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Re: [Xen-devel] Live migrate with Linux >= 4.13 domU causes kernel time jumps and TCP connection stalls.


  • To: Hans van Kranenburg <hans@xxxxxxxxxxx>, Hans van Kranenburg <Hans.van.Kranenburg@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Fri, 28 Dec 2018 11:15:17 +0100
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  • Cc: Igor Yurchenko <Igor.Yurchenko@xxxxxxxxxx>
  • Delivery-date: Fri, 28 Dec 2018 10:15:29 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 27/12/2018 22:12, Hans van Kranenburg wrote:
> So,
> 
> On 12/24/18 1:32 AM, Hans van Kranenburg wrote:
>>
>> On 12/21/18 6:54 PM, Hans van Kranenburg wrote:
>>>
>>> We've been tracking down a live migration bug during the last three days
>>> here at work, and here's what we found so far.
>>>
>>> 1. Xen version and dom0 linux kernel version don't matter.
>>> 2. DomU kernel is >= Linux 4.13.
>>>
>>> When using live migrate to another dom0, this often happens:
>>>
>>> [   37.511305] Freezing user space processes ... (elapsed 0.001 seconds)
>>> done.
>>> [   37.513316] OOM killer disabled.
>>> [   37.513323] Freezing remaining freezable tasks ... (elapsed 0.001
>>> seconds) done.
>>> [   37.514837] suspending xenstore...
>>> [   37.515142] xen:grant_table: Grant tables using version 1 layout
>>> [18446744002.593711] OOM killer enabled.
>>> [18446744002.593726] Restarting tasks ... done.
>>> [18446744002.604527] Setting capacity to 6291456
>>
>> Tonight, I've been through 29 bisect steps to figure out a bit more. A
>> make defconfig with enabling Xen PV for domU reproduces the problem
>> already, so a complete cycle with compiling and testing had only to take
>> about 7 minutes.
>>
>> So, it appears that this 18 gazillion seconds of uptime is a thing that
>> started happening earlier than the TCP situation already. All of the
>> test scenarios resulted in these huge uptime numbers in dmesg. Not all
>> of them result in TCP connections hanging.
>>
>>> As a side effect, all open TCP connections stall, because the timestamp
>>> counters of packets sent to the outside world are affected:
>>>
>>> https://syrinx.knorrie.org/~knorrie/tmp/tcp-stall.png
>>
>> This is happening since:
>>
>> commit 9a568de4818dea9a05af141046bd3e589245ab83
>> Author: Eric Dumazet <edumazet@xxxxxxxxxx>
>> Date:   Tue May 16 14:00:14 2017 -0700
>>
>>     tcp: switch TCP TS option (RFC 7323) to 1ms clock
>>
>> [...]
>>
>>> [...]
>>>
>>> 3. Since this is related to time and clocks, the last thing today we
>>> tried was, instead of using default settings, put "clocksource=tsc
>>> tsc=stable:socket" on the xen command line and "clocksource=tsc" on the
>>> domU linux kernel line. What we observed after doing this, is that the
>>> failure happens less often, but still happens. Everything else applies.
>>
>> Actually, it seems that the important thing is that uptime of the dom0s
>> is not very close to each other. After rebooting all four back without
>> tsc options, and then a few hours later rebooting one of them again, I
>> could easily reproduce again when live migrating to the later rebooted
>> server.
>>
>>> Additional question:
>>>
>>> It's 2018, should we have these "clocksource=tsc tsc=stable:socket" on
>>> Xen and "clocksource=tsc" anyways now, for Xen 4.11 and Linux 4.19
>>> domUs? All our hardware has 'TscInvariant = true'.
>>>
>>> Related: https://news.ycombinator.com/item?id=13813079
>>
>> This is still interesting.
>>
>> ---- >8 ----
>>
>> Now, the next question is... is 9a568de481 bad, or shouldn't there be 18
>> gazillion whatever uptime already... In Linux 4.9, this doesn't happen,
>> so next task will be to find out where that started.
> 
> And that's...
> 
> commit f94c8d116997597fc00f0812b0ab9256e7b0c58f
> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date:   Wed Mar 1 15:53:38 2017 +0100
> 
>     sched/clock, x86/tsc: Rework the x86 'unstable' sched_clock() interface
> 
> a.k.a. v4.11-rc2~30^2
> 
> Before this commit, time listed in dmesg seems to follow uptime of the
> domU, and after it, time in dmesg seems to jump around up and down when
> live migrating to different dom0s, with the occasional/frequent jump to
> a number above 18000000000 which then also shows the TCP timestamp
> breakage since 9a568de4.
> 
> So, next question is... what now? Any ideas appreciated.
> 
> Can anyone else reproduce this? I have super-common HP DL360 hardware
> and mostly default settings, so it shouldn't be that hard.
> 
> Should I mail some other mailinglist with a question? Which one? Does
> any of you Xen developers have more experience with time keeping code?

My gut feeling tells me that above patch was neglecting Xen by setting
a non-native TSC clock too often to "stable" (the "only call
clear_sched_clock_stable() when we mark TSC unstable when we use
native_sched_clock()" part of the commit message).

I can have a more thorough look after Jan. 7th.


Juergen

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