linux-kernel - Re: [RFC] Thread Migration Preemption

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Message-ID: <20070710233938.GT11166@waste.org>
Date:	Tue, 10 Jul 2007 18:39:39 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Thread Migration Preemption

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.
 
> 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.

-- 
Mathematics is the supreme nostalgia of our time.
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