Re: Expensive tcp_collapse with high tcp_rmem limit

[Daniel Dao <dqminh@cloudflare.com>]

On Wed, Jan 5, 2022 at 1:38 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Jan 5, 2022 at 4:15 AM Daniel Dao <dqminh@cloudflare.com> wrote:
> >
> > Hello,
> >
> > We are looking at increasing the maximum value of TCP receive buffer in order
> > to take better advantage of high BDP links. For historical reasons (
> > https://blog.cloudflare.com/the-story-of-one-latency-spike/), this was set to
> > a lower than default value.
> >
> > We are still occasionally seeing long time spent in tcp_collapse, and the time
> > seems to be proportional with max rmem. For example, with net.ipv4.tcp_rmem = 8192 2097152 16777216,
> > we observe tcp_collapse latency with the following bpftrace command:
> >
>
> I suggest you add more traces, like the payload/truesize ratio when
> these events happen.
> and tp->rcv_ssthresh, sk->sk_rcvbuf
>
> TCP stack by default assumes a conservative [1] payload/truesize ratio of 50%

I forgot to add that for this experiment we also set tcp_adv_win_scale
= -2 to see if it
reduces the chance of triggering tcp_collapse

>
> Meaning that a 16MB sk->rcvbuf would translate to a TCP RWIN of 8MB.
>
> I suspect that you use XDP, and standard MTU=1500.
> Drivers in XDP mode use one page (4096 bytes on x86) per incoming frame.
> In this case, the ratio is ~1428/4096 = 35%
>
> This is one of the reason we switched to a 4K MTU at Google, because we
> have an effective ratio close to 100% (even if XDP was used)
>
> [1] The 50% ratio of TCP is defeated with small MSS, and malicious traffic.

I updated the bpftrace script to get data on len/truesize on collapsed skb

  kprobe:tcp_collapse {
    $sk = (struct sock *) arg0;
    $tp = (struct tcp_sock *) arg0;
    printf("tid %d: rmem_alloc=%ld sk_rcvbuf=%ld rcv_ssthresh=%ld\n", tid,
        $sk->sk_backlog.rmem_alloc.counter, $sk->sk_rcvbuf, $tp->rcv_ssthresh);
    printf("tid %d: advmss=%ld wclamp=%ld rcv_wnd=%ld\n", tid, $tp->advmss,
        $tp->window_clamp, $tp->rcv_wnd);
    @start[tid] = nsecs;
  }

  kretprobe:tcp_collapse /@start[tid] != 0/ {
    $us = (nsecs - @start[tid])/1000;
    @us = hist($us);
    printf("tid %d: %ld us\n", tid, $us);
    delete(@start[tid]);
  }

  kprobe:tcp_collapse_one {
    $skb = (struct sk_buff *) arg1;
    printf("tid %d: s=%ld len=%ld truesize=%ld\n", tid, sizeof(struct
sk_buff), $skb->len, $skb->truesize);
  }

  interval:s:6000 { exit(); }

Here is the output:

  tid 0: rmem_alloc=16780416 sk_rcvbuf=16777216 rcv_ssthresh=2920
  tid 0: advmss=1460 wclamp=4194304 rcv_wnd=450560
  tid 0: len=3316 truesize=15808
  tid 0: len=4106 truesize=16640
  tid 0: len=3967 truesize=16512
  tid 0: len=2988 truesize=15488
  ...
  tid 0: len=5279 truesize=17664
  tid 0: len=425 truesize=2048
  tid 0: 17176 us

The skb looks indeed bloated (len=3316, truesize=15808), so collapsing
definitely
helps. It just took a long time to go through thousands of 16KB skb

>
>
> >   bpftrace -e 'kprobe:tcp_collapse { @start[tid] = nsecs; } kretprobe:tcp_collapse /@start[tid] != 0/ { $us = (nsecs - @start[tid])/1000; @us = hist($us); delete(@start[tid]); printf("%ld us\n", $us);} interval:s:6000 { exit(); }'
> >   Attaching 3 probes...
> >   15496 us
> >   14301 us
> >   12248 us
> >   @us:
> >   [8K, 16K)              3 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> >
> > Spending up to 16ms with 16MiB maximum receive buffer seems high.  Are there any
> > recommendations on possible approaches to reduce the tcp_collapse latency ?
> > Would clamping the duration of a tcp_collapse call be reasonable, since we only
> > need to spend enough time to free space to queue the required skb ?
>
> It depends if the incoming skb is queued in in-order queue or
> out-of-order queue.
> For out-of-orders, we have a strategy in tcp_prune_ofo_queue() which
> should work reasonably well after commit
> 72cd43ba64fc17 tcp: free batches of packets in tcp_prune_ofo_queue()
>
> Given the nature of tcp_collapse(), limiting it to even 1ms of processing time
> would still allow for malicious traffic to hurt you quite a lot.

I don't yet understand why we have cases of bloated skbs. But it seems
like adapting the
batch prune strategy in tcp_prune_ofo_queue() to tcp_collapse makes sense to me.

I think every collapsed skb saves us truesize - len (?), and we can
set goal to free up 12.5% of sk_rcvbuf
same as tcp_prune_ofo_queue()