linux-kernel - cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver for apm)

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Date:	Wed, 30 Mar 2011 22:17:43 -0400 (EDT)
From:	Len Brown <lenb@...nel.org>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc:	Trinabh Gupta <trinabh@...ux.vnet.ibm.com>, arjan@...ux.intel.com,
	Stephen Rothwell <sfr@...b.auug.org.au>, peterz@...radead.org,
	suresh.b.siddha@...el.com, benh@...nel.crashing.org,
	venki@...gle.com, ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com
Subject: cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver
 for apm)

> > > 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.

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.

cheers,
-Len Brown, Intel Open Source Technology Center

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