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SubjectRe: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling
On Wed, Feb 03, 2021 at 05:51:15PM +0100, Peter Zijlstra wrote:
>
> I'm slowly starting to go through this...
>
> On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> > +static bool sched_core_empty(struct rq *rq)
> > +{
> > + return RB_EMPTY_ROOT(&rq->core_tree);
> > +}
> > +
> > +static struct task_struct *sched_core_first(struct rq *rq)
> > +{
> > + struct task_struct *task;
> > +
> > + task = container_of(rb_first(&rq->core_tree), struct task_struct, core_node);
> > + return task;
> > +}
>
> AFAICT you can do with:
>
> static struct task_struct *sched_core_any(struct rq *rq)
> {
> return rb_entry(rq->core_tree.rb_node, struct task_struct, code_node);
> }
>
> > +static void sched_core_flush(int cpu)
> > +{
> > + struct rq *rq = cpu_rq(cpu);
> > + struct task_struct *task;
> > +
> > + while (!sched_core_empty(rq)) {
> > + task = sched_core_first(rq);
> > + rb_erase(&task->core_node, &rq->core_tree);
> > + RB_CLEAR_NODE(&task->core_node);
> > + }
> > + rq->core->core_task_seq++;
> > +}
>
> However,
>
> > + for_each_possible_cpu(cpu) {
> > + struct rq *rq = cpu_rq(cpu);
> > +
> > + WARN_ON_ONCE(enabled == rq->core_enabled);
> > +
> > + if (!enabled || (enabled && cpumask_weight(cpu_smt_mask(cpu)) >= 2)) {
> > + /*
> > + * All active and migrating tasks will have already
> > + * been removed from core queue when we clear the
> > + * cgroup tags. However, dying tasks could still be
> > + * left in core queue. Flush them here.
> > + */
> > + if (!enabled)
> > + sched_core_flush(cpu);
> > +
> > + rq->core_enabled = enabled;
> > + }
> > + }
>
> I'm not sure I understand. Is the problem that we're still schedulable
> during do_exit() after cgroup_exit() ? It could be argued that when we
> leave the cgroup there, we should definitely leave the tag group too.

That is, did you forget to implement cpu_cgroup_exit()?

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