Re: [Xen-devel] Workings/effectiveness of the xen-acpi-processor driver

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Apr 26, 2012 at 06:25:28PM +0200, Stefan Bader wrote:
> On 26.04.2012 17:50, Konrad Rzeszutek Wilk wrote:
> > On Wed, Apr 25, 2012 at 03:00:58PM +0200, Stefan Bader wrote:
> >> Since there have been requests about that driver to get backported into 
> >> 3.2, I
> >> was interested to find out what or how much would be gained by that.
> >>
> >> The first system I tried was an AMD based one (8 core Opteron 6128@2GHz). 
> >> Which
> >> was not very successful as the drivers bail out of the init function 
> >> because the
> >> first call to acpi_processor_register_performance() returns -ENODEV. There 
> >> is
> >> some frequency scaling when running without Xen, so I need to do some more
> >> debugging there.
> > 
> > Did you back-port the other components - the ones that turn off the native
> > frequency scalling?
> > 
> >       provide disable_cpufreq() function to disable the API.
> >     xen/acpi-processor: Do not depend on CPU frequency scaling drivers.
> >       xen/cpufreq: Disable the cpu frequency scaling drivers from loading
> >>
> 
> Yes, here is the full set for reference:
> 
> * xen/cpufreq: Disable the cpu frequency scaling drivers from loading.
> * xen/acpi: Remove the WARN's as they just create noise.
> * xen/acpi: Fix Kconfig dependency on CPU_FREQ
> * xen/acpi-processor: Do not depend on CPU frequency scaling drivers.
> * xen/acpi-processor: C and P-state driver that uploads said data to hyper
> * provide disable_cpufreq() function to disable the API.

And (Linus just pulled it), you also need this one:
 df88b2d96e36d9a9e325bfcd12eb45671cbbc937 (xen/enlighten: Disable MWAIT_LEAF so 
that acpi-pad won't be loaded.)

> 
> >> The second system was an Intel one (4 core i7 920@xxxxxxx) which was
> >> successfully loading the driver. Via xenpm I can see the various 
> >> frequencies and
> >> also see them being changed. However the cpuidle data out of xenpm looks a 
> >> bit odd:
> >>
> >> #> xenpm get-cpuidle-states 0
> >> Max C-state: C7
> >>
> >> cpu id               : 0
> >> total C-states       : 2
> >> idle time(ms)        : 10819311
> >> C0                   : transition [00000000000000000001]
> >>                        residency  [00000000000000005398 ms]
> >> C1                   : transition [00000000000000000001]
> >>                        residency  [00000000000010819311 ms]
> >> pc3                  : [00000000000000000000 ms]
> >> pc6                  : [00000000000000000000 ms]
> >> pc7                  : [00000000000000000000 ms]
> >> cc3                  : [00000000000000000000 ms]
> >> cc6                  : [00000000000000000000 ms]
> >>
> >> Also gathering samples over 30s does look like only C0 and C1 are used. 
> >> This
> > 
> > Yes. 
> >> might be because C1E support is enabled in BIOS but when looking at the
> >> intel_idle data in sysfs when running without a hypervisor will show C3 
> >> and C6
> >> for the cores. That could have been just a wrong output, so I plugged in a 
> >> power
> >> meter and compared a kernel running natively and running as dom0 (with and
> >> without the acpi-processor driver).
> >>
> >> Native: 175W
> >> dom0:   183W (with only marginal difference between with or without the
> >>               processor driver)
> >> [yes, the system has a somewhat high base consumption which I attribute to 
> >> a
> >> ridiculously dimensioned graphics subsystem to be running a text console]
> >>
> >> This I would take as C3 and C6 really not being used and the frequency 
> >> scaling

So the other thing I forgot to note is that C3->C6 have a detrimental
effect on some Intel boxes with Xen. We haven't figured out exactly which ones
and the bug is definitly in the hypervisor. The bug is that when the CPU goes in
those states the NIC ends up being unresponsive. Its like the interrupts stopped
being ACKed. If I run 'xenpm set-max-cstate 2' the issue disappears.

> > 
> > To go in deeper modes there is also a need to backport a Xen unstable
> > hypercall which will allow the kernel to detect the other states besides
> > C0-C2.
> > 
> > "XEN_SET_PDC query was implemented in c/s 23783:
> >     "ACPI: add _PDC input override mechanism".
> >     
> 
> I see. There is a kernel patch about enabling MWAIT that refers to that...

Were there any special things you ran when checking the output? Just plugging
and looking at the results?
> 
> > 
> >> having no impact on the idle system is not that much surprising. But if 
> >> that was
> >> true it would also limit the usefulness of the turbo mode which I 
> >> understand
> >> would also be limited by the c-state of the other cores.
> > 
> > Hm, I should double-check that - but somehow I thought that Xen independetly
> > checks for TurboMode and if the P-states are in, then they are activated.

I did a bit of checking around and it does seem that is the case. From what
I have gathered the TurboMode kicks in when the CPU is C0 mode (which should
be obvious), and when the other cores are in anything but C0 mode. And sure
enough that seems to be the case. But I can't get the concrete details whether
the "but C0 mode" means that TurboMode will work better if the C mode is legacy
C1, C2, C3 or the CPU C-states (so MWAIT enabled). Trying to find out from
Len Brown more details..
> > 
> Turbo mode should be enabled. I had been only looking at a generic overview
> about it on Intel site which sounded like it  would make more of a difference 
> on
> how much one core could get overclocked related to how many cores are active
> (and I translated active or not into deeper c-states or not).
> Looking at the verbose output of turbostat it seems not to make that much
> difference whether 2-4 cores are running. A single core alone could get one 
> more
> increment in clock stepping. That does not immediately sound a lot. And of
> course how much or long the higher clock is used depends on other factors as
> well and is not under OS control.
> 
> In the end it is probably quite dynamic and hard to come up with hard facts to
> prove its value. Though if I can lower the idle power usage by reaching a bit
> further, that would greatly help to justify the effort and potential risk of
> backporting...

I understand. I wish I could give you the exact percentage points by which
the power usage will drop. But I think the more substantial reason benefit of
these patches is performance gains. The ones that Ian Campbell ran and were
posted on Phorenix site paint that they are beneficial.

> 
> >>
> >> Do I misread the data I see? Or maybe its a known limitation? In case it is
> >> worth doing more research I'll gladly try things and gather more data.
> > 
> > Just missing some patches. 
> > 
> > Oh, and this one:
> >       xen/acpi: Fix Kconfig dependency on CPU_FREQ
> > 
> > Hmm.. I think a patch disappeared somewhere.

That was the one I referenced at the beginning of this email.

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