Re: Expensive tcp_collapse with high tcp_rmem limit

[Eric Dumazet <edumazet@google.com>]

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%

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.


>   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.