Messages in this thread | | | Date | Sat, 1 Feb 2014 10:31:47 -0500 (EST) | From | Nicolas Pitre <> | Subject | RE: [RFC PATCH 3/3] idle: store the idle state index in the struct rq |
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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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