netdev - Re: [PATCH net-next 6/6] sched: Eliminate use of flow_keys in sch

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Message-ID: <1425257932.5130.93.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sun, 01 Mar 2015 16:58:52 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, fw@...len.de
Subject: Re: [PATCH net-next 6/6] sched: Eliminate use of flow_keys in
 sch_sfq

On Sun, 2015-03-01 at 14:09 -0800, Tom Herbert wrote:
> Call qdisc_skb_get_hash instead of doing skb_flow_dissect and then
> jhash by hand.

This defeats one of the perturbation goal :

If two flows hashes into same hash, then skb_get_hash_perturb(skb,
q->perturbation) will also give same hash forever and map to same hash
bucket.

Ideally, we could add a 'u32 perturbation' salt to
__skb_get_hash()/__flow_hash_from_keys()/__flow_hash_3words instead of
using a net_get_random_once(&hashrnd, sizeof(hashrnd)); 

but I guess all these functions are becoming very fat.



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