Hi,
On Tue, Feb 16, 2016 at 07:56:23PM -0500, Mathieu Desnoyers wrote:
> This reverts commit 348e3435cbefa815bd56a5205c1412b5afe7b92e.
> It breaks HTB classful qdiscs on the loopback interface.
>
> It has been broken since kernel v4.2. The offending commit has
> been identified by bissection of the issue with the following
> test-case. It appears that the loopback interface does indeed
> still have tx_queue_len == 0.
>
> Reverting the commit on a v4.4.1 kernel fixes the issue.
Indeed, this is ugly. Affected are all drivers not calling ether_setup()
in their setup callback, and therefore not initializing tx_queue_len.
As the commit to be reverted shows, there is no common fallback value
for tx_queue_len - most qdisc implementations used 1, but HTB fell back
to 2 and PLUG to 100. But as the removed comment in sch_plug.c stated:
| /* We will set a default limit of 100 pkts (~150kB)
| * in case tx_queue_len is not available. The
| * default value is completely arbitrary.
| */
It seems not to be overly important to fallback to the exact value each
qdisc had before. Therefore I guess the following change should
appropriately fix the issue at hand:
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7440,8 +7440,10 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
dev->priv_flags = IFF_XMIT_DST_RELEASE | IFF_XMIT_DST_RELEASE_PERM;
setup(dev);
- if (!dev->tx_queue_len)
+ if (!dev->tx_queue_len) {
dev->priv_flags |= IFF_NO_QUEUE;
+ dev->tx_queue_len = 1;
+ }
dev->num_tx_queues = txqs;
dev->real_num_tx_queues = txqs;
Unless there is concern, I will formally submit this later.
Thanks, Phil
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