Re: cpus_allowed

Robert Love (
13 Oct 2001 12:22:15 -0400

On Sat, 2001-10-13 at 19:59, Rolf Fokkens wrote:
> I was curious about "CPU affinity" in linux. I found some patches which add
> affinity in task_struct but later I found out that "cpus_allowed" in
> task_struct almost does the same thing.
> It resulted in some new curiosity: where's cpus_allowed initialized? I can
> only find an assignment to cpus_allowed for softirq's but no initialization
> for other processes. I assume the correct init value would be "0xffffffff" or
> -1. Can't find it though.
> I'm sure I'm overlooking something, but that doesn't help me finding the
> answer. So would someone be so kind to enlighten me?

It is initialized to -1 (0xffffffff) by struct definition at
linux/kernel/sched.h. Since it is a mask, this means all CPUs

It isn't used much. The softirq code uses it to try to keep some tasks
on one CPU, for performance reasons. The Tux in-kernel httpd uses it
for performance reasons. Some patches use it, too.

Most of the CPU affinity patches you see were written before
cpus_allowed. They go through all sorts of trouble to do what the OS
now does on its own. If you want to change CPU affinity then you just
need a patch that adds a syscall or proc interface for setting the
cpus_allowed mask.

I wrote a proc interface but for the life of me I can't find it. If you
can I would be happy to update it to your kernel. It isn't very old,

Andrew Morton wrote has a similar patch that is quite good. Get it at

Robert Love

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