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xen-devel
[Xen-devel] Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver for apm
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To: |
Len Brown <lenb@xxxxxxxxxx>, arjan@xxxxxxxxxxxxxxx |
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Subject: |
[Xen-devel] Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver for apm |
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From: |
Trinabh Gupta <trinabh@xxxxxxxxxxxxxxxxxx> |
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Date: |
Thu, 24 Mar 2011 19:58:29 +0530 |
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Cc: |
Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>, ak@xxxxxxxxxxxxxxx, suresh.b.siddha@xxxxxxxxx, venki@xxxxxxxxxx, peterz@xxxxxxxxxxxxx, benh@xxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx |
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On 03/24/2011 02:02 AM, Len Brown wrote:
Also wondering why you would ever have a different idle routine on
different cpus?
Yes, this is an ongoing debate. Apparently it is a possibility
because of ACPI bugs. CPU's can have asymmetric C-states
and overall different idle routines on different cpus. Please
refer to http://lkml.org/lkml/2009/9/24/132 and
https://lkml.org/lkml/2011/2/10/37 for a discussion around this.
Althought the ACPI specification allows the BIOS to tell the OS
about different C-states per-processor, I know of zero system
in the field and zero systems in development that require that
capability. That isn't a guarantee that capability will never
be used, but I'm not holding my breath.
If there are systems with broken tables that make them
appear asymetric, then we should have a workaround that handles
that case, rather than complicating the normal code for
the broken case.
So I recommend deleting the extra per-cpu registration stuff
unless there is some other architecture that requires it
and can't hadle the asymmetry in another way.
Yes, lets go forward with removal of per-cpu registration
and handle rare case of asymmetry in some other may.
Using intersection or union of C-states for each cpu may
be a solution. Using intersection or lowest common C-state
has the corner case that we could have packages/cores
supporting a new lower C-state in case of thermal limit and
they would want OS to go to this state. Using intersection
or lowest common C-state may prevent this.
Another option is to use union of C-states;
but I am not sure what happens if a CPU uses a state that
is not reported for it???
Maybe there is some other way to handle asymmetry ??
I have posted a patch series that does global registration
i.e same idle routines for each cpu. Please check
http://lkml.org/lkml/2011/3/22/161 . That series applies on
top of this series. Global registration significantly
simplifies the design, but still we are not sure about the
direction to take.
I'll review that.
Thanks; please review especially the data structure changes
https://lkml.org/lkml/2011/3/22/162
-Trinabh
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