Re: [patch 00/18] SMP: Boot and CPU hotplug refactoring - Part 1

On Fri, 20 Apr 2012 16:18:04 +0200 (CEST), Thomas Gleixner [off-list ref] wrote:

On Fri, 20 Apr 2012, Srivatsa S. Bhat wrote:

quoted

already done.. Myself and Nikunj had some initial design/ideas on reducing
the duplication in architecture code, related to managing the setting
of the cpu in the online mask, sending out CPU_STARTING notifiers etc
from generic code..

The whole notifier business needs a redesign as well, because we don't
have a way to express proper dependencies, we add random notifier
points and the teardown path is ass backwards. The whole thing wants
to be a tree which can be walked in either direction and from any
point. Right now we cut the trunk first and keep the single limb up
with a helicopter and start dismantling it.

But there are two ways to do it.  One is to eliminate the need for
callbacks.  The other is to make a full dependency-based callback
system.

Let's try the first before the second.  I implemented a full dep-based
system for initcalls once, and it was not as nice as I'd hoped.

Flat notifiers are not working for this as they do not allow a tree
structure and prevent us to do things in parallel.

Not sure whether calling notifiers in parallel is going to be a big win:
they'll end up fighting over the cpu we're taking down.  But I could be
wrong.

That really needs to be completely reworked. There is also a lot of
stuff which wants to be moved into the starting/dying CPU
context. Right now we kinda do that by trampling on the CPU with a
high prio stomper thread, but that's really just a bandaid and steady
cause of trouble.

If you look at facilities which use kthreads, then there is lots other
setup which does not need a notifier at all, as it can be done in the
context of the thread when we have a way to start/park those threads
at the right time in the up/down process.

That's a very nice idea.  Should be simpler for kthreads park/unpark
themselves, rather than having some notifier to kill them.

The original concept of stopping the machine for cpu hotplug and trying
not to effect any other kernel code has jumped the shark: I think we
need to look seriously at a complete rewrite where we don't use
stop_machine.

Cheers,
Rusty.