netdev - Re: [PATCH net] net_sched: sch_fq: properly set sk->sk_pacing

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Date:   Mon, 23 Dec 2019 14:18:17 -0500
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net] net_sched: sch_fq: properly set sk->sk_pacing_status

On Mon, Dec 23, 2019 at 2:13 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> If fq_classify() recycles a struct fq_flow because
> a socket structure has been reallocated, we do not
> set sk->sk_pacing_status immediately, but later if the
> flow becomes detached.
>
> This means that any flow requiring pacing (BBR, or SO_MAX_PACING_RATE)
> might fallback to TCP internal pacing, which requires a per-socket
> high resolution timer, and therefore more cpu cycles.
>
> Fixes: 218af599fa63 ("tcp: internal implementation for pacing")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Nice catch! Thanks for the fix!

> ---
>  net/sched/sch_fq.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
> index b1c7e726ce5d1ae139f765c5b92dfdaea9bee258..ff4c5e9d0d7778d86f20f4bd67cc627eed0713d9 100644
> --- a/net/sched/sch_fq.c
> +++ b/net/sched/sch_fq.c
> @@ -301,6 +301,9 @@ static struct fq_flow *fq_classify(struct sk_buff *skb, struct fq_sched_data *q)
>                                      f->socket_hash != sk->sk_hash)) {
>                                 f->credit = q->initial_quantum;
>                                 f->socket_hash = sk->sk_hash;
> +                               if (q->rate_enable)
> +                                       smp_store_release(&sk->sk_pacing_status,
> +                                                         SK_PACING_FQ);
>                                 if (fq_flow_is_throttled(f))
>                                         fq_flow_unset_throttled(q, f);
>                                 f->time_next_packet = 0ULL;
> @@ -322,8 +325,12 @@ static struct fq_flow *fq_classify(struct sk_buff *skb, struct fq_sched_data *q)
>
>         fq_flow_set_detached(f);
>         f->sk = sk;
> -       if (skb->sk == sk)
> +       if (skb->sk == sk) {
>                 f->socket_hash = sk->sk_hash;
> +               if (q->rate_enable)
> +                       smp_store_release(&sk->sk_pacing_status,
> +                                         SK_PACING_FQ);
> +       }
>         f->credit = q->initial_quantum;
>
>         rb_link_node(&f->fq_node, parent, p);
> @@ -428,17 +435,9 @@ static int fq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>         f->qlen++;
>         qdisc_qstats_backlog_inc(sch, skb);
>         if (fq_flow_is_detached(f)) {
> -               struct sock *sk = skb->sk;
> -
>                 fq_flow_add_tail(&q->new_flows, f);
>                 if (time_after(jiffies, f->age + q->flow_refill_delay))
>                         f->credit = max_t(u32, f->credit, q->quantum);
> -               if (sk && q->rate_enable) {
> -                       if (unlikely(smp_load_acquire(&sk->sk_pacing_status) !=
> -                                    SK_PACING_FQ))
> -                               smp_store_release(&sk->sk_pacing_status,
> -                                                 SK_PACING_FQ);
> -               }
>                 q->inactive_flows--;
>         }
>
> --
> 2.24.1.735.g03f4e72817-goog
>