linux-kernel - [PATCH 1/1 v2] Honor state disabling in the cpuidle ladder governor

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Date:	Wed, 18 Jul 2012 16:09:45 +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>,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: [PATCH 1/1 v2] Honor state disabling in the cpuidle ladder governor
 - documented

On 07/18/2012 01:48 PM, Deepthi Dharwar wrote:
> On 07/18/2012 04:32 PM, Carsten Emde wrote:
>> 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.
>
> Agree, as per the ladder design.
>
>> 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?
>
> No, I dont think thats necessary, current code suffices it.
> The disable flag is knob we are giving to the user . So may be just
> document  the  intended use of disable flag working
> alongside design of ladder governor.
Here comes v2 with a related section added to the documentation.

	-Carsten.

View attachment "drivers-cpuidle-ladder-honor-disabling-with-doc.patch" of type "text/x-patch" (3858 bytes)