netdev - Re: [PATCH] tcp: fix TCP socks unreleased in BBR mode

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Date:   Wed, 3 Jun 2020 10:07:55 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Jason Xing <kerneljasonxing@...il.com>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, liweishi@...ishou.com,
        Shujin Li <lishujin@...ishou.com>
Subject: Re: [PATCH] tcp: fix TCP socks unreleased in BBR mode

On Wed, Jun 3, 2020 at 9:55 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Jun 3, 2020 at 5:02 AM Neal Cardwell <ncardwell@...gle.com> wrote:
> >
> > On Wed, Jun 3, 2020 at 1:44 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Tue, Jun 2, 2020 at 10:05 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > > >
> > > > Hi Eric,
> > > >
> > > > I'm still trying to understand what you're saying before. Would this
> > > > be better as following:
> > > > 1) discard the tcp_internal_pacing() function.
> > > > 2) remove where the tcp_internal_pacing() is called in the
> > > > __tcp_transmit_skb() function.
> > > >
> > > > If we do so, we could avoid 'too late to give up pacing'. Meanwhile,
> > > > should we introduce the tcp_wstamp_ns socket field as commit
> > > > (864e5c090749) does?
> > > >
> > >
> > > Please do not top-post on netdev mailing list.
> > >
> > >
> > > I basically suggested double-checking which point in TCP could end up
> > > calling tcp_internal_pacing()
> > > while the timer was already armed.
> > >
> > > I guess this is mtu probing.
> >
> > Perhaps this could also happen from some of the retransmission code
> > paths that don't use tcp_xmit_retransmit_queue()? Perhaps
> > tcp_retransmit_timer() (RTO) and  tcp_send_loss_probe() TLP? It seems
> > they could indirectly cause a call to __tcp_transmit_skb() and thus
> > tcp_internal_pacing() without first checking if the pacing timer was
> > already armed?
>
> I feared this, (see recent commits about very low pacing rates) :/
>
> I am not sure we need to properly fix all these points for old
> kernels, since EDT model got rid of these problems.

Agreed.

> Maybe we can try to extend the timer.

Sounds good.

> Something like :
>
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index cc4ba42052c21b206850594db6751810d8fc72b4..626b9f4f500f7e5270d8d59e6eb16dbfa3efbc7c
> 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -966,6 +966,8 @@ enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
>
>  static void tcp_internal_pacing(struct sock *sk, const struct sk_buff *skb)
>  {
> +       struct tcp_sock *tp = tcp_sk(sk);
> +       ktime_t expire, now;
>         u64 len_ns;
>         u32 rate;
>
> @@ -977,12 +979,29 @@ static void tcp_internal_pacing(struct sock *sk,
> const struct sk_buff *skb)
>
>         len_ns = (u64)skb->len * NSEC_PER_SEC;
>         do_div(len_ns, rate);
> -       hrtimer_start(&tcp_sk(sk)->pacing_timer,
> -                     ktime_add_ns(ktime_get(), len_ns),
> +
> +       now = ktime_get();
> +       /* If hrtimer is already armed, then our caller has not
> +        * used tcp_pacing_check().
> +        */
> +       if (unlikely(hrtimer_is_queued(&tp->pacing_timer))) {
> +               expire = hrtimer_get_softexpires(&tp->pacing_timer);
> +               if (ktime_after(expire, now))
> +                       now = expire;
> +               if (hrtimer_try_to_cancel(&tp->pacing_timer) == 1)
> +                       __sock_put(sk);
> +       }
> +       hrtimer_start(&tp->pacing_timer, ktime_add_ns(now, len_ns),
>                       HRTIMER_MODE_ABS_PINNED_SOFT);
>         sock_hold(sk);
>  }
>
> +static bool tcp_pacing_check(const struct sock *sk)
> +{
> +       return tcp_needs_internal_pacing(sk) &&
> +              hrtimer_is_queued(&tcp_sk(sk)->pacing_timer);
> +}
> +
>  static void tcp_update_skb_after_send(struct tcp_sock *tp, struct sk_buff *skb)
>  {
>         skb->skb_mstamp = tp->tcp_mstamp;
> @@ -2117,6 +2136,9 @@ static int tcp_mtu_probe(struct sock *sk)
>         if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
>                 return -1;
>
> +       if (tcp_pacing_check(sk))
> +               return -1;
> +
>         /* We're allowed to probe.  Build it now. */
>         nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC, false);
>         if (!nskb)
> @@ -2190,11 +2212,6 @@ static int tcp_mtu_probe(struct sock *sk)
>         return -1;
>  }
>
> -static bool tcp_pacing_check(const struct sock *sk)
> -{
> -       return tcp_needs_internal_pacing(sk) &&
> -              hrtimer_is_queued(&tcp_sk(sk)->pacing_timer);
> -}
>
>  /* TCP Small Queues :
>   * Control number of packets in qdisc/devices to two packets / or ~1 ms.

Thanks for your fix, Eric. This fix looks good to me! I agree that
this fix is good enough for older kernels.

thanks,
neal