[PATCH -tip V3 8/8] workqueue: Fix affinity of kworkers when attaching into pool

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

When worker_attach_to_pool() is called, we should not put the workers
to pool->attrs->cpumask when there is not CPU online in it.

We have to use wq_online_cpumask in worker_attach_to_pool() to check
if pool->attrs->cpumask is valid rather than cpu_online_mask or
cpu_active_mask due to gaps between stages in cpu hot[un]plug.

So for that late-spawned per-CPU kworker case: the outgoing CPU should have
already been cleared from wq_online_cpumask, so it gets its affinity reset
to the possible mask and the subsequent wakeup will ensure it's put on an
active CPU.

To use wq_online_cpumask in worker_attach_to_pool(), we need to protect
wq_online_cpumask in wq_pool_attach_mutex and we modify workqueue_online_cpu()
and workqueue_offline_cpu() to enlarge wq_pool_attach_mutex protected
region. We also put updating wq_online_cpumask and [re|un]bind_workers()
in the same wq_pool_attach_mutex protected region to make the update
for percpu workqueue atomically.

Cc: Qian Cai <cai@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Vincent Donnefort <vincent.donnefort@xxxxxxx>
Link: https://lore.kernel.org/lkml/20201210163830.21514-3-valentin.schneider@xxxxxxx/
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 kernel/workqueue.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index dd32398edf55..25d50050257c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -310,7 +310,7 @@ static bool workqueue_freezing;		/* PL: have wqs started freezing? */
 /* PL: allowable cpus for unbound wqs and work items */
 static cpumask_var_t wq_unbound_cpumask;
 
-/* PL: online cpus (cpu_online_mask with the going-down cpu cleared) */
+/* PL&A: online cpus (cpu_online_mask with the going-down cpu cleared) */
 static cpumask_var_t wq_online_cpumask;
 
 /* CPU where unbound work was last round robin scheduled from this CPU */
@@ -1848,11 +1848,11 @@ 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);
+	/* Is there any cpu in pool->attrs->cpumask online? */
+	if (cpumask_intersects(pool->attrs->cpumask, wq_online_cpumask))
+		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask) < 0);
+	else
+		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
 
 	/*
 	 * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
@@ -5082,13 +5082,12 @@ int workqueue_online_cpu(unsigned int cpu)
 	int pi;
 
 	mutex_lock(&wq_pool_mutex);
-	cpumask_set_cpu(cpu, wq_online_cpumask);
 
-	for_each_cpu_worker_pool(pool, cpu) {
-		mutex_lock(&wq_pool_attach_mutex);
+	mutex_lock(&wq_pool_attach_mutex);
+	cpumask_set_cpu(cpu, wq_online_cpumask);
+	for_each_cpu_worker_pool(pool, cpu)
 		rebind_workers(pool);
-		mutex_unlock(&wq_pool_attach_mutex);
-	}
+	mutex_unlock(&wq_pool_attach_mutex);
 
 	/* update CPU affinity of workers of unbound pools */
 	for_each_pool(pool, pi) {
@@ -5118,14 +5117,13 @@ int workqueue_offline_cpu(unsigned int cpu)
 	if (WARN_ON(cpu != smp_processor_id()))
 		return -1;
 
-	for_each_cpu_worker_pool(pool, cpu) {
-		mutex_lock(&wq_pool_attach_mutex);
-		unbind_workers(pool);
-		mutex_unlock(&wq_pool_attach_mutex);
-	}
-
 	mutex_lock(&wq_pool_mutex);
+
+	mutex_lock(&wq_pool_attach_mutex);
 	cpumask_clear_cpu(cpu, wq_online_cpumask);
+	for_each_cpu_worker_pool(pool, cpu)
+		unbind_workers(pool);
+	mutex_unlock(&wq_pool_attach_mutex);
 
 	/* update CPU affinity of workers of unbound pools */
 	for_each_pool(pool, pi) {
-- 
2.19.1.6.gb485710b