Messages in this thread | | | From | Eric Dumazet <> | Date | Thu, 6 Jan 2022 10:52:07 -0800 | Subject | Re: Expensive tcp_collapse with high tcp_rmem limit |
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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.
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