linux-kernel - Re: [PATCH 1/1 v3] Honor state disabling in the cpuidle ladder governor

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Message-ID: <5007EC10.1060508@linux.vnet.ibm.com>
Date:	Thu, 19 Jul 2012 16:44:24 +0530
From:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
To:	Carsten Emde <C.Emde@...dl.org>
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: Re: [PATCH 1/1 v3] Honor state disabling in the cpuidle ladder governor
 - with sanitizer

On 07/18/2012 08:08 PM, Carsten Emde wrote:

> 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.
> It's not necessary - but maybe better. Here comes v3 with a sanitizer.
> Is this too ugly?
> 


The v2, with the documentation in place seems sufficient.
Yup, this adds unnecessary fields which are not much use
coz the same can be achieved with just disable flag check.

Also, any reason why the patch is being sent as an attachment ?
Sending patches as an attachment is not a recommended practice.

Cheers,
Deepthi


>     -Carsten.
> 


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