netdev - Re: [PATCH v2 net-next 4/5] xps_flows: XPS for packets that don't have a socket

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Date:   Wed, 28 Sep 2016 21:54:11 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, kernel-team@...com,
        rick.jones2@....com, alexander.duyck@...il.com
Subject: Re: [PATCH v2 net-next 4/5] xps_flows: XPS for packets that don't
 have a socket

On Wed, 2016-09-28 at 20:54 -0700, Tom Herbert wrote:
> xps_flows maintains a per device flow table that is indexed by the
> skbuff hash. The table is only consulted when there is no queue saved in
> a transmit socket for an skbuff.
> 
> Each entry in the flow table contains a queue index and a queue
> pointer. The queue pointer is set when a queue is chosen using a
> flow table entry. This pointer is set to the head pointer in the
> transmit queue (which is maintained by BQL).
> 
> The new function get_xps_flows_index that looks up flows in the
> xps_flows table. The entry returned gives the last queue a matching flow
> used. The returned queue is compared against the normal XPS queue. If
> they are different, then we only switch if the tail pointer in the TX
> queue has advanced past the pointer saved in the entry. In this
> way OOO should be avoided when XPS wants to use a different queue.

There is something I do not understand with this.

If this OOO avoidance is tied to BQL, it means that packets sitting in a
qdisc wont be part of the detection.

So packets of flow X could possibly be queued on multiple qdiscs.