Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events
From: Paul E. McKenney
Date: Thu Feb 24 2011 - 16:11:28 EST
On Thu, Feb 24, 2011 at 09:58:32PM +0100, 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.
> > 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
You do indeed need to know that the CPU will be powered off for quite
some time to be worth the extra work. And a number of embedded devices
do have this level of foreknowledge -- for example, they might use the
second CPU only when the user is doing some computationally intensive
task, so that the device would normally be using only a single CPU.
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