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


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