Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

[Lai Jiangshan <jiangshanlai@gmail.com>]

On Tue, Jan 12, 2021 at 10:51 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Mark the per-cpu workqueue workers as KTHREAD_IS_PER_CPU.
>
> Workqueues have unfortunate semantics in that per-cpu workers are not
> default flushed and parked during hotplug, however a subset does
> manual flush on hotplug and hard relies on them for correctness.
>
> Therefore play silly games..
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Tested-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/workqueue.c |   11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1861,6 +1861,8 @@ static void worker_attach_to_pool(struct
>          */
>         if (pool->flags & POOL_DISASSOCIATED)
>                 worker->flags |= WORKER_UNBOUND;
> +       else
> +               kthread_set_per_cpu(worker->task, true);
>
>         list_add_tail(&worker->node, &pool->workers);
>         worker->pool = pool;
> @@ -1883,6 +1885,7 @@ static void worker_detach_from_pool(stru
>
>         mutex_lock(&wq_pool_attach_mutex);
>
> +       kthread_set_per_cpu(worker->task, false);
>         list_del(&worker->node);
>         worker->pool = NULL;
>
> @@ -4919,8 +4922,10 @@ static void unbind_workers(int cpu)
>
>                 raw_spin_unlock_irq(&pool->lock);
>
> -               for_each_pool_worker(worker, pool)
> +               for_each_pool_worker(worker, pool) {
> +                       kthread_set_per_cpu(worker->task, false);
>                         WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
> +               }
>
>                 mutex_unlock(&wq_pool_attach_mutex);
>
> @@ -4972,9 +4977,11 @@ static void rebind_workers(struct worker
>          * of all workers first and then clear UNBOUND.  As we're called
>          * from CPU_ONLINE, the following shouldn't fail.
>          */
> -       for_each_pool_worker(worker, pool)
> +       for_each_pool_worker(worker, pool) {
>                 WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
>                                                   pool->attrs->cpumask) < 0);
> +               kthread_set_per_cpu(worker->task, true);

Will the schedule break affinity in the middle of these two lines due to
patch4 allowing it and result in Paul's reported splat.

> +       }
>
>         raw_spin_lock_irq(&pool->lock);
>
>
>