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

[Gautham Shenoy]

Hi Rafael,

On 4/15/07, Rafael J. Wysocki <rjw@xxxxxxx> 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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