Re: [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle

[George Dunlap <George.Dunlap@citrix.com>]

> On Mar 12, 2020, at 1:55 PM, Andrew Cooper <Andrew.Cooper3@citrix.com> wrote:
> 
> On 12/03/2020 13:44, Dario Faggioli wrote:
>> There have been report of stalls of guest vCPUs, when Credit2 was used.
>> It seemed like these vCPUs were not getting scheduled for very long
>> time, even under light load conditions (e.g., during dom0 boot).
>> 
>> Investigations led to the discovery that --although rarely-- it can
>> happen that a vCPU manages to run for very long timeslices. In Credit2,
>> this means that, when runtime accounting happens, the vCPU will lose a
>> large quantity of credits. This in turn may lead to the vCPU having less
>> credits than the idle vCPUs (-2^30). At this point, the scheduler will
>> pick the idle vCPU, instead of the ready to run vCPU, for a few
>> "epochs", which often times is enough for the guest kernel to think the
>> vCPU is not responding and crashing.
>> 
>> An example of this situation is shown here. In fact, we can see d0v1
>> sitting in the runqueue while all the CPUs are idle, as it has
>> -1254238270 credits, which is smaller than -2^30 = −1073741824:
>> 
>>    (XEN) Runqueue 0:
>>    (XEN)   ncpus              = 28
>>    (XEN)   cpus               = 0-27
>>    (XEN)   max_weight         = 256
>>    (XEN)   pick_bias          = 22
>>    (XEN)   instload           = 1
>>    (XEN)   aveload            = 293391 (~111%)
>>    (XEN)   idlers: 00,00000000,00000000,00000000,00000000,00000000,0fffffff
>>    (XEN)   tickled: 00,00000000,00000000,00000000,00000000,00000000,00000000
>>    (XEN)   fully idle cores: 00,00000000,00000000,00000000,00000000,00000000,0fffffff
>>    [...]
>>    (XEN) Runqueue 0:
>>    (XEN) CPU[00] runq=0, sibling=00,..., core=00,...
>>    (XEN) CPU[01] runq=0, sibling=00,..., core=00,...
>>    [...]
>>    (XEN) CPU[26] runq=0, sibling=00,..., core=00,...
>>    (XEN) CPU[27] runq=0, sibling=00,..., core=00,...
>>    (XEN) RUNQ:
>>    (XEN)     0: [0.1] flags=0 cpu=5 credit=-1254238270 [w=256] load=262144 (~100%)
>> 
>> We certainly don't want, under any circumstance, this to happen.
>> Therefore, let's use INT_MIN for the credits of the idle vCPU, in
>> Credit2, to be sure that no vCPU can get below that value.
>> 
>> NOTE: investigations have been done about _how_ it is possible for a
>> vCPU to execute for so long that its credits becomes so low. While still
>> not completely clear, there are evidence that:
>> - it only happens very rarely
>> - it appears to be both machine and workload specific
>> - it does not look to be a Credit2 (e.g., as it happens when running
>>  with Credit1 as well) issue, or a scheduler issue
> 
> On what basis?
> 
> Everything reported to xen-devel appears to suggests it is a credit2
> problem.  It doesn't manifest on versions of Xen before credit2 became
> the default, and switching back to credit1 appears to mitigate the problem.
> 
> Certainly as far as XenServer is concerned, we haven't seen symptoms
> like this in a decade of running credit1.

One reason could be because the symptoms are different.  On credit1, credits and “priority” are separated; it’s not possible in credit1 for a vcpu to end up with a lower priority than the idle domain, and no matter how low the credits become, a vcpu will always end up with some “peers” at the same priority level, meaning it always has a chance at some cpu.

What Dario is saying (if I understand him correctly) is that the *proximate* cause (allowing a vcpu to have an effective priority of less than idle) is certainly credit2-only; but the *deeper* cause (vcpus racking up massive amounts of negative credit) is not.

 -George
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