Re: [PATCH 6/6] sched: Simplify set_affinity_pending refcounts

Peter Zijlstra
2021-02-24T15:49:27Z

On Wed, Feb 24, 2021 at 01:24:45PM +0100, Peter Zijlstra wrote:
> @@ -2199,12 +2199,16 @@ static int affine_move_task(struct rq *r
> push_task = get_task_struct(p);
> }
>
> + /*
> + * If there are pending waiters, but no pending stop_work,
> + * then complete now.
> + */
> pending = p->migration_pending;
> + if (pending && !pending->stop_pending) {
> p->migration_pending = NULL;
> complete = true;
> }

> @@ -2282,12 +2286,13 @@ static int affine_move_task(struct rq *r
> if (task_on_rq_queued(p))
> rq = move_queued_task(rq, rf, p, dest_cpu);
>
> + if (!pending->stop_pending) {
> + p->migration_pending = NULL;
> + complete = true;
> + }
> }
> task_rq_unlock(rq, p, rf);

Elsewhere Valentin argued something like the below ought to be possible.
I've not drawn diagrams yet, but if I understood his argument right it
should be possible.

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1c56ac4df2c9..3ffbd1b76f3e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2204,9 +2204,10 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
* then complete now.
*/
pending = p->migration_pending;
- if (pending && !pending->stop_pending) {
+ if (pending) {
p->migration_pending = NULL;
- complete = true;
+ if (!pending->stop_pending)
+ complete = true;
}

task_rq_unlock(rq, p, rf);
@@ -2286,10 +2287,9 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
if (task_on_rq_queued(p))
rq = move_queued_task(rq, rf, p, dest_cpu);

- if (!pending->stop_pending) {
- p->migration_pending = NULL;
+ p->migration_pending = NULL;
+ if (!pending->stop_pending)
complete = true;
- }
}
task_rq_unlock(rq, p, rf);