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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Roger.

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