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

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Message-Id: <1242729538.26820.497.camel@twins>
Date:	Tue, 19 May 2009 12:38:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"menage@...gle.com" <menage@...gle.com>
Subject: Re: [PATCH]cpuset: add new API to change cpuset top group's cpus

On Tue, 2009-05-19 at 10:56 +0200, Peter Zijlstra wrote:
> 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.

OK, so perhaps I should have use different words. But the point is, we
don't need a new interface to force a cpu idle. Hotplug does that.

Furthermore, we should not want anything outside of that, either the cpu
is there available for work, or its not -- halfway measures don't make
sense.

Furthermore, we already have power aware scheduling which tries to
aggregate idle time on cpu/core/packages so as to maximize the idle time
power savings. Use it there.

> > 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.
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