cpufreq status information

cpufreq status information

[Jan Beulich]

Trying to understand whether CPU frequency scaling is actually working on
a system currently requires (afaics) source patches, as there is no way to
get the current state of a CPU. Even if this is intentional, this doesn't seem
very helpful when considering to make this functionality available to
customers: I'm certain quite a few will ask how they can tell whether this
is actually working.

Now, apart from the simple job of adding a sub-hypercall to retrieve the
necessary bits, I'm wondering whether this wouldn't be just one more
element that would much better be surfaced to the guest via the vCPU
info structure (or, as that's size constrained, a new construct to make
guest-read-only information available via a shared page). Other
(potential) items to make available this same way would e.g. be guest-
accessible last-exception-from/-to MSR values (as the values read would
be meaningless if read through rdmsr).

So I'm basically considering to add a generic mechanism first, and then
make cpufreq the first user of it. The question just is - use a completely
new (guest-ro) per-vCPU page, perhaps with chained descriptors rather
than a fixed layout, or extend the vCPU info structure, but e.g. require
the guest to use VCPUOP_register_vcpu_info to gain access to all
structure fields.

Thanks, Jan

RE: cpufreq status information

[Tian, Kevin]

>From: Jan Beulich
>Sent: 2008年9月8日 21:12
>
>Trying to understand whether CPU frequency scaling is actually 
>working on
>a system currently requires (afaics) source patches, as there 
>is no way to
>get the current state of a CPU. Even if this is intentional, 

What do you mean by current state of CPU? If cpufreq is enabled,
user should be able to retrieve statistics information by sysctl
path.

>this doesn't seem
>very helpful when considering to make this functionality available to
>customers: I'm certain quite a few will ask how they can tell 
>whether this
>is actually working.
>
>Now, apart from the simple job of adding a sub-hypercall to 
>retrieve the
>necessary bits, I'm wondering whether this wouldn't be just one more
>element that would much better be surfaced to the guest via the vCPU
>info structure (or, as that's size constrained, a new construct to make
>guest-read-only information available via a shared page). Other
>(potential) items to make available this same way would e.g. be guest-
>accessible last-exception-from/-to MSR values (as the values read would
>be meaningless if read through rdmsr).

Not quite understand. Cpufreq is physical cpu stuff, and do you aim
to expose physical information through vcpu specific shared page?
Then that would add fixed requirement on dom0 vcpu number to
physical cpus, which is intentially avoided in current design.

I guess I may get your intent wrong though.

Thanks,
Kevin

>
>So I'm basically considering to add a generic mechanism first, and then
>make cpufreq the first user of it. The question just is - use 
>a completely
>new (guest-ro) per-vCPU page, perhaps with chained descriptors rather
>than a fixed layout, or extend the vCPU info structure, but 
>e.g. require
>the guest to use VCPUOP_register_vcpu_info to gain access to all
>structure fields.
>
>Thanks, Jan
>

RE: cpufreq status information

[Jan Beulich]

>>> "Tian, Kevin" <kevin.tian@intel.com> 08.09.08 15:22 >>>
>>From: Jan Beulich
>>Sent: 2008年9月8日 21:12
>>
>>Trying to understand whether CPU frequency scaling is actually 
>>working on
>>a system currently requires (afaics) source patches, as there 
>>is no way to
>>get the current state of a CPU. Even if this is intentional, 
>
>What do you mean by current state of CPU? If cpufreq is enabled,
>user should be able to retrieve statistics information by sysctl
>path.

How? I can't see where the current frequency a CPU is running at
is being exposed.

>>this doesn't seem
>>very helpful when considering to make this functionality available to
>>customers: I'm certain quite a few will ask how they can tell 
>>whether this
>>is actually working.
>>
>>Now, apart from the simple job of adding a sub-hypercall to 
>>retrieve the
>>necessary bits, I'm wondering whether this wouldn't be just one more
>>element that would much better be surfaced to the guest via the vCPU
>>info structure (or, as that's size constrained, a new construct to make
>>guest-read-only information available via a shared page). Other
>>(potential) items to make available this same way would e.g. be guest-
>>accessible last-exception-from/-to MSR values (as the values read would
>>be meaningless if read through rdmsr).
>
>Not quite understand. Cpufreq is physical cpu stuff, and do you aim
>to expose physical information through vcpu specific shared page?
>Then that would add fixed requirement on dom0 vcpu number to
>physical cpus, which is intentially avoided in current design.

The intent is to expose the frequency of the pCPU the particular vCPU
is currently running on, perhaps only in Dom0.

Jan

Re: cpufreq status information

[Keir Fraser]

On 8/9/08 14:12, "Jan Beulich" <jbeulich@novell.com> wrote:

> Trying to understand whether CPU frequency scaling is actually working on
> a system currently requires (afaics) source patches, as there is no way to
> get the current state of a CPU. Even if this is intentional, this doesn't seem
> very helpful when considering to make this functionality available to
> customers: I'm certain quite a few will ask how they can tell whether this
> is actually working.

Well, they might notice that their battery lasts longer than an hour. :-)

> So I'm basically considering to add a generic mechanism first, and then
> make cpufreq the first user of it. The question just is - use a completely
> new (guest-ro) per-vCPU page, perhaps with chained descriptors rather
> than a fixed layout, or extend the vCPU info structure, but e.g. require
> the guest to use VCPUOP_register_vcpu_info to gain access to all
> structure fields.

I'm skeptical about throwing in more shared-memory structures between Xen
and guests anyway. It doesn't even seem a good fit here since this info is
not of great interest to most guests in my opinion. I presume most users'
curiosities will be satisfied by being able to poll CPU frequencies/voltages
from dom0 via a hypercall.

 -- Keir

RE: cpufreq status information

[Tian, Kevin]

>From: Jan Beulich [mailto:jbeulich@novell.com] 
>Sent: 2008年9月8日 21:33
>>>
>>>Trying to understand whether CPU frequency scaling is actually 
>>>working on
>>>a system currently requires (afaics) source patches, as there 
>>>is no way to
>>>get the current state of a CPU. Even if this is intentional, 
>>
>>What do you mean by current state of CPU? If cpufreq is enabled,
>>user should be able to retrieve statistics information by sysctl
>>path.
>
>How? I can't see where the current frequency a CPU is running at
>is being exposed.

common/sysctl.c: XEN_SYSCTL_get_pmstat

>
>>>this doesn't seem
>>>very helpful when considering to make this functionality available to
>>>customers: I'm certain quite a few will ask how they can tell 
>>>whether this
>>>is actually working.
>>>
>>>Now, apart from the simple job of adding a sub-hypercall to 
>>>retrieve the
>>>necessary bits, I'm wondering whether this wouldn't be just one more
>>>element that would much better be surfaced to the guest via the vCPU
>>>info structure (or, as that's size constrained, a new 
>construct to make
>>>guest-read-only information available via a shared page). Other
>>>(potential) items to make available this same way would e.g. 
>be guest-
>>>accessible last-exception-from/-to MSR values (as the values 
>read would
>>>be meaningless if read through rdmsr).
>>
>>Not quite understand. Cpufreq is physical cpu stuff, and do you aim
>>to expose physical information through vcpu specific shared page?
>>Then that would add fixed requirement on dom0 vcpu number to
>>physical cpus, which is intentially avoided in current design.
>
>The intent is to expose the frequency of the pCPU the particular vCPU
>is currently running on, perhaps only in Dom0.
>

Then you have to pin dom0 vCPU to corresponding pCPU, and have
dom0 with same number as pCPU. I don't think such limitation
necessary for just retrieving some pCPU information.

Or if you still enable vcpu migration, you have to fake virtual freq
change notification within dom0 at vcpu migration as pCPU may 
scale its own freq individually.

Thanks,
Kevin

Re: cpufreq status information

[Keir Fraser]

On 8/9/08 14:50, "Tian, Kevin" <kevin.tian@intel.com> wrote:

>> The intent is to expose the frequency of the pCPU the particular vCPU
>> is currently running on, perhaps only in Dom0.
>> 
> 
> Then you have to pin dom0 vCPU to corresponding pCPU, and have
> dom0 with same number as pCPU. I don't think such limitation
> necessary for just retrieving some pCPU information.
> 
> Or if you still enable vcpu migration, you have to fake virtual freq
> change notification within dom0 at vcpu migration as pCPU may
> scale its own freq individually.

Greater virtualisation of cpufreq info (making it available to arbitrary
guests, with pcpu->vcpu remapping) is just not very useful in my opinion.
It's one of those features that more hacker-ish users might think they want
to decorate their desktop.

After all, guest performance is at least as affected by CPU (and other
resources) contention from other guests as it is by power-management
governors in the hyeprvisor. Indeed, if the governors are doing their job
right then guest performance should not be considerably impacted by them
even in absolute terms.

 -- Keir

RE: cpufreq status information

[Tian, Kevin]

>From: Keir Fraser [mailto:keir.fraser@eu.citrix.com] 
>Sent: 2008年9月8日 22:01
>
>On 8/9/08 14:50, "Tian, Kevin" <kevin.tian@intel.com> wrote:
>
>>> The intent is to expose the frequency of the pCPU the 
>particular vCPU
>>> is currently running on, perhaps only in Dom0.
>>> 
>> 
>> Then you have to pin dom0 vCPU to corresponding pCPU, and have
>> dom0 with same number as pCPU. I don't think such limitation
>> necessary for just retrieving some pCPU information.
>> 
>> Or if you still enable vcpu migration, you have to fake virtual freq
>> change notification within dom0 at vcpu migration as pCPU may
>> scale its own freq individually.
>
>Greater virtualisation of cpufreq info (making it available to 
>arbitrary
>guests, with pcpu->vcpu remapping) is just not very useful in 
>my opinion.
>It's one of those features that more hacker-ish users might 
>think they want
>to decorate their desktop.

Agree.

>
>After all, guest performance is at least as affected by CPU (and other
>resources) contention from other guests as it is by power-management
>governors in the hyeprvisor. Indeed, if the governors are 
>doing their job
>right then guest performance should not be considerably 
>impacted by them
>even in absolute terms.

Exactly. :-)

Thanks,
Kevin

RE: cpufreq status information

[Jan Beulich]

>>> "Tian, Kevin" <kevin.tian@intel.com> 08.09.08 15:50 >>>
>>From: Jan Beulich [mailto:jbeulich@novell.com] 
>>How? I can't see where the current frequency a CPU is running at
>>is being exposed.
>
>common/sysctl.c: XEN_SYSCTL_get_pmstat

Ah, okay, I missed that. But - I can't use this from the kernel anyway,
and tools that track the frequency (i.e. KDE sysguard) would need to
be modified in order to make use of this. I'd really prefer /proc/cpuinfo
to correctly reflect this at least in Dom0. And even beyond that - I can't
seem to find any users of the APIs in tools/libxc/xc_pm.c, so these
really appear to be dead stubs.

>Then you have to pin dom0 vCPU to corresponding pCPU, and have
>dom0 with same number as pCPU. I don't think such limitation
>necessary for just retrieving some pCPU information.
>
>Or if you still enable vcpu migration, you have to fake virtual freq
>change notification within dom0 at vcpu migration as pCPU may 
>scale its own freq individually.

Why? All I care about is a snapshot value. It doesn't matter whether
it's stale by the time I get it, there's nothing going to be calculated
from it, apart from statistics.

Jan

Re: cpufreq status information

[Jan Beulich]

>>> Keir Fraser <keir.fraser@eu.citrix.com> 08.09.08 16:00 >>>
>After all, guest performance is at least as affected by CPU (and other
>resources) contention from other guests as it is by power-management
>governors in the hyeprvisor. Indeed, if the governors are doing their job
>right then guest performance should not be considerably impacted by them
>even in absolute terms.

Right you say 'if' - what if not? How do I tell, especially when I can't touch
the system and easily put a patched hypervisor and/or kernel on. If we
get a complaint from a customer that he thinks frequency scaling doesn't
do what it's expected to, we'll need to have a simple mechanism at hand
to determine what's going on with his box.

And even for development purposes I think a one-look proof that things
work as expected is quite useful (I'm doing the same for a few other basic
things - the interrupt rate being one of those that helped spot problems
that otherwise would have gone unnoticed for a much longer period of
time).

Jan

Re: cpufreq status information

[Keir Fraser]

On 8/9/08 15:30, "Jan Beulich" <jbeulich@novell.com> wrote:

>>>> Keir Fraser <keir.fraser@eu.citrix.com> 08.09.08 16:00 >>>
>> After all, guest performance is at least as affected by CPU (and other
>> resources) contention from other guests as it is by power-management
>> governors in the hyeprvisor. Indeed, if the governors are doing their job
>> right then guest performance should not be considerably impacted by them
>> even in absolute terms.
> 
> Right you say 'if' - what if not? How do I tell, especially when I can't touch
> the system and easily put a patched hypervisor and/or kernel on. If we
> get a complaint from a customer that he thinks frequency scaling doesn't
> do what it's expected to, we'll need to have a simple mechanism at hand
> to determine what's going on with his box.
> 
> And even for development purposes I think a one-look proof that things
> work as expected is quite useful (I'm doing the same for a few other basic
> things - the interrupt rate being one of those that helped spot problems
> that otherwise would have gone unnoticed for a much longer period of
> time).

Okay, I'll grant you that this is a useful scenario. For this purpose the
existing sysctl, plus a simple lashed-up dom0 userspace utility to dump the
statistics, would be perfectly sufficient.

Another possibility would be to generate xentrace events when CPU
frequencies change. Then, if you already buy into using xentrace to get
accurate logs about what's happening and when (which we do for our own
product development and maintenance), then the CPU frequency events would be
nicely integrated into that.

The advantage of the latter is that the frequency changes are interleaved
with other stuff you care about, like what's being scheduled, what's on run
queues etc. Since obviously frequency information all by itself is not so
interesting.

 -- Keir

RE: cpufreq status information

[Tian, Kevin]

>From: Jan Beulich [mailto:jbeulich@novell.com] 
>Sent: 2008年9月8日 22:24
>
>>>> "Tian, Kevin" <kevin.tian@intel.com> 08.09.08 15:50 >>>
>>>From: Jan Beulich [mailto:jbeulich@novell.com] 
>>>How? I can't see where the current frequency a CPU is running at
>>>is being exposed.
>>
>>common/sysctl.c: XEN_SYSCTL_get_pmstat
>
>Ah, okay, I missed that. But - I can't use this from the kernel anyway,
>and tools that track the frequency (i.e. KDE sysguard) would need to
>be modified in order to make use of this. I'd really prefer 
>/proc/cpuinfo
>to correctly reflect this at least in Dom0. And even beyond 
>that - I can't
>seem to find any users of the APIs in tools/libxc/xc_pm.c, so these
>really appear to be dead stubs.

They're not dead stubs, and we have internal tools as a modified 
xen PowerTop version and future we can expect more. This is not
a cpufreq only design choice, and similar thing goes for other stuff
like MCA and CPU hotplug: whether we want to reuse all existing
dom0 Linux interfaces (which however pushes requirement on a
vcpu placement within dom0 for each pcpu), or we use some side-
band hypercall with existing tools supporting Xen hypercall interface
to retrieve pcpu related information in a batch. By far we choose the
latter for cpufreq.

Actually even by keeping same interface, existing dom0 tools may 
have to be modified more or less for specific physical information 
as those tools haven't system knowledge. For example, PowerTop 
uses /proc/interrupts to derive break events for Cx residency, which 
however only reflects virtual interrupts within dom0.

Even for /proc/cpuinfo, then you finally require a mangled version
with mixed physical and virtual bits?

>
>>Then you have to pin dom0 vCPU to corresponding pCPU, and have
>>dom0 with same number as pCPU. I don't think such limitation
>>necessary for just retrieving some pCPU information.
>>
>>Or if you still enable vcpu migration, you have to fake virtual freq
>>change notification within dom0 at vcpu migration as pCPU may 
>>scale its own freq individually.
>
>Why? All I care about is a snapshot value. It doesn't matter whether
>it's stale by the time I get it, there's nothing going to be calculated
>from it, apart from statistics.
>

Then such physical info may conflict with other code snippet, if 
existing in guest, which simply gets freq info from some self 
calibrated logic. If we can't assure a consistent view about freq
within guest, what's the point then?

Thanks,
Kevin 

RE: cpufreq status information

[Tian, Kevin]

>From: Keir Fraser [mailto:keir.fraser@eu.citrix.com] 
>Sent: 2008年9月8日 22:35
>> 
>> Right you say 'if' - what if not? How do I tell, especially 
>when I can't touch
>> the system and easily put a patched hypervisor and/or kernel 
>on. If we
>> get a complaint from a customer that he thinks frequency 
>scaling doesn't
>> do what it's expected to, we'll need to have a simple 
>mechanism at hand
>> to determine what's going on with his box.
>> 
>> And even for development purposes I think a one-look proof 
>that things
>> work as expected is quite useful (I'm doing the same for a 
>few other basic
>> things - the interrupt rate being one of those that helped 
>spot problems
>> that otherwise would have gone unnoticed for a much longer period of
>> time).
>
>Okay, I'll grant you that this is a useful scenario. For this 
>purpose the
>existing sysctl, plus a simple lashed-up dom0 userspace 
>utility to dump the
>statistics, would be perfectly sufficient.
>
>Another possibility would be to generate xentrace events when CPU
>frequencies change. Then, if you already buy into using xentrace to get
>accurate logs about what's happening and when (which we do for our own
>product development and maintenance), then the CPU frequency 
>events would be
>nicely integrated into that.
>
>The advantage of the latter is that the frequency changes are 
>interleaved
>with other stuff you care about, like what's being scheduled, 
>what's on run
>queues etc. Since obviously frequency information all by 
>itself is not so
>interesting.
>

Yes, we already have many examples within dom0 to retrieve xen
specific information bypassing dom0. By the way, we have the plan
to add xentrace events for all processor PM stuff, including freq
change and also cpu idle states transition (like break causes, etc.)

Thanks,
Kevin

RE: cpufreq status information

[Tian, Kevin]

>From: Jan Beulich [mailto:jbeulich@novell.com] 
>Sent: 2008年9月8日 22:31
>
>>>> Keir Fraser <keir.fraser@eu.citrix.com> 08.09.08 16:00 >>>
>>After all, guest performance is at least as affected by CPU (and other
>>resources) contention from other guests as it is by power-management
>>governors in the hyeprvisor. Indeed, if the governors are 
>doing their job
>>right then guest performance should not be considerably 
>impacted by them
>>even in absolute terms.
>
>Right you say 'if' - what if not? How do I tell, especially 
>when I can't touch
>the system and easily put a patched hypervisor and/or kernel on. If we
>get a complaint from a customer that he thinks frequency 
>scaling doesn't
>do what it's expected to, we'll need to have a simple mechanism at hand
>to determine what's going on with his box.
>

Extend this complaint to the case, where a customer complains that he
thinks frequency scaling on (v)cpu 0 doesn't do what it's expected to, how
do you check what's going on if there's no fixed mapping between vcpu
and pcpu, and no tool talking to xen directly...

Thanks,
Kevin

Re: cpufreq status information

[Keir Fraser]

On 8/9/08 15:49, "Tian, Kevin" <kevin.tian@intel.com> wrote:

>> The advantage of the latter is that the frequency changes are
>> interleaved
>> with other stuff you care about, like what's being scheduled,
>> what's on run
>> queues etc. Since obviously frequency information all by
>> itself is not so
>> interesting.
>> 
> 
> Yes, we already have many examples within dom0 to retrieve xen
> specific information bypassing dom0. By the way, we have the plan
> to add xentrace events for all processor PM stuff, including freq
> change and also cpu idle states transition (like break causes, etc.)

It's good to know there are other users and developers of xentrace!

 -- Keir