linux-kernel - RE: [patch 1/2] x86: Add pm_play_dead funcptr to power-efficiently offline CPUs

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Date:	Sat, 23 May 2009 08:07:55 -0700
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>
Subject: RE: [patch 1/2] x86: Add pm_play_dead funcptr to power-efficiently
 offline CPUs

 

>-----Original Message-----
>From: Peter Zijlstra [mailto:a.p.zijlstra@...llo.nl] 
>Sent: Saturday, May 23, 2009 3:45 AM
>To: Pallipadi, Venkatesh
>Cc: mingo@...e.hu; tglx@...utronix.de; hpa@...or.com; 
>linux-kernel@...r.kernel.org; lenb@...nel.org; Li, Shaohua; 
>svaidy@...ux.vnet.ibm.com
>Subject: Re: [patch 1/2] x86: Add pm_play_dead funcptr to 
>power-efficiently offline CPUs
>
>On Fri, 2009-05-22 at 16:19 -0700, venkatesh.pallipadi@...el.com wrote:
>> plain text document attachment
>> (0001-x86-Add-pm_play_dead-funcptr-to-power-efficiently-o.patch)
>> Add a funcptr pm_play_dead (similar to pm_idle) that can take
>> the offline CPUs to the most power efficient idle state.
>> 
>> This patch just adds the func pointer. The pointer will get 
>initialized
>> by patch that follows.
>
>Since the pm_idle function pointer has given us so much grief, I don't
>think its wise to repeat that particular disaster.
>
>I'd much rather see a framework where idle functions can be registered,
>and selected from based on criteria such as wakeup latency as provided
>by the pm_qos stuff, and power saving.
>
>This framework should be shared between hotplug-idle and the regular
>idle routines. Hotplug would of course not care about things 
>like wakeup
>latency and might therefore pick another idle routine.
>

Yes. Thought about that approach. There are issues with that approach however.
- It would be ugly as we have to add logic in various low level idle handlers we have today mwait_idle, default_idle, acpi_mwait_idle, acpi_ioport_idle to make them aware of offline idle case and disable interrupt and not enable interrupt while going idle and not to wake up on interrupt.
- There are limitations on what we can and cannot do when offline. We have had issues with calling acpi methods and/or IO port accesses, during suspend before, so CPU going on suspend only uses C1. Specifically, with the above approach we will have:
1) CPU 1 going offline and starts using regular ACPI IO port C-states.
2) CPU 0 starts suspend and after some points it is unsafe to access ACPI io port accesses, which can potentially result in crash/hangs.

So, I would like to keep ACPI out of play_dead. And keeping idle and play_dead separate seemed to be cleaner way to do it.

Thanks,
Venki

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