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

[Thomas Rosenstein]

On 5 Nov 2020, at 13:38, Toke Høiland-Jørgensen wrote:

> "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).

Guess I'll go the easy route and let it be ...

I'll update all routers to the 5.9.4 and see if it fixes the traffic 
flow - will report back once more after that.

>
>>>>> 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...
>

Thanks for the support :)

> -Toke