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

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Date:	Sat, 1 Feb 2014 10:31:47 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	"Brown, Len" <len.brown@...el.com>
cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Preeti Murthy <preeti.lkml@...il.com>,
	"mingo@...hat.com" <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

On Sat, 1 Feb 2014, Brown, Len wrote:

> > Right now (on ARM at least but I imagine this is pretty universal), the
> > biggest impact on information accuracy for a CPU depends on what the
> > other CPUs are doing.  The most obvious example is cluster power down.
> > For a cluster to be powered down, all the CPUs sharing this cluster must
> > also be powered down.  And all those CPUs must have agreed to a possible
> > cluster power down in advance as well.  But it is not because an idle
> > CPU has agreed to the extra latency imposed by a cluster power down that
> > the cluster has actually powered down since another CPU in that cluster
> > might still be running, in which case the recorded latency information
> > for that idle CPU would be higher than it would be in practice at that
> > moment.
> 
> That will not work.

What will not work?

> When a CPU goes idle, it uses the CURRENT criteria for entering that state.
> If the criteria change after it has entered the state, are you going
> to wake it up so it can re-evaluate?  No.

That's not what I'm saying at all.

> That is why the state must describe the worst case latency
> that CPU may see when waking from the state on THAT entry.

No disagreement there.  Isn't that what I'm saying?

> That is why we use the package C-state numbers to describe
> core C-states on IA.

And your point is?


Nicolas
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