workqueue: implement apply_workqueue_attrs()

Implement apply_workqueue_attrs() which applies workqueue_attrs to the
specified unbound workqueue by creating a new pwq (pool_workqueue)
linked to worker_pool with the specified attributes.

A new pwq is linked at the head of wq->pwqs instead of tail and
__queue_work() verifies that the first unbound pwq has positive refcnt
before choosing it for the actual queueing.  This is to cover the case
where creation of a new pwq races with queueing.  As base ref on a pwq
won't be dropped without making another pwq the first one,
__queue_work() is guaranteed to make progress and not add work item to
a dead pwq.

init_and_link_pwq() is updated to return the last first pwq the new
pwq replaced, which is put by apply_workqueue_attrs().

Note that apply_workqueue_attrs() is almost identical to unbound pwq
part of alloc_and_link_pwqs().  The only difference is that there is
no previous first pwq.  apply_workqueue_attrs() is implemented to
handle such cases and replaces unbound pwq handling in
alloc_and_link_pwqs().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index c270b4e..e152394 100644 (file)
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -410,6 +410,8 @@ extern void destroy_workqueue(struct workqueue_struct *wq);
 
 struct workqueue_attrs *alloc_workqueue_attrs(gfp_t gfp_mask);
 void free_workqueue_attrs(struct workqueue_attrs *attrs);
+int apply_workqueue_attrs(struct workqueue_struct *wq,
+                         const struct workqueue_attrs *attrs);
 
 extern bool queue_work_on(int cpu, struct workqueue_struct *wq,
                        struct work_struct *work);

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 16fb674..2a67fbb 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1228,7 +1228,7 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
        if (unlikely(wq->flags & WQ_DRAINING) &&
            WARN_ON_ONCE(!is_chained_work(wq)))
                return;
-
+retry:
        /* pwq which will be used unless @work is executing elsewhere */
        if (!(wq->flags & WQ_UNBOUND)) {
                if (cpu == WORK_CPU_UNBOUND)
@@ -1262,6 +1262,25 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
                spin_lock(&pwq->pool->lock);
        }
 
+       /*
+        * pwq is determined and locked.  For unbound pools, we could have
+        * raced with pwq release and it could already be dead.  If its
+        * refcnt is zero, repeat pwq selection.  Note that pwqs never die
+        * without another pwq replacing it as the first pwq or while a
+        * work item is executing on it, so the retying is guaranteed to
+        * make forward-progress.
+        */
+       if (unlikely(!pwq->refcnt)) {
+               if (wq->flags & WQ_UNBOUND) {
+                       spin_unlock(&pwq->pool->lock);
+                       cpu_relax();
+                       goto retry;
+               }
+               /* oops */
+               WARN_ONCE(true, "workqueue: per-cpu pwq for %s on cpu%d has 0 refcnt",
+                         wq->name, cpu);
+       }
+
        /* pwq determined, queue */
        trace_workqueue_queue_work(req_cpu, pwq, work);
 
@@ -3425,7 +3444,8 @@ static void pwq_unbound_release_workfn(struct work_struct *work)
 
 static void init_and_link_pwq(struct pool_workqueue *pwq,
                              struct workqueue_struct *wq,
-                             struct worker_pool *pool)
+                             struct worker_pool *pool,
+                             struct pool_workqueue **p_last_pwq)
 {
        BUG_ON((unsigned long)pwq & WORK_STRUCT_FLAG_MASK);
 
@@ -3445,13 +3465,58 @@ static void init_and_link_pwq(struct pool_workqueue *pwq,
        mutex_lock(&wq->flush_mutex);
        spin_lock_irq(&workqueue_lock);
 
+       if (p_last_pwq)
+               *p_last_pwq = first_pwq(wq);
        pwq->work_color = wq->work_color;
-       list_add_tail_rcu(&pwq->pwqs_node, &wq->pwqs);
+       list_add_rcu(&pwq->pwqs_node, &wq->pwqs);
 
        spin_unlock_irq(&workqueue_lock);
        mutex_unlock(&wq->flush_mutex);
 }
 
+/**
+ * apply_workqueue_attrs - apply new workqueue_attrs to an unbound workqueue
+ * @wq: the target workqueue
+ * @attrs: the workqueue_attrs to apply, allocated with alloc_workqueue_attrs()
+ *
+ * Apply @attrs to an unbound workqueue @wq.  If @attrs doesn't match the
+ * current attributes, a new pwq is created and made the first pwq which
+ * will serve all new work items.  Older pwqs are released as in-flight
+ * work items finish.  Note that a work item which repeatedly requeues
+ * itself back-to-back will stay on its current pwq.
+ *
+ * Performs GFP_KERNEL allocations.  Returns 0 on success and -errno on
+ * failure.
+ */
+int apply_workqueue_attrs(struct workqueue_struct *wq,
+                         const struct workqueue_attrs *attrs)
+{
+       struct pool_workqueue *pwq, *last_pwq;
+       struct worker_pool *pool;
+
+       if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
+               return -EINVAL;
+
+       pwq = kmem_cache_zalloc(pwq_cache, GFP_KERNEL);
+       if (!pwq)
+               return -ENOMEM;
+
+       pool = get_unbound_pool(attrs);
+       if (!pool) {
+               kmem_cache_free(pwq_cache, pwq);
+               return -ENOMEM;
+       }
+
+       init_and_link_pwq(pwq, wq, pool, &last_pwq);
+       if (last_pwq) {
+               spin_lock_irq(&last_pwq->pool->lock);
+               put_pwq(last_pwq);
+               spin_unlock_irq(&last_pwq->pool->lock);
+       }
+
+       return 0;
+}
+
 static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 {
        bool highpri = wq->flags & WQ_HIGHPRI;
@@ -3468,26 +3533,12 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
                        struct worker_pool *cpu_pools =
                                per_cpu(cpu_worker_pools, cpu);
 
-                       init_and_link_pwq(pwq, wq, &cpu_pools[highpri]);
+                       init_and_link_pwq(pwq, wq, &cpu_pools[highpri], NULL);
                }
+               return 0;
        } else {
-               struct pool_workqueue *pwq;
-               struct worker_pool *pool;
-
-               pwq = kmem_cache_zalloc(pwq_cache, GFP_KERNEL);
-               if (!pwq)
-                       return -ENOMEM;
-
-               pool = get_unbound_pool(unbound_std_wq_attrs[highpri]);
-               if (!pool) {
-                       kmem_cache_free(pwq_cache, pwq);
-                       return -ENOMEM;
-               }
-
-               init_and_link_pwq(pwq, wq, pool);
+               return apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
        }
-
-       return 0;
 }
 
 static int wq_clamp_max_active(int max_active, unsigned int flags,