linux-kernel - Re: [v7 PATCH 0/7]: cpuidle/x86/POWER: Cleanup idle power management code in x86, cleanup drivers/cpuidle/cpuidle.c and introduce cpuidle to POWER.

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Date:	Wed, 7 Oct 2009 16:56:48 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan@...radead.org>
Cc:	arun@...ux.vnet.ibm.com, Joel Schopp <jschopp@...tin.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-arch@...r.kernel.org
Subject: Re: [v7 PATCH 0/7]: cpuidle/x86/POWER: Cleanup idle power
 management code in x86, cleanup drivers/cpuidle/cpuidle.c and introduce
 cpuidle to POWER.

* Peter Zijlstra <a.p.zijlstra@...llo.nl> [2009-10-06 20:04:39]:

> On Tue, 2009-10-06 at 22:05 +0530, Arun R Bharadwaj wrote:
> 
> > Also, the per-cpu nature of registration/unregistration of cpuidle
> > has been maintained as ACPI needs this.
> 
> Right, so can't we ditch that and have acpi default to the lowest common
> C-state and warn when various cpus report different C-states?

Hi Peter,

As Arjan mentioned previously, the per-cpu registration has to stay
for x86 for now due to legacy ACPI compatibility.  Breaking that may
break lot of existing users and we do not have a clean fallback
method.  

As far as powerpc is concerned, we can work with a single global
registration.  However we would like to have the same interface across
different archs.

With the new re-factoring (v7), Arun has killed most of the list
traversal and linking between various cpu's cpuidle_driver structures.
Now we have a per-cpu stack of registered devices and we lookup the
structs using online cpumasks.  The cpuidle_driver structure has list
of idle routing pointers (struct cpuidle_state) and rest of it is
statistics that needs to be maintained at a per-cpu level anyway.  All
that is duplicated here is the array of idle routines (struct
cpuidle_state) on each cpu.

The objective of the refactoring is to have a single common idle
routine management framework (remove pm_idle) and we have it done
through cpuidle registration framework.  We can incrementally remove
the per-cpu registration later easily by splitting the cpuidle_driver
structure.

--Vaidy

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