linux-kernel - Re: [RFC][PATCH][EXPERIMENTAL] CPU hotplug with frozen tasks

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Date:	Mon, 16 Apr 2007 09:50:14 +0000
From:	"Gautham Shenoy" <ego.lkml@...il.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	LKML <linux-kernel@...r.kernel.org>, "Pavel Machek" <pavel@....cz>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Gautham R Shenoy" <ego@...ibm.com>,
	"Srivatsa Vaddagiri" <vatsa@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Oleg Nesterov" <oleg@...sign.ru>
Subject: Re: [RFC][PATCH][EXPERIMENTAL] CPU hotplug with frozen tasks

Hi Rafael,

On 4/15/07, Rafael J. Wysocki <rjw@...k.pl> wrote:
> Hi,
>
> As I said before, we have a problem with using the CPU hotplug for suspending
> because of the notifiers that are called from within cpu_up()/cpu_down() and
> (sometimes) assume that the system is fully functional.
>

Right. In order to use freezer for CPU hotplug, we need to perform
that audit anyway.

> One obvious solution of this problem would be to make the notifiers behave
> differently if tasks are frozen, but for this purpose we'd need to tell them
> that this is the case.  In principle, we could do it in many different ways
> (eg. by using a global variable, with the help of suspend notifiers etc.), but
> IMO one of the cleanest methods woud be to use some special values for the
> notifications occuring while tasks are frozen (eg. CPU_DEAD_FROZEN instead of
> CPU_DEAD etc.).  In that case the notifiers could react in some special ways
> to the "FROZEN" notfifications and that would allow us to simplify some code
> paths (eg. in the microcode driver).
>

Agreed.

> The appended patch introduces such "FROZEN" notfifications, modifies the CPU
> hotplug core to use them and updates all of the users of CPU hotplug notifiers
> to recognize them.  For now, they are treated in the same way as the
> corresponding "normal" notifications, but I'm going to modify the microcode
> driver to really use them and I believe that some other subsystems can benefit
> from using them as well.
>

Ok. A minor doubt.

When you say FROZEN, do you mean frozen due to suspend ? If yes, then
it makes sense. Otherwise once cpu-hotplug starts using the freezer
(hopefully it will someday soon
:-)) won't this patch become redundant ? [Except of course fixing a few glitches
due to the assumption that the system is fully functional, when it's
actually frozen.]

I am of the opinion that we should have notifications which help the
cpu-hotplug aware
subsystems differentiate between a normal cpu-hotplug  and a
cpu-hotplug initiated by
suspend. Thereby they can handle it accordingly and not destroy any
percpu resources
and reuse them instead during resume.

Am I missing something?

> The patch is totally experimental and untested, although it's been successfully
> compiled on x86_64 and it's main purpose is to show what exactly I mean. :-)
>
> Comments welcome.
>

Other than that, I am ok with the patch.

> Greetings,
> Rafael
>

Thanks and Regards
gautham.
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