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

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Message-ID: <20140131090230.GM5002@laptop.programming.kicks-ass.net>
Date:	Fri, 31 Jan 2014 10:02:30 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Preeti Murthy <preeti.lkml@...il.com>
Cc:	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>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

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 :-)
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