[PATCH -v3 5/9] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

From: Peter Zijlstra
Date: Thu Jan 21 2021 - 05:39:15 EST


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@xxxxxxxxxxxxx>
---
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, pool->cpu);

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, -1);
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, -1);
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) {
+ kthread_set_per_cpu(worker->task, pool->cpu);
WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
pool->attrs->cpumask) < 0);
+ }

raw_spin_lock_irq(&pool->lock);