Re: Expensive tcp_collapse with high tcp_rmem limit

[Eric Dumazet <edumazet@google.com>]

On Thu, Jan 6, 2022 at 4:32 AM Daniel Dao <dqminh@cloudflare.com> wrote:
>
> 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

Ouch.
What kind of NIC driver is used on your host ?

>   ...
>   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.
>

Except that you would still have to parse the linear list.

> 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()

I think that you should first look if you are under some kind of attack [1]

Eventually you would still have to make room, involving expensive copies.

12% of 16MB is still a lot of memory to copy.

[1] Detecting an attack signature could allow you to zap the socket
and save ~16MB of memory per flow.