netdev - [net-next PATCH 2/3] net: sched: set tx_queue_len to default when changing noqueue device's qdisc

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Message-Id: <1438203103-27013-3-git-send-email-phil@nwl.cc>
Date:	Wed, 29 Jul 2015 22:51:42 +0200
From:	Phil Sutter <phil@....cc>
To:	netdev@...r.kernel.org
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>,
	Cong Wang <cwang@...pensource.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>
Subject: [net-next PATCH 2/3] net: sched: set tx_queue_len to default when changing noqueue device's qdisc

Virtual interfaces don't necessarily need a qdisc attached to them. This
is signalled by setting dev->tx_queue_len to zero upon initialisation. The
problems begin when a user still adds a qdisc, as then the special value
is used as a regular one causing massive packet drops as soon as the
device sees a bit of traffic load.

This patch solves the issue by setting tx_queue_len to the global
default for physical devices if it is zero and a qdisc is added. Another
benefit of this is that veth devices can default to noqueue while still
preserving the expected behaviour when adding a qdisc.

The drawback of this patch is that the noqueue state won't be restored
after a user has removed the custom qdisc, as the information about
whether this is legitimate for the given device is lost.

Signed-off-by: Phil Sutter <phil@....cc>
---
 net/sched/sch_api.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index f06aa01..79b8900 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -1315,6 +1315,10 @@ graft:
 			qdisc_destroy(q);
 		return err;
 	}
+	/* circumvent noqueue hack for virtual interfaces if
+	 * user desires to use a qdisc on it */
+	if (!dev->tx_queue_len)
+		dev->tx_queue_len = CONFIG_DEFAULT_TX_QUEUE_LEN;

 	return 0;
 }
-- 
2.1.2

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