[PATCH v5 3/5] workqueue: Make too_many_workers() return the worker excess

[Valentin Schneider <vschneid@redhat.com>]

Later patches will need the logic implemented within too_many_workers() to
get the amount of workers to delete. Rather than duplicate the logic,
rework too_many_workers() to return the count of workers to delete - its
return value can be used as a boolean value, so no change in behaviour
intended.

The function currently returns true when
  (nr_idle - 2) * MAX_IDLE_WORKERS_RATIO >= nr_busy
thus, the desired number of idle workers is expressed by
  (nr_idle - 2) * MAX_IDLE_WORKERS_RATIO == nr_busy - 1
IOW
   nr_idle == ((nr_busy - 1) / MAX_IDLE_WORKERS_RATIO) + 2

MAX_IDLE_WORKERS_RATIO being a compile-time power of 2, we can leave that
as a division.

While at it, rename too_many_workers() to worker_cull_count().

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 kernel/workqueue.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8185a42848c50..4fc8085f3fe17 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -816,14 +816,20 @@ static bool need_to_create_worker(struct worker_pool *pool)
 	return need_more_worker(pool) && !may_start_working(pool);
 }
 
-/* Do we have too many workers and should some go away? */
-static bool too_many_workers(struct worker_pool *pool)
+/* How many idle workers should we get rid of, if any? */
+static unsigned int worker_cull_count(struct worker_pool *pool)
 {
 	bool managing = pool->flags & POOL_MANAGER_ACTIVE;
 	int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
 	int nr_busy = pool->nr_workers - nr_idle;
 
-	return nr_idle > 2 && (nr_idle - 2) * MAX_IDLE_WORKERS_RATIO >= nr_busy;
+	lockdep_assert_held(&pool->lock);
+
+	/*
+	 * We keep at least 2 spare idle workers, but overall aim to keep at
+	 * most (1 / MAX_IDLE_WORKERS_RATIO) workers idle.
+	 */
+	return max(0, nr_idle - 2 - ((nr_busy - 1) / MAX_IDLE_WORKERS_RATIO));
 }
 
 /*
@@ -1806,7 +1812,7 @@ static void worker_enter_idle(struct worker *worker)
 	/* idle_list is LIFO */
 	list_add(&worker->entry, &pool->idle_list);
 
-	if (too_many_workers(pool) && !timer_pending(&pool->idle_timer))
+	if (worker_cull_count(pool) && !timer_pending(&pool->idle_timer))
 		mod_timer(&pool->idle_timer, jiffies + IDLE_WORKER_TIMEOUT);
 
 	/* Sanity check nr_running. */
@@ -2025,7 +2031,7 @@ static void idle_worker_timeout(struct timer_list *t)
 
 	raw_spin_lock_irq(&pool->lock);
 
-	while (too_many_workers(pool)) {
+	while (worker_cull_count(pool)) {
 		struct worker *worker;
 		unsigned long expires;
 
-- 
2.31.1