Re: Xen C-state Issues

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.08.2021 07:37, Elliott Mitchell wrote:
> On Mon, Aug 23, 2021 at 09:12:52AM +0200, Jan Beulich wrote:
>> On 21.08.2021 18:25, Elliott Mitchell wrote:
>>> ACPI C-state support might not see too much use, but it does see some.
>>>
>>> With Xen 4.11 and Linux kernel 4.19, I found higher C-states only got
>>> enabled for physical cores for which Domain 0 had a corresponding vCPU.
>>> On a machine where Domain 0 has 5 vCPUs, but 8 reported cores, the
>>> additional C-states would only be enabled on cores 0-4.
>>>
>>> This can be worked around by giving Domain 0 vCPUs equal to cores, but
>>> then offlining the extra vCPUs.  I'm guessing this is a bug with the
>>> Linux 4.19 xen_acpi_processor module.
>>>
>>>
>>>
>>> Appears Xen 4.14 doesn't work at all with Linux kernel 4.19's ACPI
>>> C-state support.  This combination is unable to enable higher C-states
>>> on any core.  Since Xen 4.14 and Linux 4.19 are *both* *presently*
>>> supported it seems patch(es) are needed somewhere for this combination.
>>
>> Hmm, having had observed the same quite some time ago, I thought I had
>> dealt with these problems. Albeit surely not in Xen 4.11 or Linux 4.19.
>> Any chance you could check up-to-date versions of both Xen and Linux
>> (together)?
> 
> I can believe you got this fixed, but the Linux fixes never got
> backported.
> 
> Of the two, higher C-states working with Linux 4.19 and Xen 4.11, but
> not Linux 4.19 and Xen 4.14 is more concerning to me.

I'm afraid without you providing detail (full verbosity logs) and
ideally checking with 4.15 or yet better -unstable it's going to be
hard to judge whether that's a bug, and if so where it might sit.

Jan