On Mon, Jan 11, 2021 at 06:16:39PM +0100, Peter Zijlstra wrote:
>
> While thinking more about this, I'm thinking a big part of the problem
> is that we're not dinstinguishing between geniuine per-cpu kthreads and
> kthreads that just happen to be per-cpu.
>
> Geniuine per-cpu kthreads are kthread_bind() and have PF_NO_SETAFFINITY,
> but sadly a lot of non-per-cpu kthreads, that might happen to still be
> per-cpu also have that -- again workqueue does that even to it's unbound
> workers :-(
>
> Now, anything created by smpboot, is created through
> kthread_create_on_cpu() and that additionally sets to_kthread(p)->flags
> KTHREAD_IS_PER_CPU.
>
> And I'm thinking that might be sufficient, if we modify
> is_per_cpu_kthread() to check that, then we only match smpboot threads
> (which include the hotplug and stopper threads, but notably not the idle
> thread)
>
> Sadly it appears like io_uring() uses kthread_create_on_cpu() without
> then having any hotplug crud on, so that needs additinoal frobbing.
>
> Also, init_task is PF_KTHREAD but doesn't have a struct kthread on.. and
> I suppose bound workqueues don't go through this either.
>
> Let me rummage around a bit...
>
> This seems to not insta-explode... opinions?
It passes quick tests on -rcu both with and without the rcutorture fixes,
which is encouraging. I will start a more vigorous test in about an hour.
Thanx, Paul
> ---
> include/linux/kthread.h | 3 +++
> kernel/kthread.c | 25 ++++++++++++++++++++++++-
> kernel/sched/core.c | 2 +-
> kernel/sched/sched.h | 4 ++--
> kernel/smpboot.c | 1 +
> kernel/workqueue.c | 12 +++++++++---
> 6 files changed, 40 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/kthread.h b/include/linux/kthread.h
> index 65b81e0c494d..fdd5a52e35d8 100644
> --- a/include/linux/kthread.h
> +++ b/include/linux/kthread.h
> @@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cpu(int
> (*threadfn)(void *data),
> unsigned int cpu,
> const char *namefmt);
>
> +void kthread_set_per_cpu(struct task_struct *k, bool set);
> +bool kthread_is_per_cpu(struct task_struct *k);
> +
> /**
> * kthread_run - create and wake a thread.
> * @threadfn: the function to run until signal_pending(current).
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index a5eceecd4513..7f081530e459 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -493,11 +493,34 @@ struct task_struct *kthread_create_on_cpu(int
> (*threadfn)(void *data),
> return p;
> kthread_bind(p, cpu);
> /* CPU hotplug need to bind once again when unparking the thread. */
> - set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
> to_kthread(p)->cpu = cpu;
> return p;
> }
>
> +void kthread_set_per_cpu(struct task_struct *k, bool set)
> +{
> + struct kthread *kthread = to_kthread(k);
> + if (!kthread)
> + return;
> +
> + if (set) {
> + WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
> + WARN_ON_ONCE(k->nr_cpus_allowed != 1);
> + set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> + } else {
> + clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> + }
> +}
> +
> +bool kthread_is_per_cpu(struct task_struct *k)
> +{
> + struct kthread *kthread = to_kthread(k);
> + if (!kthread)
> + return false;
> +
> + return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> +}
> +
> /**
> * kthread_unpark - unpark a thread created by kthread_create().
> * @k: thread created by kthread_create().
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 15d2562118d1..e71f9e44789e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7277,7 +7277,7 @@ static void balance_push(struct rq *rq)
> * Both the cpu-hotplug and stop task are in this case and are
> * required to complete the hotplug process.
> */
> - if (is_per_cpu_kthread(push_task) || is_migration_disabled(push_task)) {
> + if (rq->idle == push_task || is_per_cpu_kthread(push_task) ||
> is_migration_disabled(push_task)) {
> /*
> * If this is the idle task on the outgoing CPU try to wake
> * up the hotplug control thread which might wait for the
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 12ada79d40f3..3679f63e0aa2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2697,10 +2697,10 @@ static inline bool is_per_cpu_kthread(struct
> task_struct *p)
> if (!(p->flags & PF_KTHREAD))
> return false;
>
> - if (p->nr_cpus_allowed != 1)
> + if (!(p->flags & PF_NO_SETAFFINITY))
> return false;
>
> - return true;
> + return kthread_is_per_cpu(p);
> }
> #endif
>
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index 2efe1e206167..b0abe575a524 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht,
> unsigned int cpu)
> kfree(td);
> return PTR_ERR(tsk);
> }
> + kthread_set_per_cpu(tsk, true);
> /*
> * Park the thread so that it could start right on the CPU
> * when it is available.
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 9880b6c0e272..824276e4fb2e 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1861,6 +1861,8 @@ static void worker_attach_to_pool(struct worker *worker,
> */
> 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;
> @@ -4919,8 +4921,10 @@ static void unbind_workers(int cpu)
>
> raw_spin_unlock_irq(&pool->lock);
>
> - for_each_pool_worker(worker, pool)
> - WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> cpu_active_mask) < 0);
> + 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 +4976,11 @@ static void rebind_workers(struct worker_pool *pool)
> * 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);
> + }
>
> raw_spin_lock_irq(&pool->lock);
>
