[Bloat] Router congestion, slow ping/ack times with kernel 5.4.60

[Toke Høiland-Jørgensen]

"Thomas Rosenstein" <thomas.rosenstein at creamfinance.com> writes:

> On 5 Nov 2020, at 12:21, Toke Høiland-Jørgensen wrote:
>
>> "Thomas Rosenstein" <thomas.rosenstein at creamfinance.com> writes:
>>
>>>> If so, this sounds more like a driver issue, or maybe something to 
>>>> do
>>>> with scheduling. Does it only happen with ICMP? You could try this
>>>> tool
>>>> for a userspace UDP measurement:
>>>
>>> It happens with all packets, therefore the transfer to backblaze with 
>>> 40
>>> threads goes down to ~8MB/s instead of >60MB/s
>>
>> Huh, right, definitely sounds like a kernel bug; or maybe the new 
>> kernel
>> is getting the hardware into a state where it bugs out when there are
>> lots of flows or something.
>>
>> You could try looking at the ethtool stats (ethtool -S) while running
>> the test and see if any error counters go up. Here's a handy script to
>> monitor changes in the counters:
>>
>> https://github.com/netoptimizer/network-testing/blob/master/bin/ethtool_stats.pl
>>
>>> I'll try what that reports!
>>>
>>>> Also, what happens if you ping a host on the internet (*through* the
>>>> router instead of *to* it)?
>>>
>>> Same issue, but twice pronounced, as it seems all interfaces are
>>> affected.
>>> So, ping on one interface and the second has the issue.
>>> Also all traffic across the host has the issue, but on both sides, so
>>> ping to the internet increased by 2x
>>
>> Right, so even an unloaded interface suffers? But this is the same 
>> NIC,
>> right? So it could still be a hardware issue...
>>
>>> Yep default that CentOS ships, I just tested 4.12.5 there the issue 
>>> also
>>> does not happen. So I guess I can bisect it then...(really don't want 
>>> to
>>> 😃)
>>
>> Well that at least narrows it down :)
>
> I just tested 5.9.4 seems to also fix it partly, I have long stretches 
> where it looks good, and then some increases again. (3.10 Stock has them 
> too, but not so high, rather 1-3 ms)
>
> for example:
>
> 64 bytes from x.x.x.x: icmp_seq=10 ttl=64 time=0.169 ms
> 64 bytes from x.x.x.x: icmp_seq=11 ttl=64 time=5.53 ms
> 64 bytes from x.x.x.x: icmp_seq=12 ttl=64 time=9.44 ms
> 64 bytes from x.x.x.x: icmp_seq=13 ttl=64 time=0.167 ms
> 64 bytes from x.x.x.x: icmp_seq=14 ttl=64 time=3.88 ms
>
> and then again:
>
> 64 bytes from x.x.x.x: icmp_seq=15 ttl=64 time=0.569 ms
> 64 bytes from x.x.x.x: icmp_seq=16 ttl=64 time=0.148 ms
> 64 bytes from x.x.x.x: icmp_seq=17 ttl=64 time=0.286 ms
> 64 bytes from x.x.x.x: icmp_seq=18 ttl=64 time=0.257 ms
> 64 bytes from x.x.x.x: icmp_seq=19 ttl=64 time=0.220 ms
> 64 bytes from x.x.x.x: icmp_seq=20 ttl=64 time=0.125 ms
> 64 bytes from x.x.x.x: icmp_seq=21 ttl=64 time=0.188 ms
> 64 bytes from x.x.x.x: icmp_seq=22 ttl=64 time=0.202 ms
> 64 bytes from x.x.x.x: icmp_seq=23 ttl=64 time=0.195 ms
> 64 bytes from x.x.x.x: icmp_seq=24 ttl=64 time=0.177 ms
> 64 bytes from x.x.x.x: icmp_seq=25 ttl=64 time=0.242 ms
> 64 bytes from x.x.x.x: icmp_seq=26 ttl=64 time=0.339 ms
> 64 bytes from x.x.x.x: icmp_seq=27 ttl=64 time=0.183 ms
> 64 bytes from x.x.x.x: icmp_seq=28 ttl=64 time=0.221 ms
> 64 bytes from x.x.x.x: icmp_seq=29 ttl=64 time=0.317 ms
> 64 bytes from x.x.x.x: icmp_seq=30 ttl=64 time=0.210 ms
> 64 bytes from x.x.x.x: icmp_seq=31 ttl=64 time=0.242 ms
> 64 bytes from x.x.x.x: icmp_seq=32 ttl=64 time=0.127 ms
> 64 bytes from x.x.x.x: icmp_seq=33 ttl=64 time=0.217 ms
> 64 bytes from x.x.x.x: icmp_seq=34 ttl=64 time=0.184 ms
>
>
> For me it looks now that there was some fix between 5.4.60 and 5.9.4 ... 
> anyone can pinpoint it?

$ git log --no-merges --oneline v5.4.60..v5.9.4|wc -l
72932

Only 73k commits; should be easy, right? :)

(In other words no, I have no idea; I'd suggest either (a) asking on
netdev, (b) bisecting or (c) using 5.9+ and just making peace with not
knowing).

>>>> How did you configure the new kernel? Did you start from scratch, or
>>>> is
>>>> it based on the old centos config?
>>>
>>> first oldconfig and from there then added additional options for IB,
>>> NVMe, etc (which I don't really need on the routers)
>>
>> OK, so you're probably building with roughly the same options in terms
>> of scheduling granularity etc. That's good. Did you enable spectre
>> mitigations etc on the new kernel? What's the output of
>> `tail /sys/devices/system/cpu/vulnerabilities/*` ?
>
> mitigations are off

Right, I just figured maybe you were hitting some threshold that
involved a lot of indirect calls which slowed things down due to
mitigations. Guess not, then...

-Toke