Re: [PATCH]cpuset: add new API to change cpuset top group's cpus

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2009-05-19 at 16:48 +0800, Shaohua Li wrote:
> On Tue, May 19, 2009 at 04:40:54PM +0800, Peter Zijlstra wrote:
> > On Tue, 2009-05-19 at 15:39 +0800, Shaohua Li wrote:
> > > ACPI 4.0 defines processor aggregator device. The device can notify OS to idle
> > > some CPUs to save power. This isn't to hot remove cpus, but just makes cpus
> > > idle.
> > > 
> > > This patch adds one API to change cpuset top group's cpus. If we want to
> > > make one cpu idle, simply remove the cpu from cpuset top group's cpu list,
> > > then all tasks will be migrate to other cpus, and other tasks will not be
> > > migrated to this cpu again. No functional changes.
> > > 
> > > We will use this API in new ACPI processor aggregator device driver later.
> > 
> > I don't think so. There really is a lot more to do than move processes
> > about.
> no processor running is good enough for us, we don't care about interrupts/softirq/
> timers so far.

Well, I don't care for this interface.

> > Furthermore, I object to being able to remove online cpus from the top
> > cpuset, that just doesn't make sense.
> > 
> > I'd suggest using hotplug for this.

> cpu hotplug involves too much things, and we are afraid it's not reliable.

Then make it more reliable instead of providing ugly ass shit like this.

> Besides, a hot removed cpu will do a dead loop halt, which isn't power saving
> efficient. To make hot removed cpu enters deep C-state is in whish list for a
> long time, but still not available. The acpi_processor_idle is a module, and
> cpuidle governor potentially can't handle offline cpu.

Then fix that hot-unplug idle loop. I agree that the hlt thing is silly,
and I've no idea why its still there, seems like a much better candidate
for your efforts than this.