthwhile and use a static key
or wrap this in a likely().

Also the lockless case drops the TCQ_F_CAN_BYPASS logic. The reason
for this is synchronizing a qlen counter across threads proves to
cost more than doing the enqueue/dequeue operations when tested with
pktgen.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 20 insertions, 10 deletions

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index f6803e1d6783..ec757f66896a 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -174,7 +174,8 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
 	int ret = NETDEV_TX_BUSY;
 
 	/* And release qdisc */
-	spin_unlock(root_lock);
+	if (root_lock)
+		spin_unlock(root_lock);
 
 	/* Note that we validate skb (GSO, checksum, ...) outside of locks */
 	if (validate)
@@ -187,10 +188,13 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
 
 		HARD_TX_UNLOCK(dev, txq);
 	} else {
-		spin_lock(root_lock);
+		if (root_lock)
+			spin_lock(root_lock);
 		return qdisc_qlen(q);
 	}
-	spin_lock(root_lock);
+
+	if (root_lock)
+		spin_lock(root_lock);
 
 	if (dev_xmit_complete(ret)) {
 		/* Driver sent out skb successfully or skb was consumed */
@@ -231,9 +235,9 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
  */
 static inline int qdisc_restart(struct Qdisc *q, int *packets)
 {
+	spinlock_t *root_lock = NULL;
 	struct netdev_queue *txq;
 	struct net_device *dev;
-	spinlock_t *root_lock;
 	struct sk_buff *skb;
 	bool validate;
 
@@ -242,7 +246,9 @@ static inline int qdisc_restart(struct Qdisc *q, int *packets)
 	if (unlikely(!skb))
 		return 0;
 
-	root_lock = qdisc_lock(q);
+	if (!(q->flags & TCQ_F_NOLOCK))
+		root_lock = qdisc_lock(q);
+
 	dev = qdisc_dev(q);
 	txq = skb_get_tx_queue(dev, skb);
 
@@ -880,14 +886,18 @@ static bool some_qdisc_is_busy(struct net_device *dev)
 
 		dev_queue = netdev_get_tx_queue(dev, i);
 		q = dev_queue->qdisc_sleeping;
-		root_lock = qdisc_lock(q);
 
-		spin_lock_bh(root_lock);
+		if (q->flags & TCQ_F_NOLOCK) {
+			val = test_bit(__QDISC_STATE_SCHED, &q->state);
+		} else {
+			root_lock = qdisc_lock(q);
+			spin_lock_bh(root_lock);
 
-		val = (qdisc_is_running(q) ||
-		       test_bit(__QDISC_STATE_SCHED, &q->state));
+			val = (qdisc_is_running(q) ||
+			       test_bit(__QDISC_STATE_SCHED, &q->state));
 
-		spin_unlock_bh(root_lock);
+			spin_unlock_bh(root_lock);
+		}
 
 		if (val)
 			return true;