Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events

[Thomas Gleixner]

On Thu, 24 Feb 2011, Peter Zijlstra wrote:

> On Thu, 2011-02-24 at 21:47 +0100, Thomas Gleixner wrote:
> > On Thu, 24 Feb 2011, Peter Zijlstra wrote:
> > 
> > > On Thu, 2011-02-24 at 15:24 -0500, Nicolas Pitre wrote:
> > > 
> > > > Most SMP ARM processors are going to use it soon.  Powering down idle 
> > > > cores provides substantial power saving.
> > > 
> > > And why can't regular idle paths be used? CPU hotplug is a massively
> > > expensive operation.
> > 
> > To achieve the same result from idle, you need to exclude the core
> > from any unwanted wakeup. At the moment cpu unplug is the only way to
> > achieve that.
> 
> Right, everything is a nail because all we have is a hammer like.
> 
> > If you want to do the same from idle, then we need the isolation
> > features Frederic is working on for RT/HPC.
> > 
> > They allow us to isolate cores completely for totaly different
> > reasons, but it could be resused to provide full isolation of a core
> > in a very deep power state.
> 
> Exactly.
> 
> > That would solve the problem w/o going through kstompmachine
> 
> Right, kstopmachine is a large part of the problem, but cpu hotplug
> really does an insane amount of work if all you want is to idle the
> core.

As far as I can tell the isolation stuff covers lot's of the problems
including RCU, but there is still a way to go. And the good new is
that, when a cpu is idle it has not much state to preserve if its
properly isolated and the memory is not going away.

Thanks,

	tglx

 
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