workqueue: Restrict affinity change to rescuer

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

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index cce3433..894bb88 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1849,12 +1849,6 @@ 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
         * definition for details.
@@ -1864,6 +1858,9 @@ static void worker_attach_to_pool(struct worker *worker,
        else
                kthread_set_per_cpu(worker->task, pool->cpu);
 
+       if (worker->rescue_wq)
+               set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+
        list_add_tail(&worker->node, &pool->workers);
        worker->pool = pool;