linux-kernel - Re: [RFC PATCH V1 0/2] cpuidle: global registration of idle states with per-cpu statistics

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Message-ID: <20110325171533.GA19214@dirshya.in.ibm.com>
Date:	Fri, 25 Mar 2011 22:45:33 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Len Brown <lenb@...nel.org>
Cc:	Trinabh Gupta <trinabh@...ux.vnet.ibm.com>, arjan@...ux.intel.com,
	peterz@...radead.org, suresh.b.siddha@...el.com,
	benh@...nel.crashing.org, venki@...gle.com, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V1 0/2] cpuidle: global registration of idle states
 with per-cpu statistics

* Len Brown <lenb@...nel.org> [2011-03-25 03:28:59]:

> On Tue, 22 Mar 2011, Trinabh Gupta wrote:
> 
> > This patch series is an early RFC to discuss the feasibility of
> > avoiding registering of idle states from each cpu.
> > 
> > The core change is to split the cpuidle_device structure into parts
> > that can be global and parts that has to remain per-cpu.  The per-cpu
> > pieces are mostly generic statistics that can be independent of
> > current running driver.
> > 
> > Motivation:
> > * Simplify the cpuidle subsystem framework and have
> >   registration/unregistration done by single cpu.
> > 
> > * Minimise the data structure that needs to be maintained for multiple
> >   cpuidle drivers
> > 
> > * Reference: https://lkml.org/lkml/2011/2/10/37
> > 
> > Advantages:
> > * Make the cpuidle framework simple for most use cases where C-States
> >   are symmetric.  In case there are asymmetric C-States detected,
> >   fallback mechanism should be incorporated to maintain the system
> >   functional
> >   https://lkml.org/lkml/2011/2/10/257
> >   https://lkml.org/lkml/2011/2/10/37
> > 
> > * Non x86 archs that does not have asymmetric C-States like POWER, may
> >   not need the fallback mechanism and hence the framework will be
> >   simple for most use cases.
> > 
> > Disadvantages:
> > * Asymmetric C-States are part of x86 ACPI specification.  Incorrect
> >   handling may functionally affect the system
> 
> I think this is a non-issue.

Hi Len,

Thanks for the confirmation.  This gives us the right direction to
proceed with the cleanup.

> > * Incorporating per-cpu masks for each state to allow/dis-allow global
> >   states on subset of CPUs may result in an implementation that is
> >   not better than current solution of having per-cpu states.
> 
> I don't think this is needed.
> 
> > This patch series applies on top of the pm_idle cleanup patch
> > https://lkml.org/lkml/2011/3/22/150 (cpuidle: Cleanup pm_idle and 
> > include driver/cpuidle.c in-kernel)
> > 
> > This patch series is tested on x86 Nehalem system with multiple ACPI
> > C-States.
> > 
> > This patch series has limitations of not handling multiple driver
> > registration and switching between drivers on all CPUs mainly due to
> > incomplete handling of per-cpu enable/disable and driver_data.
> 
> I think this series is more important than the feature of multiple
> driver/system support, and thus should come first.

Removing pm_idle() gets us to the situation where we have to support
multiple registration to allow default_idle driver to register and do
mwait.  Your cleanup patch to x86 idle will make the transition much
simple.  Lets us have removal of pm_idle() and removal of per-cpu
registration in the same patch series as the next step.

--Vaidy

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