On Wed, Jan 13, 2021 at 7:11 PM Peter Zijlstra <pet...@infradead.org> wrote:
>
> On Tue, Jan 12, 2021 at 11:38:12PM +0800, Lai Jiangshan wrote:
>
> > But the hard problem is "how to suppress the warning of
> > online&!active in __set_cpus_allowed_ptr()" for late spawned
> > unbound workers during hotplug.
>
> I cannot see create_worker() go bad like that.
>
> The thing is, it uses:
>
> kthread_bind_mask(, pool->attr->cpumask)
> worker_attach_to_pool()
> set_cpus_allowed_ptr(, pool->attr->cpumask)
>
> which means set_cpus_allowed_ptr() must be a NOP, because the affinity
> is already set by kthread_bind_mask(). Further, the first wakeup of that
> worker will then hit:
>
> select_task_rq()
> is_cpu_allowed()
> is_per_cpu_kthread() -- false
> select_fallback_rq()
>
>
> So normally that really isn't a problem. I can only see a tiny hole
> there, where someone changes the cpumask between kthread_bind_mask() and
> set_cpus_allowed_ptr(). AFAICT that can be fixed in two ways:
>
> - add wq_pool_mutex around things in create_worker(), or
> - move the set_cpus_allowed_ptr() out of worker_attach_to_pool() and
> into rescuer_thread().
>
> Which then brings us to rescuer_thread... If we manage to trigger the
> rescuer during hotplug, then yes, I think that can go wobbly.
Oh, I forgot set_cpus_allowed_ptr() is NOP when combined with
kthread_bind_mask()(create_worker()).
So the problem becomes "how to suppress the warning of online&!active in
__set_cpus_allowed_ptr()" for late *attached unbound rescuer* workers
during hotplug.
>
> Let me consider that a bit more while I try and make sense of that splat
> Paul reported.
>
> ---
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index ec0771e4a3fb..fe05308dc472 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1844,15 +1844,19 @@ static struct worker *alloc_worker(int node)
> * cpu-[un]hotplugs.
> */
> static void worker_attach_to_pool(struct worker *worker,
> - struct worker_pool *pool)
> + struct worker_pool *pool,
> + bool set_affinity)
> {
> mutex_lock(&wq_pool_attach_mutex);
>
> - /*
> - * set_cpus_allowed_ptr() will fail if the cpumask doesn't have any
> - * online CPUs. It'll be re-applied when any of the CPUs come up.
> - */
> - set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
> + if (set_affinity) {
> + /*
> + * set_cpus_allowed_ptr() will fail if the cpumask doesn't
> have
> + * any online CPUs. It'll be re-applied when any of the CPUs
> + * come up.
> + */
> + set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
> + }
>
> /*
> * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
> @@ -1944,7 +1948,7 @@ static struct worker *create_worker(struct worker_pool
> *pool)
> kthread_bind_mask(worker->task, pool->attrs->cpumask);
>
> /* successful, attach the worker to the pool */
> - worker_attach_to_pool(worker, pool);
> + worker_attach_to_pool(worker, pool, false);
>
> /* start the newly created worker */
> raw_spin_lock_irq(&pool->lock);
> @@ -2509,7 +2513,11 @@ static int rescuer_thread(void *__rescuer)
>
> raw_spin_unlock_irq(&wq_mayday_lock);
>
> - worker_attach_to_pool(rescuer, pool);
> + /*
> + * XXX can go splat when running during hot-un-plug and
> + * the pool affinity is wobbly.
> + */
> + worker_attach_to_pool(rescuer, pool, true);
>
> raw_spin_lock_irq(&pool->lock);
>
