linux-kernel - Re: [PATCH 1/1] Honor state disabling in the cpuidle ladder governor

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Message-ID: <500697A9.6070101@osadl.org>
Date:	Wed, 18 Jul 2012 13:02:01 +0200
From:	Carsten Emde <C.Emde@...dl.org>
To:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
CC:	Len Brown <len.brown@...el.com>, Kevin Hilman <khilman@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] Honor state disabling in the cpuidle ladder governor

On 07/18/2012 08:36 AM, Deepthi Dharwar wrote:
> On 07/18/2012 12:29 AM, Carsten Emde wrote:
>
>> There are two cpuidle governors ladder and menu. While the ladder
>> governor is always available, if CONFIG_CPU_IDLE is selected, the
>> menu governor additionally requires CONFIG_NO_HZ.
>>
>> A particular C state can be disabled by writing to the sysfs file
>> /sys/devices/system/cpu/cpuN/cpuidle/stateN/disable, but this mechanism
>> is only implemented in the menu governor. Thus, in a system where
>> CONFIG_NO_HZ is not selected, the ladder governor becomes default and
>> always will walk through all sleep states - irrespective of whether the
>> C state was disabled via sysfs or not. The only way to select a specific
>> C state was to write the related latency to /dev/cpu_dma_latency and
>> keep the file open as long as this setting was required - not very
>> practical and not suitable for setting a single core in an SMP system.
>>
>> With this patch, the ladder governor only will promote to the next
>> C state, if it has not been disabled, and it will demote, if the
>> current C state was disabled.
>
> Yes, I agree that currently that disabling a particular C-state
> is not reflected in working of ladder governor. This patch is needed
> to fix it on ladder too.
>
> Also wanted to clarify on the intended implementation here,
> if there are say 5 C-states on a system, disabling 2nd
> state would also end by disabling all the remaining 3 deeper states too
> as ladder governor enters the lightest state first, and will only move
> on to the next deeper state if a idle period was long enough as
> per the implementation.
> If one is disabling only the deepest state, then it would
> work as intended.
Yes, the patch does not make the setting of the sysfs variable
"disable" coherent, i.e. if one is disabling a light state, then all
deeper states are disabled as well, but the "disable" variable does not
reflect it. Likewise, if one enables a deep state but a lighter state
still is disabled, then this has no effect.

I could implement a sanitize mechanism of the ladder governor that
takes care the "disable" variables of all deeper states are set to 1,
if a state is disabled, and those of all lighter states are set to 0,
if a state is enabled. Do you wish me to do that?

	-Carsten.
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