netdev - Re: iperf performance regression since Linux 5.18

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Message-ID: <CANn89i+6VuixihW4YyHntjj_GOKOOyXt8hHF8TJtB3bm07CZ6w@mail.gmail.com>
Date: Mon, 16 Oct 2023 20:47:39 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Stefan Wahren <wahrenst@....net>
Cc: Neal Cardwell <ncardwell@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Fabio Estevam <festevam@...il.com>, linux-imx@....com, 
	Stefan Wahren <stefan.wahren@...rgebyte.com>, Michael Heimpold <mhei@...mpold.de>, netdev@...r.kernel.org, 
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: iperf performance regression since Linux 5.18

On Mon, Oct 16, 2023 at 8:25 PM Stefan Wahren <wahrenst@....net> wrote:
>
> Hi Eric,
>
> Am 16.10.23 um 12:35 schrieb Eric Dumazet:
> > On Mon, Oct 16, 2023 at 11:49 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > Speaking of TSQ, it seems an old change (commit 75eefc6c59fd "tcp:
> > tsq: add a shortcut in tcp_small_queue_check()")
> > has been accidentally removed in 2017 (75c119afe14f "tcp: implement
> > rb-tree based retransmit queue")
> >
> > Could you try this fix:
> >
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index 9c8c42c280b7638f0f4d94d68cd2c73e3c6c2bcc..e61a3a381d51b554ec8440928e22a290712f0b6b
> > 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -2542,6 +2542,18 @@ static bool tcp_pacing_check(struct sock *sk)
> >          return true;
> >   }
> >
> > +static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
> > +{
> > +       const struct rb_node *node = sk->tcp_rtx_queue.rb_node;
> > +
> > +       /* No skb in the rtx queue. */
> > +       if (!node)
> > +               return true;
> > +
> > +       /* Only one skb in rtx queue. */
> > +       return !node->rb_left && !node->rb_right;
> > +}
> > +
> >   /* TCP Small Queues :
> >    * Control number of packets in qdisc/devices to two packets / or ~1 ms.
> >    * (These limits are doubled for retransmits)
> > @@ -2579,12 +2591,12 @@ static bool tcp_small_queue_check(struct sock
> > *sk, const struct sk_buff *skb,
> >                  limit += extra_bytes;
> >          }
> >          if (refcount_read(&sk->sk_wmem_alloc) > limit) {
> > -               /* Always send skb if rtx queue is empty.
> > +               /* Always send skb if rtx queue is empty or has one skb.
> >                   * No need to wait for TX completion to call us back,
> >                   * after softirq/tasklet schedule.
> >                   * This helps when TX completions are delayed too much.
> >                   */
> > -               if (tcp_rtx_queue_empty(sk))
> > +               if (tcp_rtx_queue_empty_or_single_skb(sk))
> >                          return false;
> >
> >                  set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
>
> This patch applied on top of Linux 6.1.49, TSO on, gso_max_size 65535,
> CONFIG_HZ_100=y
>
> root@...ragon:/boot# iperf -t 10 -i 1 -c 192.168.1.129
> ------------------------------------------------------------
> Client connecting to 192.168.1.129, TCP port 5001
> TCP window size:  192 KByte (default)
> ------------------------------------------------------------
> [  3] local 192.168.1.12 port 59714 connected with 192.168.1.129 port 5001
> [ ID] Interval       Transfer     Bandwidth
> [  3]  0.0- 1.0 sec  11.5 MBytes  96.5 Mbits/sec
> [  3]  1.0- 2.0 sec  11.4 MBytes  95.4 Mbits/sec
> [  3]  2.0- 3.0 sec  11.1 MBytes  93.3 Mbits/sec
> [  3]  3.0- 4.0 sec  11.2 MBytes  94.4 Mbits/sec
> [  3]  4.0- 5.0 sec  11.1 MBytes  93.3 Mbits/sec
> [  3]  5.0- 6.0 sec  11.2 MBytes  94.4 Mbits/sec
> [  3]  6.0- 7.0 sec  11.2 MBytes  94.4 Mbits/sec
> [  3]  7.0- 8.0 sec  11.1 MBytes  93.3 Mbits/sec
> [  3]  8.0- 9.0 sec  11.4 MBytes  95.4 Mbits/sec
> [  3]  9.0-10.0 sec  11.2 MBytes  94.4 Mbits/sec
> [  3]  0.0-10.0 sec   113 MBytes  94.4 Mbits/sec
>
> The figures are comparable to disabling TSO -> Good
>
> Thanks

Great. I suspect a very slow TX completion from fec then.

Could you use the following bpftrace program while your iperf is running ?

.bpftrace -e '
k:__dev_queue_xmit {
 $skb = (struct sk_buff *)arg0;
 if ($skb->fclone == 2) {
  @start[$skb] = nsecs;
 }
}
k:__kfree_skb {
 $skb = (struct sk_buff *)arg0;
 if ($skb->fclone == 2 && @start[$skb]) {
  @tx_compl_usecs = hist((nsecs - @start[$skb])/1000);
  delete(@start[$skb]);
 }
}
END { clear(@start); }'