[PATCH 4.19 17/17] workqueue: Restrict affinity change to rescuer

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 640f17c82460e9724fd256f0a1f5d99e7ff0bda4 ]

create_worker() will already set the right affinity using
kthread_bind_mask(), this means only the rescuer will need to change
it's affinity.

Howveer, while in cpu-hot-unplug a regular task is not allowed to run
on online&&!active as it would be pushed away quite agressively. We
need KTHREAD_IS_PER_CPU to survive in that environment.

Therefore set the affinity after getting that magic flag.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210121103506.826629830@infradead.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/workqueue.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index cd98ef48345e1..78600f97ffa72 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1728,12 +1728,6 @@ static void worker_attach_to_pool(struct worker *worker,
 {
 	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);
-
 	/*
 	 * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
 	 * stable across this function.  See the comments above the flag
@@ -1742,6 +1736,9 @@ static void worker_attach_to_pool(struct worker *worker,
 	if (pool->flags & POOL_DISASSOCIATED)
 		worker->flags |= WORKER_UNBOUND;
 
+	if (worker->rescue_wq)
+		set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+
 	list_add_tail(&worker->node, &pool->workers);
 	worker->pool = pool;
 
-- 
2.27.0