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

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Date:	Tue, 11 Nov 2014 11:20:58 +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 11:21:19PM +0100, Daniel Lezcano wrote:
> 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.

The ignoring it is _wrong_. Make that go away and you'll get rid of most
of the STATE_START crap.

The governors are the place where we combine the QoS constraints with
idle predictors and pick an idle state, polling is a valid state to
pick, and given QoS constraints it might be the only state to pick.

> 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 ?

Because the latter is actually arch specific, whereas the idle polling
thing is not. You can _always_ poll idle, its generic, its valid, and
its guaranteed the most responsive method.

The arch drivers get to add arch specific idle states; if a x86 cpuidle
driver wants to add hlt they can.


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