linux-kernel - Re: [RFC PATCH 0/6] CPU hotplug: Reverse invocation of notifiers during CPU hotplug

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Date:	Wed, 1 Aug 2012 09:10:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Rusty Russell <rusty@...tcorp.com.au>
cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>, mingo@...nel.org,
	peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
	namhyung@...nel.org, tj@...nel.org, rjw@...k.pl,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] CPU hotplug: Reverse invocation of notifiers
 during CPU hotplug

On Fri, 27 Jul 2012, Rusty Russell wrote:

> On Wed, 25 Jul 2012 18:30:41 +0200 (CEST), Thomas Gleixner <tglx@...utronix.de> wrote:
> > The problem with the current notifiers is, that we only have ordering
> > for a few specific callbacks, but we don't have the faintest idea in
> > which order all other random stuff is brought up and torn down.
> > 
> > So I started experimenting with the following:
> > 
> > struct hotplug_event {
> >        int (*bring_up)(unsigned int cpu);
> >        int (*tear_down)(unsigned int cpu);
> > };
> > 
> > enum hotplug_events {
> >      CPU_HOTPLUG_START,
> >      CPU_HOTPLUG_CREATE_THREADS,
> >      CPU_HOTPLUG_INIT_TIMERS,
> >      ...
> >      CPU_HOTPLUG_KICK_CPU,
> >      ...
> >      CPU_HOTPLUG_START_THREADS,
> >      ...
> >      CPU_HOTPLUG_SET_ONLINE,
> >      ...
> >      CPU_HOTPLUG_MAX_EVENTS,
> > };
> 
> This looks awfully like hardcoded a list of calls, without the
> readability :)

I'd love to make it a list of calls, though we have module users of
cpu hotplug which makes a list of calls a tad hard.
 
> OK, I finally got off my ass and looked at the different users of cpu
> hotplug.  Some are just doing crazy stuff, but most seem to fall into
> two types:
> 
> 1) Hardware-style cpu callbacks (CPU_UP_PREPARE & CPU_DEAD)
> 2) Live cpu callbacks (CPU_ONLINE & CPU_DOWN_PREPARE)
> 
> I think this is what Srivatsa was referring to with "physical" and
> "logical" parts.  Maybe we should explicitly split them, with the idea
> that we'd automatically call the other one if we hit an error.
> 
> struct cpu_hotplug_physical {
>        int (*coming)(unsigned int cpu);
>        void (*gone)(unsigned int cpu);
> };
> 
> struct cpu_hotplug_logical {
>        void (*arrived)(unsigned int cpu);
>        int (*going)(unsigned int cpu);
> };
> 
> Several of the live cpu callbacks seem racy to me, since we could be
> running userspace tasks before CPU_ONLINE.  It'd be nice to fix this,
> too.

Yes, I know. I wan't to change that as well. The trick here is that we
can schedule per cpu stuff on a not fully online cpu and only when all
the callbacks have been executed allow user space tasks to be
scheduled on that newly online cpu.
 
Thanks,

	tglx
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