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

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Date:	Thu, 26 Jul 2012 16:52:56 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Alan Stern <stern@...land.harvard.edu>, mingo@...nel.org,
	peterz@...radead.org, rusty@...tcorp.com.au,
	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 07/25/2012 10:00 PM, Thomas Gleixner wrote:
> On Wed, 25 Jul 2012, Srivatsa S. Bhat wrote:
>> On 07/25/2012 08:27 PM, Alan Stern wrote:
>> One of the other ideas to improve the hotplug notifier stuff that came up during some
>> of the discussions was to implement explicit dependency tracking between the notifiers
>> and perhaps get rid of the priority numbers that are currently being used to provide
>> some sort of ordering between the callbacks. Links to some of the related discussions
>> are provided below.
> 
> The current code which brings up/down a CPU (mostly architecture
> specific) code is comnpletely asymetric.
> 
> We really want a fully symetric state machine here, which also gives
> us the proper invocation points for the other subsystems callbacks.
> 
> While I thought about having a full dependency tracking system, I'm
> quite convinced by now, that hotplug is a rather linear sequence which
> does not provide much room for paralell setup/teardown.
> 
> At least we should start with a simple linear chain.
> 
> 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,
> };
> 
> Now I have two arrays:
> 
> struct hotplug_event hotplug_events_bp[CPU_HOTPLUG_MAX_EVENTS];
> struct hotplug_event hotplug_events_ap[CPU_HOTPLUG_MAX_EVENTS];
>    
> The _bp one is the list of events which are executed on the active cpu
> and the _ap ones are those executed on the hotplugged cpu.
> 
> The core code advances the events in sync steps, so both BP and AP can
> issue a stop on the process and cause a rollback.
> 
> Most of the callbacks can be added to the arrays at compile time, just
> the stuff which is in modules requires an register/unregister
> interface.
> 
> Though in any case the enum gives us a very explicit ordering of
> setup/teardown, so rollback or partial online/offline should be simple
> to achieve.
>

Is there anything I can help you with, in implementing this new design?

Regards,
Srivatsa S. Bhat

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