Re: [Xen-devel] Guest start issue on ARM (maybe related to Credit2) [Was: Re: [xen-unstable test] 113807: regressions - FAIL]

[Julien Grall <julien.grall@xxxxxxx>]

Hi Dario,

On 09/26/2017 09:51 PM, Dario Faggioli wrote:
> On Tue, 2017-09-26 at 18:28 +0100, Julien Grall wrote:
>> On 09/26/2017 08:33 AM, Dario Faggioli wrote:
>>>>
>>> Here's the logs:
>>> http://logs.test-lab.xenproject.org/osstest/logs/113816/test-armhf-
>>> armhf-xl-rtds/info.html
>>
>> It does not seem to be similar, in the credit2 case the kernel is
>> stuck at very early boot.
>> Here it seems it is running (there are grants setup).
>>
> Yes, I agree, it's not totally similar.
> 
>> This seem to be confirmed from the guest console log, I can see the
>> prompt. Interestingly
>> when the guest job fails, it has been waiting for a long time disk
>> and hvc0. Although, it
>> does not timeout.
>>
> Ah, I see what you mean, I found it in the guest console log.
> 
>> I am actually quite surprised that we start a 4 vCPUs guest on a 2
>> pCPUs platform. The total of
>> vCPUs is 6 (2 DOM0 + 4 DOMU). The processors in are not the greatest
>> for testing. So I was
>> wondering if we end up to have too many vCPUs running on the platform
>> and making it unreliable
>> the test?
>>
> Well, doing that, with this scheduler, is certainly *not* the best
> recipe for determinism and reliability.
> 
> In fact, RTDS is a non-work conserving scheduler. This means that (with
> default parameters) each vCPU gets at most 40% CPU time, even if there
> are idle cycles.
> 
> With 6 vCPU, there's a total demand of 240% of CPU time, and with 2
> pCPUs, there's at most 200% of that, which means we're in overload
> (well, at least that's the case if/when all the vCPUs try to execute
> for their guaranteed 40%).
> 
> Things *should really not* explode (like as in Xen crashes) if that
> happens; actually, from a scheduler perspective, it should really not
> be too big of a deal (especially if the overload is transient, like I
> guess it should be in this case). However, it's entirely possible that
> some specific vCPUs failing to be scheduler for a certain amount of
> time, causes something _inside_ the guest to timeout, or get stuck or
> wedged, which may be what happens here.

Looking at the log I don't see any crash of Xen and it seems to
be responsive.

I don't know much about the scheduler and how to interpret the logs:

Sep 25 22:43:21.495119 (XEN) Domain info:
Sep 25 22:43:21.503073 (XEN)    domain: 0
Sep 25 22:43:21.503100 (XEN) [    0.0 ] cpu 0, (10000000, 4000000), 
cur_b=3895333 cur_d=1611120000000 last_start=1611116505875
Sep 25 22:43:21.511080 (XEN)             onQ=0 runnable=0 flags=0 effective 
hard_affinity=0-1
Sep 25 22:43:21.519082 (XEN) [    0.1 ] cpu 1, (10000000, 4000000), 
cur_b=3946375 cur_d=1611130000000 last_start=1611126446583
Sep 25 22:43:21.527023 (XEN)             onQ=0 runnable=1 flags=0 effective 
hard_affinity=0-1
Sep 25 22:43:21.535063 (XEN)    domain: 5
Sep 25 22:43:21.535089 (XEN) [    5.0 ] cpu 0, (10000000, 4000000), 
cur_b=3953875 cur_d=1611120000000 last_start=1611110106041
Sep 25 22:43:21.543073 (XEN)             onQ=0 runnable=0 flags=0 effective 
hard_affinity=0-1
Sep 25 22:43:21.551078 (XEN) [    5.1 ] cpu 1, (10000000, 4000000), 
cur_b=3938167 cur_d=1611140000000 last_start=1611130169791
Sep 25 22:43:21.559063 (XEN)             onQ=0 runnable=0 flags=0 effective 
hard_affinity=0-1
Sep 25 22:43:21.559096 (XEN) [    5.2 ] cpu 1, (10000000, 4000000), 
cur_b=3952500 cur_d=1611140000000 last_start=1611130107958
Sep 25 22:43:21.575067 (XEN)             onQ=0 runnable=0 flags=0 effective 
hard_affinity=0-1
Sep 25 22:43:21.575101 (XEN) [    5.3 ] cpu 0, (10000000, 4000000), 
cur_b=3951875 cur_d=1611120000000 last_start=1611110154166
Sep 25 22:43:21.583196 (XEN)             onQ=0 runnable=0 flags=0 effective 
hard_affinity=0-1

Also, it seems to fail fairly reliably, so it might be possible
to set up a reproducer.

Cheers,

-- 
Julien Grall

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