netdev - Re: Why do we use RX queue mapping for TX?

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Message-ID: <CAHA+R7PbLRw-oA4v9ivZ0s-++ALx99zddJLDqDoemyBTc2XZ1g@mail.gmail.com>
Date:	Wed, 18 Feb 2015 15:57:17 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: Why do we use RX queue mapping for TX?

On Wed, Feb 18, 2015 at 3:14 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2015-02-18 at 14:11 -0800, Cong Wang wrote:
>> On Wed, Feb 18, 2015 at 1:18 PM, Cong Wang <cwang@...pensource.com> wrote:
>> >
>> > 2) This breaks the queue mapping specified by an skbedit action,
>> > since for TX queues the index starts with 0 while for RX it starts with 1
>> > (for some reason I don't see yet). There is at least a mismatch.
>>
>> So queue 0 is reserved for TX, at least for bonding queue mapping.
>
> It seems you missed that bonding ndo_select_queue() is rather special.
>
>

I am aware of it. I even would guess (means not digging the history)
skb_edit was invented for bonding queue mapping, since I don't see it
even works as a general TC action on the physical interface. We pick
the tx queue prior to getting the Qdisc, therefore too late to set
skb->queue_mapping to specify a hardware TX queue. This is
another story I planned to bring it up to David.

However, this still doesn't seem to be a reason to break people who
don't use bonding at all? At least we just want to map skb's to different
hardware TX queues by setting skb->queue_mapping (before
dev_queue_xmit() of course) and sysfs reports the queues starting with
index 0. This is why I complain. :)
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