[RFC PATCH 04/13] net: sched: provide atomic qlen helpers for bypass case

[John Fastabend]

The qlen is used by the core/dev.c to determine if a packet
can skip the qdisc on qdiscs with bypass enabled. In these
cases a per cpu qlen value can cause one cpu to bypass a
qdisc that has packets in it.

To avoid this case use the simplest solution I could come
up with for now and add an atomic qlen value to the qdisc
to use in these cases.

Signed-off-by: John Fastabend <john.r.fastab...@intel.com>
---
 include/net/sch_generic.h |   21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index f69da4b..193cf8c 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -78,6 +78,7 @@ struct Qdisc {
         */
        struct sk_buff          *gso_skb ____cacheline_aligned_in_smp;
        struct sk_buff_head     q;
+       atomic_t                qlen_atomic;
        struct gnet_stats_basic_packed bstats;
        seqcount_t              running;
        struct gnet_stats_queue qstats;
@@ -247,6 +248,11 @@ static inline void qdisc_cb_private_validate(const struct 
sk_buff *skb, int sz)
        BUILD_BUG_ON(sizeof(qcb->data) < sz);
 }
 
+static inline int qdisc_qlen_atomic(const struct Qdisc *q)
+{
+       return atomic_read(&q->qlen_atomic);
+}
+
 static inline int qdisc_qlen_cpu(const struct Qdisc *q)
 {
        return this_cpu_ptr(q->cpu_qstats)->qlen;
@@ -254,8 +260,11 @@ static inline int qdisc_qlen_cpu(const struct Qdisc *q)
 
 static inline int qdisc_qlen(const struct Qdisc *q)
 {
+       /* current default is to use atomic ops for qdisc qlen when
+        * running with TCQ_F_NOLOCK.
+        */
        if (q->flags & TCQ_F_NOLOCK)
-               return qdisc_qlen_cpu(q);
+               return qdisc_qlen_atomic(q);
 
        return q->q.qlen;
 }
@@ -595,6 +604,16 @@ static inline void qdisc_qstats_cpu_backlog_inc(struct 
Qdisc *sch,
        q->backlog += qdisc_pkt_len(skb);
 }
 
+static inline void qdisc_qstats_atomic_qlen_inc(struct Qdisc *sch)
+{
+       atomic_inc(&sch->qlen_atomic);
+}
+
+static inline void qdisc_qstats_atomic_qlen_dec(struct Qdisc *sch)
+{
+       atomic_dec(&sch->qlen_atomic);
+}
+
 static inline void qdisc_qstats_cpu_qlen_inc(struct Qdisc *sch)
 {
        this_cpu_ptr(sch->cpu_qstats)->qlen++;