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xen-devel
[Xen-devel] Re: cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: c
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To: |
Len Brown <lenb@xxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: cpuidle asymmetry (was 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: |
Fri, 01 Apr 2011 12:32:01 +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, Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx>, arjan@xxxxxxxxxxxxxxx |
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Fri, 01 Apr 2011 00:03:17 -0700 |
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On 03/31/2011 07:47 AM, Len Brown wrote:
Maybe there is some other way to handle asymmetry ??
I mis-spoke on asymmetry.
Moorestown is already an example of an asymmetric system,
since its deepest c-state is available on cpu0, but not on cpu1.
So it needs different tables for each cpu.
I think what would work is a default c-state table for the system,
and the ability of a per-cpu override table. I think that would
gracefully handle the case of many identical cpus, and also systems
with different tables per cpu.
Hi Len,
What would happen if a cpu enters a state which wasn't
reported for it? I am wondering if an approach like union
of states of each would work.
Can we handle asymmetry through checking and demotion inside the
routine itself; just like you are proposing as dev->prepare
alternative? But I guess this may not be efficient if this
happens often.
I am not sure if having a per-cpu override would be very tidy
(ideas ?); and much better than per-cpu stuff. So just want to check
what would be the best way forward?
The same goes for write-access to the tables.
In the typical case, a single table can be shared for the entire system
and nobody will be writing to it. However, with the governor changes
to call dev->prepare and sift through all the states to find the
legal one with the lowest power_usage... There is software today
out of tree that updates that power_usage entry from prepare().
As I mentioned, I'm not fond of that mechanism - it looks racey
to me. I'd rather see the capability of a drivers idle handler
to demote to another handler in the driver and for the accounting
to not get messed up when that happens. I think the way to do that
is to let the driver do the accounting rather than doing it in
the cpuidle caller.
I agree with this; we should move update of statistics inside the
driver routines (enter routines). They already take device/stats
structure as parameter.
Thanks,
-Trinabh
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