Date: Wed, 30 Mar 2011 22:17:43 -0400 (EDT)
From: Len Brown <lenb@kernel.org>
To: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Cc: Trinabh Gupta <trinabh@linux.vnet.ibm.com>, arjan@linux.intel.com,
        Stephen Rothwell <sfr@canb.auug.org.au>, peterz@infradead.org,
        suresh.b.siddha@intel.com, benh@kernel.crashing.org, venki@google.com,
        ak@linux.intel.com, linux-kernel@vger.kernel.org,
        xen-devel@lists.xensource.com
Subject: cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver
 for apm)
In-reply-to: <20110325180156.GC19214@dirshya.in.ibm.com>
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> > > 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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