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SubjectRe: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU
On Sun, Jan 17, 2021 at 12:14:34AM +0800, Lai Jiangshan wrote:

> BP: AP: worker:
> cpus_write_lock()
> bringup_cpu() work_item_func()
> bringup_wait_for_ap get_online_cpus()
> kthread_park(worker)

Thanks, pictures are easier. Agreed, that a problem.

I've also found another problem I think. rescuer_thread becomes part of
for_each_pool_worker() between worker_attach_to_pool() and
worker_detach_from_pool(), so it would try and do kthread_park() on
rescuer, when things align. And rescuer_thread() doesn't have a
kthread_parkme().

And we already rely on this 'ugly' thing of first doing
kthread_set_per_cpu() and fixing up the affinity later for the rescuer.

Let me restart the SRCU-P testing with the below delta applied.

---
kernel/workqueue.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1db769b116a1..894bb885b40b 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2368,7 +2368,6 @@ static int worker_thread(void *__worker)
/* tell the scheduler that this is a workqueue worker */
set_pf_worker(true);
woke_up:
- kthread_parkme();
raw_spin_lock_irq(&pool->lock);

/* am I supposed to die? */
@@ -2426,7 +2425,7 @@ static int worker_thread(void *__worker)
move_linked_works(work, &worker->scheduled, NULL);
process_scheduled_works(worker);
}
- } while (keep_working(pool) && !kthread_should_park());
+ } while (keep_working(pool));

worker_set_flags(worker, WORKER_PREP);
sleep:
@@ -2438,12 +2437,9 @@ static int worker_thread(void *__worker)
* event.
*/
worker_enter_idle(worker);
- set_current_state(TASK_IDLE);
+ __set_current_state(TASK_IDLE);
raw_spin_unlock_irq(&pool->lock);
-
- if (!kthread_should_park())
- schedule();
-
+ schedule();
goto woke_up;
}

@@ -4979,9 +4975,9 @@ static void rebind_workers(struct worker_pool *pool)
* from CPU_ONLINE, the following shouldn't fail.
*/
for_each_pool_worker(worker, pool) {
- WARN_ON_ONCE(kthread_park(worker->task) < 0);
kthread_set_per_cpu(worker->task, pool->cpu);
- kthread_unpark(worker->task);
+ WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
+ pool->attrs->cpumask) < 0);
}

raw_spin_lock_irq(&pool->lock);
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