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    SubjectRe: [PATCH 08/35] autonuma: introduce kthread_bind_node()
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    On Tue, 2012-05-29 at 18:11 +0200, Andrea Arcangeli wrote:
    > 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.

    Yeah, and you did so without mentioning that in your changelog.
    Furthermore I object to that change. I object even more strongly to
    doing it without mention and keeping a misleading comment near the
    definition.

    > 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).

    Just teach each knuma_migrated what node it represents and don't use
    numa_node_id().

    That way you can change the affinity just fine, it'll be sub-optimal,
    copying memory from node x to node y through node z, but it'll still
    work correctly.

    numa isn't special in the way per-cpu stuff is special.


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