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Subject[PATCH 26/31] workqueue: implement apply_workqueue_attrs()
Date
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>
---
include/linux/workqueue.h | 2 ++
kernel/workqueue.c | 91 ++++++++++++++++++++++++++++++++++++-----------
2 files changed, 73 insertions(+), 20 deletions(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 0341403..c8c3bf4 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -409,6 +409,8 @@ 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 4c67967..36fcf9c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1225,7 +1225,7 @@ 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)
@@ -1259,6 +1259,25 @@ 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);

@@ -3418,7 +3437,8 @@ 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);

@@ -3438,13 +3458,58 @@ 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;
@@ -3461,26 +3526,12 @@ 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,
--
1.8.1.2


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