netdev - Re: fq_codel and skb->hash

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Date:   Wed, 18 Jan 2017 13:21:26 -0700
From:   Andrew Collins <acollins@...dlepoint.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Tom Herbert <tom@...bertland.com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: fq_codel and skb->hash

On 01/17/2017 08:14 AM, Eric Dumazet wrote:
> Right, but that might add overhead in cases we do not need skb->hash
> after IPsec . I've heard IPsec is already quite slow :/

I've been running with the following change locally with good results:

--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -57,7 +57,6 @@ struct fq_codel_sched_data {
         struct fq_codel_flow *flows;    /* Flows table [flows_cnt] */
         u32             *backlogs;      /* backlog table [flows_cnt] */
         u32             flows_cnt;      /* number of flows */
-       u32             perturbation;   /* hash perturbation */
         u32             quantum;        /* psched_mtu(qdisc_dev(sch)); */
         u32             drop_batch_size;
         u32             memory_limit;
@@ -75,9 +74,7 @@ struct fq_codel_sched_data {
  static unsigned int fq_codel_hash(const struct fq_codel_sched_data *q,
                                   struct sk_buff *skb)
  {
-       u32 hash = skb_get_hash_perturb(skb, q->perturbation);
-
-       return reciprocal_scale(hash, q->flows_cnt);
+       return reciprocal_scale(skb_get_hash(skb), q->flows_cnt);
  }

  static unsigned int fq_codel_classify(struct sk_buff *skb, struct Qdisc *sch,
@@ -482,7 +479,6 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt)
         q->memory_limit = 32 << 20; /* 32 MBytes */
         q->drop_batch_size = 64;
         q->quantum = psched_mtu(qdisc_dev(sch));
-       q->perturbation = prandom_u32();
         INIT_LIST_HEAD(&q->new_flows);
         INIT_LIST_HEAD(&q->old_flows);
         codel_params_init(&q->cparams);

Any interest in me spinning a real patch for this?  I agree that it'd be better
if we were guaranteed to get a pre-encryption flow hash for any IPsec traffic,
but in my particular case I don't care, as I control the HW and can make it give
me a hash. :)

Thanks,
Andrew Collins