Re: Low TCP throughput due to vmpressure with swap enabled

Ivan Babrou
2022-12-06T00:51:04Z

On Mon, Dec 5, 2022 at 3:57 PM Ivan Babrou <ivan@cloudflare.com> wrote:
>
> On Mon, Nov 28, 2022 at 10:07 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Tue, Nov 22, 2022 at 05:28:24PM -0800, Ivan Babrou wrote:
> > > On Tue, Nov 22, 2022 at 2:11 PM Ivan Babrou <ivan@cloudflare.com> wrote:
> > > >
> > > > On Tue, Nov 22, 2022 at 12:05 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > > >
> > > > > On Mon, Nov 21, 2022 at 04:53:43PM -0800, Ivan Babrou wrote:
> > > > > > Hello,
> > > > > >
> > > > > > We have observed a negative TCP throughput behavior from the following commit:
> > > > > >
> > > > > > * 8e8ae645249b mm: memcontrol: hook up vmpressure to socket pressure
> > > > > >
> > > > > > It landed back in 2016 in v4.5, so it's not exactly a new issue.
> > > > > >
> > > > > > The crux of the issue is that in some cases with swap present the
> > > > > > workload can be unfairly throttled in terms of TCP throughput.
> > > > >
> > > > > Thanks for the detailed analysis, Ivan.
> > > > >
> > > > > Originally, we pushed back on sockets only when regular page reclaim
> > > > > had completely failed and we were about to OOM. This patch was an
> > > > > attempt to be smarter about it and equalize pressure more smoothly
> > > > > between socket memory, file cache, anonymous pages.
> > > > >
> > > > > After a recent discussion with Shakeel, I'm no longer quite sure the
> > > > > kernel is the right place to attempt this sort of balancing. It kind
> > > > > of depends on the workload which type of memory is more imporant. And
> > > > > your report shows that vmpressure is a flawed mechanism to implement
> > > > > this, anyway.
> > > > >
> > > > > So I'm thinking we should delete the vmpressure thing, and go back to
> > > > > socket throttling only if an OOM is imminent. This is in line with
> > > > > what we do at the system level: sockets get throttled only after
> > > > > reclaim fails and we hit hard limits. It's then up to the users and
> > > > > sysadmin to allocate a reasonable amount of buffers given the overall
> > > > > memory budget.
> > > > >
> > > > > Cgroup accounting, limiting and OOM enforcement is still there for the
> > > > > socket buffers, so misbehaving groups will be contained either way.
> > > > >
> > > > > What do you think? Something like the below patch?
> > > >
> > > > The idea sounds very reasonable to me. I can't really speak for the
> > > > patch contents with any sort of authority, but it looks ok to my
> > > > non-expert eyes.
> > > >
> > > > There were some conflicts when cherry-picking this into v5.15. I think
> > > > the only real one was for the "!sc->proactive" condition not being
> > > > present there. For the rest I just accepted the incoming change.
> > > >
> > > > I'm going to be away from my work computer until December 5th, but
> > > > I'll try to expedite my backported patch to a production machine today
> > > > to confirm that it makes the difference. If I can get some approvals
> > > > on my internal PRs, I should be able to provide the results by EOD
> > > > tomorrow.
> > >
> > > I tried the patch and something isn't right here.
> >
> > Thanks for giving it a sping.
> >
> > > With the patch applied I'm capped at ~120MB/s, which is a symptom of a
> > > clamped window.
> > >
> > > I can't find any sockets with memcg->socket_pressure = 1, but at the
> > > same time I only see the following rcv_ssthresh assigned to sockets:
> >
> > Hm, I don't see how socket accounting would alter the network behavior
> > other than through socket_pressure=1.
> >
> > How do you look for that flag? If you haven't yet done something
> > comparable, can you try with tracing to rule out sampling errors?
>
> Apologies for a delayed reply, I took a week off away from computers.
>
> I looked with bpftrace (from my bash_history):
>
> $ sudo bpftrace -e 'kprobe:tcp_try_rmem_schedule { @sk[cpu] = arg0; }
> kretprobe:tcp_try_rmem_schedule { $arg = @sk[cpu]; if ($arg) { $sk =
> (struct sock *) $arg; $id = $sk->sk_memcg->css.cgroup->kn->id;
> $socket_pressure = $sk->sk_memcg->socket_pressure; if ($id == 21379) {
> printf("id = %d, socket_pressure = %d\n", $id, $socket_pressure); } }
> }'
>
> I tried your patch on top of v6.1-rc8 (where it produced no conflicts)
> in my vm and it still gave me low numbers and nothing in
> /sys/kernel/debug/tracing/trace. To be extra sure, I changed it from
> trace_printk to just printk and it still didn't show up in dmesg, even
> with constant low throughput:
>
> ivan@vm:~$ curl -o /dev/null https://sim.cfperf.net/cached-assets/zero-5g.bin
> % Total % Received % Xferd Average Speed Time Time Time Current
> Dload Upload Total Spent Left Speed
> 14 4768M 14 685M 0 0 12.9M 0 0:06:08 0:00:52 0:05:16 13.0M
>
> I still saw clamped rcv_ssthresh:
>
> $ sudo ss -tinm dport 443
> State Recv-Q Send-Q
> Local Address:Port
> Peer Address:Port Process
> ESTAB 0 0
> 10.2.0.15:35800
> 162.159.136.82:443
> skmem:(r0,rb2577228,t0,tb46080,f0,w0,o0,bl0,d0) cubic rto:201
> rtt:0.42/0.09 ato:40 mss:1460 pmtu:1500 rcvmss:1440 advmss:1460
> cwnd:10 bytes_sent:12948 bytes_acked:12949 bytes_received:2915062731
> segs_out:506592 segs_in:2025111 data_segs_out:351 data_segs_in:2024911
> send 278095238bps lastsnd:824 lastrcv:154 lastack:154 pacing_rate
> 556190472bps delivery_rate 47868848bps delivered:352 app_limited
> busy:147ms rcv_rtt:0.011 rcv_space:82199 rcv_ssthresh:5840
> minrtt:0.059 snd_wnd:65535 tcp-ulp-tls rxconf: none txconf: none
>
> I also tried with my detection program for ebpf_exporter (fexit based version):
>
> * https://github.com/cloudflare/ebpf_exporter/pull/172/files
>
> Which also showed signs of a clamped window:
>
> # HELP ebpf_exporter_tcp_window_clamps_total Number of times that TCP
> window was clamped to a low value
> # TYPE ebpf_exporter_tcp_window_clamps_total counter
> ebpf_exporter_tcp_window_clamps_total 53887
>
> In fact, I can replicate this with just curl to a public URL and fio running,

I sprinkled some more printk around to get to the bottom of this:

static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
{
if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
memcg->socket_pressure) {
printk("socket pressure[1]: %lu", memcg->socket_pressure);
return true;
}
do {
if (memcg->socket_pressure) {
printk("socket pressure[2]: %lu",
memcg->socket_pressure);
return true;
}
} while ((memcg = parent_mem_cgroup(memcg)));
return false;
}

And now I can see plenty of this:

[ 108.156707][ T5175] socket pressure[2]: 4294673429
[ 108.157050][ T5175] socket pressure[2]: 4294673429
[ 108.157301][ T5175] socket pressure[2]: 4294673429
[ 108.157581][ T5175] socket pressure[2]: 4294673429
[ 108.157874][ T5175] socket pressure[2]: 4294673429
[ 108.158254][ T5175] socket pressure[2]: 4294673429

I think the first result below is to blame:

$ rg '.->socket_pressure' mm
mm/memcontrol.c
5280: memcg->socket_pressure = jiffies;
7198: memcg->socket_pressure = 0;
7201: memcg->socket_pressure = 1;
7211: memcg->socket_pressure = 0;
7215: memcg->socket_pressure = 1;

While we set socket_pressure to either zero or one in
mem_cgroup_charge_skmem, it is still initialized to jiffies on memcg
creation. Zero seems like a more appropriate starting point. With that
change I see it working as expected with no TCP speed bumps. My
ebpf_exporter program also looks happy and reports zero clamps in my
brief testing.

Since it's not "socket pressure[1]" in dmesg output, then it's
probably one of the parent cgroups that is not getting charged for
socket memory that is reporting memory pressure.

I also think we should downgrade socket_pressure from "unsigned long"
to "bool", as it only holds zero and one now.