Re: [Xen-devel] CPU frequency throttling based on the temperature

[Jan Beulich <JBeulich@xxxxxxxx>]

On 25.07.2019 16:17, Roger Pau Monné  wrote:
> On Thu, Jul 25, 2019 at 01:59:22PM +0000, Jan Beulich wrote:
>> On 25.07.2019 15:47, Roger Pau Monné  wrote:
>>> On Thu, Jul 25, 2019 at 09:29:01AM -0400, Fredy P. wrote:
>>>> On Thu, 2019-07-25 at 15:13 +0200, Roger Pau Monné wrote:
>>>>> On Thu, Jul 25, 2019 at 12:54:46PM +0000, Jan Beulich wrote:
>>>>>> On 25.07.2019 14:44,  Fredy P.  wrote:
>>>>>>> On Wed, 2019-07-24 at 17:41 +0200, Roger Pau Monné wrote:
>>>>>>>>>> What hardware interface does thermald (or the driver in
>>>>>>>>>> Linux if
>>>>>>>>>> there's one) use to get the temperature data?
>>>>>>>
>>>>>>> In our initial POC using Xen 4.8.x we where using Linux coretemp
>>>>>>> driver
>>>>>>> reading by example /class/sys/hwmon/hwmon0/temp3_input but it got
>>>>>>> deprecated at commit 72e038450d3d5de1a39f0cfa2d2b0f9b3d43c6c6
>>>>>>
>>>>>> Hmm, I wouldn't call this deprecation, but a regression. I would
>>>>>> say we want to re-expose this leaf to Dom0, the more that the
>>>>>> commit also only mentions unprivileged domains. Andrew?
>>>>>
>>>>> AFAICT from the documents provided by Fredy the temperature is read
>>>>> from a MSR that reports the current temperature of the core on which
>>>>> the MSR is read from. When running on Xen this will only work
>>>>> correctly if dom0 is given the same vCPUs as pCPUs and those are
>>>>> identity pinned.
>>>>
>>>> I just want to be sure I got it correctly, by saying "When running on
>>>> Xen this will only work correctly if ..." means in a future
>>>> implementation or that right now could work if I pin this v/pCPUS?
>>>
>>> No, right now there's no way to get this data from dom0, regardless of
>>> the pinning.
>>
>> Of course you can, using the MSR "device" Linux optionally
>> provides (plus perhaps the rdmsr utility from the msr-tools
>> package).
> 
> But you won't get coherent results, since the vCPU might be jumping
> from pCPU to pCPU, thus returning values from multiple different pCPUs
> regardless of whether all rdmsr have been executed from the same vCPU
> from dom0 PoV.

I don't understand. Earlier you said "regardless of the pinning".
That's what my response was to, i.e. I was implying vCPU-s to be
pinned.

Jan
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