netdev - [PATCH net-next] sched: manipulate __QDISC_STATE_RUNNING in qdisc_run

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Date:   Tue, 15 May 2018 10:50:31 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        John Fastabend <john.fastabend@...il.com>
Subject: [PATCH net-next] sched: manipulate __QDISC_STATE_RUNNING in qdisc_run_* helpers

Currently NOLOCK qdiscs pay a measurable overhead to atomically
manipulate the __QDISC_STATE_RUNNING. Such bit is flipped twice per
packet in the uncontended scenario with packet rate below the
line rate: on packed dequeue and on the next, failing dequeue attempt.

This changeset moves the bit manipulation into the qdisc_run_{begin,end}
helpers, so that the bit is now flipped only once per packet, with
measurable performance improvement in the uncontended scenario.

This also allows simplifying the qdisc teardown code path - since
qdisc_is_running() is now effective for each qdisc type - and avoid a
possible race between qdisc_run() and dev_deactivate_many(), as now
the some_qdisc_is_busy() can properly detect NOLOCK qdiscs being busy
dequeuing packets.

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 include/net/sch_generic.h | 10 +++++++++-
 net/core/dev.c            |  2 +-
 net/sched/sch_generic.c   | 31 +++++++++----------------------
 3 files changed, 19 insertions(+), 24 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 5154c8300262..4d2b37226e75 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -113,13 +113,19 @@ static inline void qdisc_refcount_inc(struct Qdisc *qdisc)
 
 static inline bool qdisc_is_running(const struct Qdisc *qdisc)
 {
+	if (qdisc->flags & TCQ_F_NOLOCK)
+		return test_bit(__QDISC_STATE_RUNNING, &qdisc->state);
 	return (raw_read_seqcount(&qdisc->running) & 1) ? true : false;
 }
 
 static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 {
-	if (qdisc_is_running(qdisc))
+	if (qdisc->flags & TCQ_F_NOLOCK) {
+		if (test_and_set_bit(__QDISC_STATE_RUNNING, &qdisc->state))
+			return false;
+	} else if (qdisc_is_running(qdisc)) {
 		return false;
+	}
 	/* Variant of write_seqcount_begin() telling lockdep a trylock
 	 * was attempted.
 	 */
@@ -131,6 +137,8 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
 	write_seqcount_end(&qdisc->running);
+	if (qdisc->flags & TCQ_F_NOLOCK)
+		clear_bit(__QDISC_STATE_RUNNING, &qdisc->state);
 }
 
 static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
diff --git a/net/core/dev.c b/net/core/dev.c
index 9f4390182384..7ca19f47a92a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3244,7 +3244,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 			rc = NET_XMIT_DROP;
 		} else {
 			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
-			__qdisc_run(q);
+			qdisc_run(q);
 		}
 
 		if (unlikely(to_free))
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 39c144b6ff98..ff3ce71aec93 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -373,33 +373,24 @@ bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
  */
 static inline bool qdisc_restart(struct Qdisc *q, int *packets)
 {
-	bool more, validate, nolock = q->flags & TCQ_F_NOLOCK;
 	spinlock_t *root_lock = NULL;
 	struct netdev_queue *txq;
 	struct net_device *dev;
 	struct sk_buff *skb;
+	bool validate;
 
 	/* Dequeue packet */
-	if (nolock && test_and_set_bit(__QDISC_STATE_RUNNING, &q->state))
-		return false;
-
 	skb = dequeue_skb(q, &validate, packets);
-	if (unlikely(!skb)) {
-		if (nolock)
-			clear_bit(__QDISC_STATE_RUNNING, &q->state);
+	if (unlikely(!skb))
 		return false;
-	}
 
-	if (!nolock)
+	if (!(q->flags & TCQ_F_NOLOCK))
 		root_lock = qdisc_lock(q);
 
 	dev = qdisc_dev(q);
 	txq = skb_get_tx_queue(dev, skb);
 
-	more = sch_direct_xmit(skb, q, dev, txq, root_lock, validate);
-	if (nolock)
-		clear_bit(__QDISC_STATE_RUNNING, &q->state);
-	return more;
+	return sch_direct_xmit(skb, q, dev, txq, root_lock, validate);
 }
 
 void __qdisc_run(struct Qdisc *q)
@@ -1131,17 +1122,13 @@ static bool some_qdisc_is_busy(struct net_device *dev)
 		dev_queue = netdev_get_tx_queue(dev, i);
 		q = dev_queue->qdisc_sleeping;
 
-		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);
+		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;
-- 
2.14.3