On Thu, Jul 01, 2010 at 01:43:23PM +0200, Peter Zijlstra wrote:

On Thu, 2010-07-01 at 13:19 +0200, Peter Zijlstra wrote:

quoted

On Thu, 2010-07-01 at 14:07 +0300, Michael S. Tsirkin wrote:

quoted

Author: Sridhar Samudrala [off-list ref]

sched: export sched_set/getaffinity to modules

vhost-net driver wants to copy the affinity from the
owner thread to thread it creates. Export
sched_set/get affinity to modules to make this possible
when vhost is built as a module.

quoted

Urgh,.. so why again is that a good idea?

In particular:
 - who sets the affinity of the task? 

management tools do this when they start qemu.

 - why can't it set the kernel thread's affinity too?

It can. However: the threads are started internally by the driver
when qemu does an ioctl.  What we want to do is give it a sensible
default affinity. management tool can later tweak it if it wants to.

 - what happens if someone changes the tasks' affinity?

We would normally create a cgroup including all internal
tasks, making it easy to find and change affinity for
them all if necessary.

So no, I don't think this is a sensible thing to do at all.