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Date: Wed, 22 Jun 2016 07:55:43 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Jesper Dangaard Brouer <brouer@...hat.com>
Cc: Eric Dumazet <edumazet@...gle.com>,
"David S . Miller" <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
John Fastabend <john.r.fastabend@...el.com>
Subject: Re: [PATCH net-next 0/4] net_sched: bulk dequeue and deferred drops
On Wed, 2016-06-22 at 16:47 +0200, Jesper Dangaard Brouer wrote:
> On Tue, 21 Jun 2016 23:16:48 -0700
> Eric Dumazet <edumazet@...gle.com> wrote:
>
> > First patch adds an additional parameter to ->enqueue() qdisc method
> > so that drops can be done outside of critical section
> > (after locks are released).
> >
> > Then fq_codel can have a small optimization to reduce number of cache
> > lines misses during a drop event
> > (possibly accumulating hundreds of packets to be freed).
> >
> > A small htb change exports the backlog in class dumps.
> >
> > Final patch adds bulk dequeue to qdiscs that were lacking this feature.
> >
> > This series brings a nice qdisc performance increase (more than 80 %
> > in some cases).
>
> Thanks for working on this Eric! this is great work! :-)
Thanks Jesper
I worked yesterday on bulk enqueues, but initial results are not that
great.
Here is my current patch, on top of my last series :
(A second patch would remove the busylock spinlock, of course)
include/net/sch_generic.h | 9 ++
net/core/dev.c | 135 ++++++++++++++++++++++++++++--------
2 files changed, 115 insertions(+), 29 deletions(-)
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 909aff2db2b3..1975a6fab10f 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -87,7 +87,14 @@ struct Qdisc {
int padded;
atomic_t refcnt;
- spinlock_t busylock ____cacheline_aligned_in_smp;
+ spinlock_t busylock;
+
+#ifdef CONFIG_SMP
+ struct pcpu_skb_context *prequeue0 ____cacheline_aligned_in_smp;
+#ifdef CONFIG_NUMA
+ struct pcpu_skb_context *prequeue1 ____cacheline_aligned_in_smp;
+#endif
+#endif
};
static inline bool qdisc_is_running(const struct Qdisc *qdisc)
diff --git a/net/core/dev.c b/net/core/dev.c
index aba10d2a8bc3..5f0d3fe5b109 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3065,26 +3065,117 @@ static void qdisc_pkt_len_init(struct sk_buff *skb)
}
}
-static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
- struct net_device *dev,
- struct netdev_queue *txq)
+
+#ifdef CONFIG_SMP
+
+struct pcpu_skb_context {
+ struct pcpu_skb_context *next;
+ union {
+ struct sk_buff *skb;
+ unsigned long status;
+ struct pcpu_skb_context *self;
+ };
+};
+static DEFINE_PER_CPU_ALIGNED(struct pcpu_skb_context, pcpu_skb_context);
+
+/* Provides a small queue before qdisc so that we can batch ->enqueue()
+ * under SMP stress.
+ */
+static noinline int dev_xmit_skb_slow(struct sk_buff *skb, struct Qdisc *q,
+ struct pcpu_skb_context **prequeue)
+{
+ struct pcpu_skb_context *prev, *myptr;
+ struct sk_buff *to_free = NULL;
+ spinlock_t *root_lock;
+ void *status;
+ int i, rc;
+
+ myptr = this_cpu_ptr(&pcpu_skb_context);
+ myptr->skb = skb;
+ myptr->next = NULL;
+
+ /* Take our ticket in prequeue file, a la MCS lock */
+ prev = xchg(prequeue, myptr);
+ if (prev) {
+ /* Ok, another cpu got the lock and serves the prequeue.
+ * Wait that it either processed our skb or it exhausted
+ * its budget and told us to process a batch ourself.
+ */
+ WRITE_ONCE(prev->next, myptr);
+
+ while ((status = READ_ONCE(myptr->skb)) == skb)
+ cpu_relax_lowlatency();
+
+ /* Nice ! Our skb was handled by another cpu */
+ if ((unsigned long)status < NET_XMIT_MASK)
+ return (int)(unsigned long)status;
+
+ /* Oh well, we got the responsability of next batch */
+ BUG_ON(myptr != status);
+ }
+ root_lock = qdisc_lock(q);
+ spin_lock(root_lock);
+
+ for (i = 0; i < 16; i++) {
+ bool may_release = true;
+
+ if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
+ __qdisc_drop(skb, &to_free);
+ rc = NET_XMIT_DROP;
+ } else {
+ rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
+ }
+ while (!(prev = READ_ONCE(myptr->next))) {
+ if (may_release) {
+ if (cmpxchg_release(prequeue, myptr, NULL) == myptr)
+ break;
+ may_release = false;
+ }
+ cpu_relax_lowlatency();
+ }
+ smp_store_release(&myptr->status, (unsigned long)rc);
+ myptr = prev;
+ if (!myptr)
+ break;
+ skb = READ_ONCE(myptr->skb);
+ }
+
+ qdisc_run(q);
+ spin_unlock(root_lock);
+
+ /* Give control to another cpu for following batch */
+ if (myptr)
+ smp_store_release(&myptr->self, myptr);
+
+ if (unlikely(to_free))
+ kfree_skb_list(to_free);
+
+ return (int)this_cpu_read(pcpu_skb_context.status);
+}
+#endif
+
+static int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
+ struct net_device *dev,
+ struct netdev_queue *txq)
{
spinlock_t *root_lock = qdisc_lock(q);
struct sk_buff *to_free = NULL;
- bool contended;
int rc;
qdisc_calculate_pkt_len(skb, q);
- /*
- * Heuristic to force contended enqueues to serialize on a
- * separate lock before trying to get qdisc main lock.
- * This permits qdisc->running owner to get the lock more
- * often and dequeue packets faster.
- */
- contended = qdisc_is_running(q);
- if (unlikely(contended))
- spin_lock(&q->busylock);
+#ifdef CONFIG_SMP
+ {
+ struct pcpu_skb_context **prequeue = &q->prequeue0;
+
+#ifdef CONFIG_NUMA
+ if (numa_node_id() & 1)
+ prequeue = &q->prequeue1;
+#endif
+ if (unlikely(*prequeue || qdisc_is_running(q)))
+ return dev_xmit_skb_slow(skb, q, prequeue);
+ }
+#endif
spin_lock(root_lock);
if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
__qdisc_drop(skb, &to_free);
@@ -3099,31 +3190,19 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
qdisc_bstats_update(q, skb);
- if (sch_direct_xmit(skb, q, dev, txq, root_lock, true)) {
- if (unlikely(contended)) {
- spin_unlock(&q->busylock);
- contended = false;
- }
+ if (sch_direct_xmit(skb, q, dev, txq, root_lock, true))
__qdisc_run(q);
- } else
+ else
qdisc_run_end(q);
rc = NET_XMIT_SUCCESS;
} else {
rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
- if (qdisc_run_begin(q)) {
- if (unlikely(contended)) {
- spin_unlock(&q->busylock);
- contended = false;
- }
- __qdisc_run(q);
- }
+ qdisc_run(q);
}
spin_unlock(root_lock);
if (unlikely(to_free))
kfree_skb_list(to_free);
- if (unlikely(contended))
- spin_unlock(&q->busylock);
return rc;
}
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