Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively

[Peter Zijlstra <peterz@infradead.org>]

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?

---
 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);