netdev - Re: [net-next PATCH V6 0/2] qdisc: bulk dequeue support

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Message-ID: <1412262270.16704.103.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 02 Oct 2014 08:04:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>, Amir Vadai <amirv@...lanox.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Daniel Borkmann <dborkman@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Dave Taht <dave.taht@...il.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>
Subject: Re: [net-next PATCH V6 0/2] qdisc: bulk dequeue support

On Thu, 2014-10-02 at 07:42 -0700, Tom Herbert wrote:


> Unfortunately we probably still need something. If BQL were disabled
> (by setting BQL min_limit to infinity) then we'll always dequeue all
> the packets in the qdisc. Disabling BQL might be legitimate in
> deployment if say a bug is found in a device that prevents prompt
> transmit completions for some corner case.


Unfortunately, there is nothing we can do if a ndo_start_xmit() function
is buggy.

For example, if a prior packet is queued on NIC TX ring (with xmit_more
set to 1)

Then following packet given to ndo_start_xmit() returns infamous
NETDEV_TX_BUSY, we are screwed.

mlx4 driver for example has this bug.


Thats why I advocate we have no artifical limit : We are going to catch
driver bugs sooner and fix them before xmit_more support is validated,
for each driver that claims to support it.

Amir, the problem with NETDEV_TX_BUSY in mlx4 need to be fixed.


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