Re: Xen C-state Issues

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.08.2021 03:18, Elliott Mitchell wrote:
> On Tue, Aug 24, 2021 at 08:14:41AM +0200, Jan Beulich wrote:
>> 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.
> 
> That would be a very different sort of bug report if that was found to
> be an issue.  This report is likely a problem of fixes not being
> backported to stable branches.

As you say - likely. I'd like to be sure.

> What you're writing about would be looking for bugs in development
> branches.

Very much so, in case there are issues left, or ones have got
reintroduced. That's what the primary purpose of this list is.

If you were suspecting missing fixes in the kernel, I guess xen-devel
isn't the preferred channel anyway. Otoh the stable maintainers there
would likely want concrete commits pointed out ...

Jan