netdev - fq_codel and skb->hash

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Date:   Mon, 16 Jan 2017 17:04:44 -0700
From:   Andrew Collins <acollins@...dlepoint.com>
To:     <netdev@...r.kernel.org>
CC:     <edumazet@...gle.com>
Subject: fq_codel and skb->hash

The fq_codel packet scheduler always regenerates the skb flow hash.  Is there any reason
to do this other than the recent hash perturbation additions?

I ask because I have a case where an incoming set of TCP flows is encapsulated in a
single ipsec tunnel, which is then classified on egress into a single flow by fq_codel
resulting in poor behavior.

Reusing the skb hash set in receive results in much better behavior, as the value is
determined pre-encapsulation and flows end up being distributed more naturally across
codel queues.

Is opportunistically using the pre-existing skb hash (if available) problematic?  Is
there a better way to manage flow separation in routed+encapsulated traffic?

Thanks,
Andrew Collins