netdev - [PATCH net-next v2 1/2] net: sched: add empty status flag for NOLOCK qdisc

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Message-Id: <8e2fe320cd3075be079d0bf8c46b98f32935d116.1553188500.git.pabeni@redhat.com>
Date:   Fri, 22 Mar 2019 09:30:42 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        John Fastabend <john.fastabend@...il.com>,
        Ivan Vecera <ivecera@...hat.com>
Subject: [PATCH net-next v2 1/2] net: sched: add empty status flag for NOLOCK qdisc

The queue is marked not empty after acquiring the seqlock,
and it's up to the NOLOCK qdisc clearing such flag on dequeue.
Since the empty status lays on the same cache-line of the
seqlock, it's always hot on cache during the updates.

This makes the empty flag update a little bit loosy. Given
the lack of synchronization between enqueue and dequeue, this
is unavoidable.

v1 -> v2:
 - use really an 'empty' flag instead of 'not_empty', as
   suggested by Eric

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 include/net/sch_generic.h | 11 +++++++++++
 net/sched/sch_generic.c   |  3 +++
 2 files changed, 14 insertions(+)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 31284c078d06..15c62501b243 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -113,6 +113,9 @@ struct Qdisc {
 
 	spinlock_t		busylock ____cacheline_aligned_in_smp;
 	spinlock_t		seqlock;
+
+	/* for NOLOCK qdisc, true if there are no enqueued skbs */
+	bool			empty;
 	struct rcu_head		rcu;
 };
 
@@ -143,11 +146,19 @@ static inline bool qdisc_is_running(struct Qdisc *qdisc)
 	return (raw_read_seqcount(&qdisc->running) & 1) ? true : false;
 }
 
+static inline bool qdisc_is_empty(struct Qdisc *qdisc)
+{
+	if (qdisc->flags & TCQ_F_NOLOCK)
+		return qdisc->empty;
+	return !qdisc->q.qlen;
+}
+
 static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 {
 	if (qdisc->flags & TCQ_F_NOLOCK) {
 		if (!spin_trylock(&qdisc->seqlock))
 			return false;
+		qdisc->empty = false;
 	} else if (qdisc_is_running(qdisc)) {
 		return false;
 	}
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index a117d9260558..81356ef38d1d 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -671,6 +671,8 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
 		qdisc_qstats_cpu_backlog_dec(qdisc, skb);
 		qdisc_bstats_cpu_update(qdisc, skb);
 		qdisc_qstats_atomic_qlen_dec(qdisc);
+	} else {
+		qdisc->empty = true;
 	}
 
 	return skb;
@@ -880,6 +882,7 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
 	sch->enqueue = ops->enqueue;
 	sch->dequeue = ops->dequeue;
 	sch->dev_queue = dev_queue;
+	sch->empty = true;
 	dev_hold(dev);
 	refcount_set(&sch->refcnt, 1);
 
-- 
2.20.1