linux-kernel - Re: CPU isolation question again

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Message-ID: <20120704143635.GE28294@somewhere>
Date:	Wed, 4 Jul 2012 16:36:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Luming Yu <luming.yu@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, gilad@...yossef.com,
	Len Brown <lenb@...nel.org>, shli@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: CPU isolation question again

On Wed, Jul 04, 2012 at 10:12:43PM +0800, Luming Yu wrote:
> On Wed, Jul 4, 2012 at 9:25 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > On Wed, Jul 04, 2012 at 08:42:29PM +0800, Luming Yu wrote:
> >> On Tue, Jul 3, 2012 at 7:28 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> >> > On Wed, Jun 27, 2012 at 09:22:09PM +0800, Luming Yu wrote:
> >> >> Hi there,
> >> >>
> >> >> I noticed some discussion about CPU isolation which points me to the
> >> >> patch set (https://lkml.org/lkml/2011/8/15/245). I'm currently
> >> >> preparing a RFC-patch-set to automatically pick up a few suitable CPUs
> >> >> to isolate then kick them out of service for a while. We need to
> >> >> balance between  thermal & power management And overall system
> >> >> performance during this operation as much as possible. So
> >> >> software-cpu-online-offline interface could not be a good option to
> >> >> me. But to make sure I'm not blindly running on a dead-end path, I'd
> >> >> check with experts here to ensure it makes some sense to isolate CPUs
> >> >> to this level, and the idea also makes some sense, and the most
> >> >> important is it's not implemented yet.
> >> >
> >> > I don't understand what you are trying to do and how exactly. How do you
> >> > plan to do this isolation and how do you want to balance between thermal
> >> > and power?
> >>
> >> My question could be wrong as the question arose several weeks ago
> >> when I came across
> >>  drivers/acpi/acpi_paid.c which looks like a real user who need to
> >> request system automatically
> >> pick up a few CPU to get them isolated and deactivated. Later on, I
> >> noticed tglx's cpu hot plug re-work.
> >> I realized we could reuse the interface to do isolation and deactivation work.
> >>
> >> Of cause, to pick up which ones to isolate and deactivate is another problem.
> >>
> >> cc'ed the author and ACPI maintainer of the driver as well as tglx.
> >
> > May be I'm confused because we both have our own definition of isolation.
> > I'm not sure what kind of CPU isolation you're looking for.
> 
> At first, it needs not avaiable to scheduler.  Then, it needs in
> deepest power saving mode.
> At last, it needs available to scheduler again on demand.
> Sounds very like a typical soft offline cpu, but needs to be low light weight.

I see. So indeed the latest developments made in CPU hotplug could make it a solution
for you.
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