Re: [RFC] Thread Migration Preemption

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Matt Mackall (mpm@selenic.com) wrote:
> On Wed, Jul 11, 2007 at 10:02:23AM +1000, Nick Piggin wrote:
> > >I like this patch a lot. Even if we don't add the underlying mechanism
> > >right now, adding migration_disable as an alias for preempt_disable
> > >will much better document quite a number of the users.
> > 
> > I'd have no problem with that, and it might make it easier in future to
> > justify a more complex scheme.
> 
> What do you think, Mathieu?
> 

That's an excellent idea. It should probably come with some
documentation explaining what difference should be expected between
preempt_disable and migrate_disable in the future so that people can
choose the right alternative for their code.

> Also, small nit: it ought to be migrate_disable to match the form of
> preempt_disable.

Yes, that's fine with me. :)

I though a little more about this whole concept of migrate_disable, and
I think it could be brought further. One of the main problems with per
cpu variables is that this concept cannot be ported to this grey place
called "user-space" because preemption cannot be disabled. The
equivalent (kind of) is currently per thread variables, but it can
consume a lot of space if many threads are running.

However, it could be possible, if we implement a vsyscall based
migration preemption counter accessible with read/write access from
user-space, to tie a thread to a CPU during a user-space critical path.
If we combine this with local atomic operations done in user-space, we
could have highly scalable access to per cpu data structures reentrant
with respect to signal handlers.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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