Re: [PATCH 3/4] xen: sched: reorganize cpu_disable_scheduler()

[Juergen Gross <jgross@suse.com>]

On 07/15/2015 04:54 PM, Dario Faggioli wrote:
> On Thu, 2015-07-09 at 12:45 +0200, Juergen Gross wrote:
>> On 07/09/2015 12:24 PM, Dario Faggioli wrote:
>>> On Wed, 2015-07-08 at 17:13 +0200, Dario Faggioli wrote:
>>>> On Tue, 2015-07-07 at 13:16 +0200, Juergen Gross wrote:
>>>>>> +            /*
>>>>>> +             * In case of shutdown/suspend, it is not necessary to ask the
>>>>>> +             * scheduler to chime in. In fact:
>>>>>> +             *  * there is no reason for it: the end result we are after is
>>>>>> +             *    just 'all the vcpus on the boot pcpu, and no vcpu anywhere
>>>>>> +             *    else', so let's just go for it;
>>>>>> +             *  * it's wrong, when dealing a cpupool with only non-boot pcpus,
>>>>>> +             *    as the scheduler will always fail to send the vcpus away
>>>>>> +             *    from the last online (non boot) pcpu!
>>>>>
>>>>> I'd add a comment that in shutdown/suspend case all domains are being
>>>>> paused, so we can be active in dom0/Pool-0 only.
>>>>>
>>>> Sure, I'll add this.
>>>>
>>> ...while putting such a comment together, I'm realizing that I'm not
>>> sure about what you meant, or what you wanted the comment itself to
>>> express.
>>>
>>> I mean, it is certainly true that all domains are being paused (they've
>>> been paused already, actually), but that include Dom0 too. Also, we are
>>> in Xen, in stop_machine context, so I'm not sure what you meant either
>>> with "we can be active in dom0/Pool-0 only".
>>
>> We are running on the vcpu which issued the hypercall resulting in
>> pausing the domains. A vcpu can't pause itself.
>>
> Hey, sorry it took me a bit to reply.
>
> Actually, I'm still not getting the "we are running on the vcpu" part.
> Perhaps it's all a terminology issue that we have, and it may not be
> that much worthwhile to spend a lot of time on it.
>
> However, the hypercall to suspend/shutdown the system has indeed been
> issued by a dom0 vcpu. The call chain is this:
>
> XENPF_enter_acpi_sleep:
>    acpi_enter_sleep()
>      continue_hypercall_on_cpu(0, enter_state_helper)

Interesting. I managed to miss this one.

I'm rather sure this was handled differently when I initially wrote the
cpupool stuff. So please forget my comment about Pool-0/dom0, it was
from memory and is no longer or was never correct.


Juergen