Re: [Xen-devel] [PATCH v3 29/47] xen/sched: introduce unit_runnable_state()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.09.2019 16:25, Jürgen Groß wrote:
> On 23.09.19 17:24, Jan Beulich wrote:
>> On 14.09.2019 10:52, Juergen Gross wrote:
>>> Today the vcpu runstate of a new scheduled vcpu is always set to
>>> "running" even if at that time vcpu_runnable() is already returning
>>> false due to a race (e.g. with pausing the vcpu).
>>
>> I can see this part, ...
>>
>>> With core scheduling this can no longer work as not all vcpus of a
>>> schedule unit have to be "running" when being scheduled. So the vcpu's
>>> new runstate has to be selected at the same time as the runnability of
>>> the related schedule unit is probed.
>>
>> ... but I continue having trouble here. If it has been okay to set
>> a vCPU no longer runnable to "running" nevertheless, why would the
>> same not be true for schedule units? Part of the problem may be
>> that ...
> 
> The difference is the need to drop the scheduling lock for doing the
> rendezvous. vcpu_sleep() or vcpu_wake() could now interfere with
> scheduling in a way which was not possible before.

Ah, right. But this would be worth mentioning explicitly here to
help readers, wouldn't it? Doesn't need to be more than half a
sentence ...

Jan

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