Re: Xen C-state Issues

[Elliott Mitchell <ehem+xen@xxxxxxx>]

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.

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


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