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

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Message-ID: <20141110194820.GD10501@worktop.programming.kicks-ass.net>
Date:	Mon, 10 Nov 2014 20:48:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.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 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.

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.

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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