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

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Date:	Fri, 25 Mar 2011 03:28:59 -0400 (EDT)
From:	Len Brown <lenb@...nel.org>
To:	Trinabh Gupta <trinabh@...ux.vnet.ibm.com>
Cc:	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

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.

> * 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.

thanks,
Len Brown, Intel Open Source Technology Center

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