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.

v2 -> v3:
 - qdisc_is_empty() has a const argument (Eric)

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

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 31284c0..e227475 100644 (file)
--- 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(const 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 a117d92..81356ef 100644 (file)
--- 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);