Re: [PATCH 08/35] autonuma: introduce kthread_bind_node()

Andrea Arcangeli
2012-05-29T16:13:10Z

On Tue, May 29, 2012 at 02:49:13PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-05-25 at 19:02 +0200, Andrea Arcangeli wrote:
> > /**
> > + * kthread_bind_node - bind a just-created kthread to the CPUs of a node.
> > + * @p: thread created by kthread_create().
> > + * @nid: node (might not be online, must be possible) for @k to run on.
> > + *
> > + * Description: This function is equivalent to set_cpus_allowed(),
> > + * except that @nid doesn't need to be online, and the thread must be
> > + * stopped (i.e., just returned from kthread_create()).
> > + */
> > +void kthread_bind_node(struct task_struct *p, int nid)
> > +{
> > + /* Must have done schedule() in kthread() before we set_task_cpu */
> > + if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
> > + WARN_ON(1);
> > + return;
> > + }
> > +
> > + /* It's safe because the task is inactive. */
> > + do_set_cpus_allowed(p, cpumask_of_node(nid));
> > + p->flags |= PF_THREAD_BOUND;
>
> No, I've said before, this is wrong. You should only ever use
> PF_THREAD_BOUND when its strictly per-cpu. Moving the your numa threads
> to a different node is silly but not fatal in any way.

I changed the semantics of that bitflag, now it means: userland isn't
allowed to shoot itself in the foot and mess with whatever CPU
bindings the kernel has set for the kernel thread.

It'd be a clear regress not to set PF_THREAD_BOUND there. It would be
even worse to remove the CPU binding to the node: it'd risk to copy
memory with both src and dst being in remote nodes from the CPU where
knuma_migrate runs on (there aren't just 2 node systems out there).