linux-kernel - Re: [PATCH V3 2/6] sched: idle: cpuidle: Check the latency req before idle

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Date:	Mon, 10 Nov 2014 23:21:19 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	rjw@...ysocki.net, preeti@...ux.vnet.ibm.com,
	nicolas.pitre@...aro.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	patches@...aro.org, lenb@...nel.org
Subject: Re: [PATCH V3 2/6] sched: idle: cpuidle: Check the latency req before
 idle

On 11/10/2014 08:48 PM, Peter Zijlstra wrote:
> On Mon, Nov 10, 2014 at 06:19:02PM +0100, Daniel Lezcano wrote:
>>> I really don't get why the governors should know about this though, its
>>> just another state, they should iterate all states and pick the best,
>>> given the power usage this state should really never be eligible unless
>>> we're QoS forced or whatnot.
>>
>> The governors just don't use the poll state at all, except for a couple of
>> cases in menu.c defined above in the previous email. What is the rational of
>> adding a state in the cpuidle driver and do everything we can to avoid using
>> it ? From my POV, the poll state is a special state, we should remove from
>> the driver's idle states like the arch_cpu_idle() is a specific idle state
>> only used in idle.c (but which may overlap with an idle state in different
>> archs eg. cpu_do_idle() and the 0th idle state).
>
> So I disagree, I think poll-idle is an idle mode just like all the
> others. It should be an available state to the governor and it should
> treat it like any other.

The governors are just ignoring it, except for a small timer 
optimization in menu.c (and I am still not convinced it is worth to have 
it). I don't see the point to add a state we don't want to use.

Eg. on my server it was called 2 times over 1313856 times.

> I don't tihnk the whole ARCH_HAS_CPU_RELAX thing makes any kind of
> sense, _every_ arch has some definition of it, the generic polling loop
> is always a valid idle implementation.
>
> What we can do is always populate the idle state table with it before
> calling the regular drivers.

I am not sure to understand. You want to add the poll idle loop in all 
the drivers ?

What about "safe_halt()" ? (arch_cpu_idle() for x86). It is also an idle 
state. Why not add it in the idle state table also ?

> If the arch drivers have a 'better' latency_req==0 idle routine -- note
> my argument on the ppc issue, I think its wrong -- it can replace the
> existing one.
>
> We should further remove all the special casing in the governors, its
> always a valid state, but it should hardly ever be the most desirable
> state.
>
> I think the whole arch specific idle loop is a mistake, we already have
> an (arch) interface into the idle routines, we don't need yet another.



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