linux-kernel - Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

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Date:	Fri, 31 Jan 2014 15:09:49 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Len Brown <len.brown@...el.com>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	nicolas.pitre@...aro.org, mingo@...hat.com,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC PATCH 3/3] idle: store the idle state index in the struct
 rq

Hi Peter,

On 01/31/2014 02:32 PM, Peter Zijlstra wrote:
> On Fri, Jan 31, 2014 at 02:15:47PM +0530, Preeti Murthy wrote:
>>>
>>> If the driver does its own random mapping that will break the governor
>>> logic. So yes, the states are ordered, the higher the index is, the more you
>>> save power and the higher the exit latency is.
>>
>> The above point holds true for only the ladder governor which sees the idle
>> states indexed in the increasing order of target_residency/exit_latency.
>>
>> However this is not true as far as I can see in the menu governor. It
>> acknowledges the dynamic ordering of idle states as can be seen in the
>> menu_select() function in the menu governor, where the idle state for the
>> CPU gets chosen.  You will notice that, even if it is found that the predicted
>> idle time of the CPU is smaller than the target residency of an idle state,
>> the governor continues to search for suitable idle states in the higher indexed
>> states although it should have halted if the idle states' were ordered according
>> to their target residency.. The same holds for exit_latency.
>>
>> Hence I think this patch would make sense only with additional information
>> like exit_latency or target_residency is present for the scheduler. The idle
>> state index alone will not be sufficient.
> 
> Alternatively, can we enforce sanity on the cpuidle infrastructure to
> make the index naturally ordered? If not, please explain why :-)

The commit id 71abbbf856a0e70 says that there are SOCs which could have
their target_residency and exit_latency values change at runtime. This
commit thus removed the ordering of the idle states according to their
target_residency/exit_latency. Adding Len and Arjan to the CC.

Thanks

Regards
Preeti U Murthy

> 

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