Re: Xen C-state Issues

[Elliott Mitchell <ehem+xen@xxxxxxx>]

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.  While offlining
extra vCPUs after boot is inefficient, it does work.  I'm unaware of any
potential workaround for disabled C-states.

Though on second thought some breakage with Xen showed up between
Linux 4.19.181 and 4.19.194.  Now I'm wondering whether the higher
C-states might have worked with 4.19.181...


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