netdev - Re: [net-next PATCH 1/1 V4] qdisc: bulk dequeue support for qdiscs with TCQ_F

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Date:	Wed, 24 Sep 2014 19:12:20 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	Florian Westphal <fw@...len.de>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Dave Taht <dave.taht@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [net-next PATCH 1/1 V4] qdisc: bulk dequeue support for qdiscs
 with TCQ_F_ONETXQUEUE

On Wed, 2014-09-24 at 12:22 -0700, Eric Dumazet wrote:
> On Wed, 2014-09-24 at 11:34 -0700, Tom Herbert wrote:
> > >
> > I believe drivers typically use skb->len for BQL tracking. Since
> > bytelimit is based on BQL here, it might be more correct to use
> > skb->len.

Speaking of BQL, I wonder if we now should try to not wakeup queues as
soon some room was made, and instead have a 50% threshold ?

This would probably increase probability to have bulk dequeues ;)


diff --git a/include/linux/dynamic_queue_limits.h b/include/linux/dynamic_queue_limits.h
index 5621547d631b..c0be7ff5ae97 100644
--- a/include/linux/dynamic_queue_limits.h
+++ b/include/linux/dynamic_queue_limits.h
@@ -83,6 +83,13 @@ static inline int dql_avail(const struct dql *dql)
 	return dql->adj_limit - dql->num_queued;
 }
 
+/* Returns true if a queue occupancy is less than half capacity */
+static inline bool dql_half_avail(const struct dql *dql)
+{
+	return dql->adj_limit >= (dql->num_queued << 1);
+}
+
+
 /* Record number of completed objects and recalculate the limit. */
 void dql_completed(struct dql *dql, unsigned int count);
 
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index c8e388e5fccc..1f7541284b32 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2413,7 +2413,7 @@ static inline void netdev_tx_sent_queue(struct netdev_queue *dev_queue,
 	smp_mb();
 
 	/* check again in case another CPU has just made room avail */
-	if (unlikely(dql_avail(&dev_queue->dql) >= 0))
+	if (unlikely(dql_half_avail(&dev_queue->dql)))
 		clear_bit(__QUEUE_STATE_STACK_XOFF, &dev_queue->state);
 #endif
 }
@@ -2448,7 +2448,7 @@ static inline void netdev_tx_completed_queue(struct netdev_queue *dev_queue,
 	 */
 	smp_mb();
 
-	if (dql_avail(&dev_queue->dql) < 0)
+	if (!dql_half_avail(&dev_queue->dql))
 		return;
 
 	if (test_and_clear_bit(__QUEUE_STATE_STACK_XOFF, &dev_queue->state))


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