Re: [RFC] Thread Migration Preemption

[Nick Piggin]

Matt Mackall wrote:
> On Fri, Jul 06, 2007 at 04:12:10PM +1000, Nick Piggin wrote:
>
> > Mathieu Desnoyers wrote:
> >
> > > Thread Migration Preemption
> > >
> > > This patch adds the ability to protect critical sections from migration to
> > > another CPU without disabling preemption.
> > >
> > > This will be useful to minimize the amount of preemption disabling for the 
> > > -rt
> > > patch. It will help leveraging improvements brought by the local_t types in
> > > asm/local.h (see Documentation/local_ops.txt). Note that the updates done 
> > > to
> > > variables protected by migration_disable must be either atomic or 
> > > protected from
> > > concurrent updates done by other threads.
> > >
> > > Typical use:
> > >
> > > migration_disable();
> > > local_inc(&__get_cpu_var(&my_local_t_var));
> > > migration_enable();
> > >
> > > Which will increment the variable atomically wrt the local CPU.
> > >
> > > Comments (such as how to integrate this in the already almost full
> > > preempt_count) are welcome.
> >
> > This seems like way too much stuff to add just for this type of thing. Why
> > not just disable and reenable preempt? Surely local_inc is not going to take
> > so long that disabling preemption matters.
>
>
> 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.


> > The task struct is not something we should just be carefree putting crap
> > into because it is seemingly free :(
>
>
> Sadly, it is free at the moment. We can only fit 3 task_structs in an order-1 SLAB,
> with lots of slop.

Well apart from more cacheline access, that's why I say seemingly free. But
actually it uses up space we may like for something else in future and/or
makes it harder to shrink down if any effort ever goes into that.

--
SUSE Labs, Novell Inc.
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