Messages in this thread |  | | Subject | Re: cpus_allowed | From | Robert Love <> | Date | 13 Oct 2001 12:22:15 -0400 |
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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 (obviously).
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, anyhow.
Andrew Morton wrote has a similar patch that is quite good. Get it at http://www.uow.edu.au/~andrewm/linux/#cpus_allowed
Robert Love
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