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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