linux-kernel - Re: [patch 00/18] SMP: Boot and CPU hotplug refactoring

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Date:	Tue, 22 May 2012 10:23:47 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch 00/18] SMP: Boot and CPU hotplug refactoring - Part 1

On Mon, 21 May 2012 10:25:21 +0200 (CEST), Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 21 May 2012, Rusty Russell wrote:
> 
> > On Fri, 20 Apr 2012 16:18:04 +0200 (CEST), Thomas Gleixner <tglx@...utronix.de> wrote:
> > > On Fri, 20 Apr 2012, Srivatsa S. Bhat wrote:
> > > The whole notifier business needs a redesign as well, because we don't
> > > have a way to express proper dependencies, we add random notifier
> > > points and the teardown path is ass backwards. The whole thing wants
> > > to be a tree which can be walked in either direction and from any
> > > point. Right now we cut the trunk first and keep the single limb up
> > > with a helicopter and start dismantling it.
> > 
> > But there are two ways to do it.  One is to eliminate the need for
> > callbacks.  The other is to make a full dependency-based callback
> 
> What do you mean with fully eliminating the need for callbacks. Do you
> want to put the necessary bringup/shutdown function calls just in the
> core code so we get rid of the notifiers or do you have something
> different in mind ?

Eliminate is probably too hard, but with park/unpark I can see it
getting less common.  Maybe few enough that we can simplify.

> > Not sure whether calling notifiers in parallel is going to be a big win:
> > they'll end up fighting over the cpu we're taking down.  But I could be
> > wrong.
> 
> I'm not going to aim for parallel in the first place. That was just an
> idea and if we chose the right implementation then parallelism can be
> added later.

Parallel CPUs going offline/online is probably a bigger win.  Both for
suspend/resume and powersaving.

> > The original concept of stopping the machine for cpu hotplug and trying
> > not to effect any other kernel code has jumped the shark: I think we
> > need to look seriously at a complete rewrite where we don't use
> > stop_machine.
> 
> Yep. Working on it. :)

I thought you might be :)

I'd love to review once you've got something.

Cheers,
Rusty.
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