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

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Date:	06 Jul 2007 13:59:33 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Thread Migration Preemption

Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> writes:

> Thread Migration Preemption
> 
> This patch adds the ability to protect critical sections from migration to
> another CPU without disabling preemption.

Good idea.

I sometimes think we could have avoided _much_ trouble
if that had been always default for processes running 
in kernel space.
 
> 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();

It seems strange to have a new interface for this. We already 
have get_cpu()/put_cpu(). So why not use that?

>  	unsigned long		flags;		/* low level flags */
>  	__u32			cpu;
>  	__s32			preempt_count;	/* 0 => preemptable, <0 => BUG */
> +	int			migration_count;/* 0: can migrate, <0 => BUG */

Can you turn preempt_count into a short first and use a short? That should be enough
and cache line usage wouldn't be increased. That's ok on x86; on RISCs 
int might be faster


-Andi
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