linux-kernel - Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

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Date:	Wed, 14 Feb 2007 22:43:35 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	ego@...ibm.com
Cc:	akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu, vatsa@...ibm.com,
	dipankar@...ibm.com, venkatesh.pallipadi@...el.com,
	linux-kernel@...r.kernel.org, oleg@...sign.ru
Subject: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

Hi,

On Wednesday, 14 February 2007 15:40, Gautham R Shenoy wrote:
> Hello Everybody,
> 
> This is an experiment towards process_freezer based implementation
> of cpu-hotplug. This is mainly based on ideas of Andrew Morton, 
> Ingo Molnar and Paul Mckenney featured in the discussion
> http://lkml.org/lkml/2007/1/31/323.
> 
> This is an absolute bare-minimal implementation to check the feasibility
> of using process freezer for cpu-hotplug. 
> 
> The patchset comprises of four patches.
> o PATCH 1/4: Core implementation of freezer-based-hotplug.
> o PATCH 2/4: Revert changes to workqueue to make it work with the
>              freezer-cpu-hotplug.
> o PATCH 3/4: Eliminate hotcpu subsystem mutexes from sched and slab.
> o PATCH 4/4: Eliminate lock_cpu_hotplug from the kernel.

I think two things are missing:

1) We should make sure there are not PF_NOFREEZE tasks running when a CPU
is removed (when one is added probably too).  For this purpose we can add a
parameter to freeze_processes() that will tell it to ignore PF_NOFREEZE, but
at the same time we'll have to change all kernel threads that set PF_NOFREEZE
to call try_to_freeze() anyway.  I can do that, but it will take me a couple of
days.

2) We have to change the PM code to stop using CPU hotplug for disabling
nonboot CPUs. ;-)

Greetings,
Rafael
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