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

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Date:	Fri, 20 Apr 2012 16:18:04 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
cc:	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"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 Fri, 20 Apr 2012, Srivatsa S. Bhat wrote:
> > This first part moves the idle thread management for non-boot cpus
> > into the core. fork_idle() is called in a workqueue as it is
> > implemented in a few architectures already. This is necessary when not
> > all cpus are brought up by the early boot code as otherwise we would
> > take a ref on the user task VM of the thread which brings the cpu up
> > via the sysfs interface.
> > 
> 
> 
> Do you have a git tree where you have made these patches available?
> That would be pretty useful, so that we can build on whatever you have

Not yet, but I'll stick that into a tip/ branch.

> already done.. Myself and Nikunj had some initial design/ideas on reducing
> the duplication in architecture code, related to managing the setting
> of the cpu in the online mask, sending out CPU_STARTING notifiers etc
> from generic code..

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.

Flat notifiers are not working for this as they do not allow a tree
structure and prevent us to do things in parallel.

That really needs to be completely reworked. There is also a lot of
stuff which wants to be moved into the starting/dying CPU
context. Right now we kinda do that by trampling on the CPU with a
high prio stomper thread, but that's really just a bandaid and steady
cause of trouble.

If you look at facilities which use kthreads, then there is lots other
setup which does not need a notifier at all, as it can be done in the
context of the thread when we have a way to start/park those threads
at the right time in the up/down process.

I've already done a prototype for kthread park/unpark and converted
softirq over to use it. That makes the complete softirq notifier go
away and let the core code handle the thread creation / start / park /
unpark. It's pretty hacky right now, but I'm going to push on this
next, once I have a better idea how to express the dependency tree.

Thanks,

	tglx
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