workqueue: implement attribute-based unbound worker_pool management

This patch makes unbound worker_pools reference counted and
dynamically created and destroyed as workqueues needing them come and
go.  All unbound worker_pools are hashed on unbound_pool_hash which is
keyed by the content of worker_pool->attrs.

When an unbound workqueue is allocated, get_unbound_pool() is called
with the attributes of the workqueue.  If there already is a matching
worker_pool, the reference count is bumped and the pool is returned.
If not, a new worker_pool with matching attributes is created and
returned.

When an unbound workqueue is destroyed, put_unbound_pool() is called
which decrements the reference count of the associated worker_pool.
If the refcnt reaches zero, the worker_pool is destroyed in sched-RCU
safe way.

Note that the standard unbound worker_pools - normal and highpri ones
with no specific cpumask affinity - are no longer created explicitly
during init_workqueues().  init_workqueues() only initializes
workqueue_attrs to be used for standard unbound pools -
unbound_std_wq_attrs[].  The pools are spawned on demand as workqueues
are created.

v2: - Comment added to init_worker_pool() explaining that @pool should
      be in a condition which can be passed to put_unbound_pool() even
      on failure.

    - pool->refcnt reaching zero and the pool being removed from
      unbound_pool_hash should be dynamic.  pool->refcnt is converted
      to int from atomic_t and now manipulated inside workqueue_lock.

    - Removed an incorrect sanity check on nr_idle in
      put_unbound_pool() which may trigger spuriously.

    All changes were suggested by Lai Jiangshan.

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

1 files changed, 224 insertions, 13 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index b0d3cbb83f63..3fe2c79bf166 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -41,6 +41,7 @@
 #include <linux/debug_locks.h>
 #include <linux/lockdep.h>
 #include <linux/idr.h>
+#include <linux/jhash.h>
 #include <linux/hashtable.h>
 #include <linux/rculist.h>
 
@@ -80,6 +81,7 @@ enum {
 
         NR_STD_WORKER_POOLS     = 2,            /* # standard pools per cpu */
 
+        UNBOUND_POOL_HASH_ORDER = 6,            /* hashed by pool->attrs */
         BUSY_WORKER_HASH_ORDER  = 6,            /* 64 pointers */
 
         MAX_IDLE_WORKERS_RATIO  = 4,            /* 1/4 of busy can be idle */
@@ -149,6 +151,8 @@ struct worker_pool {
         struct ida              worker_ida;     /* L: for worker IDs */
 
         struct workqueue_attrs  *attrs;         /* I: worker attributes */
+        struct hlist_node       hash_node;      /* R: unbound_pool_hash node */
+        int                     refcnt;         /* refcnt for unbound pools */
 
         /*
          * The current concurrency level.  As it's likely to be accessed
@@ -156,6 +160,12 @@ struct worker_pool {
          * cacheline.
          */
         atomic_t                nr_running ____cacheline_aligned_in_smp;
+
+        /*
+         * Destruction of pool is sched-RCU protected to allow dereferences
+         * from get_work_pool().
+         */
+        struct rcu_head         rcu;
 } ____cacheline_aligned_in_smp;
 
 /*
@@ -218,6 +228,11 @@ struct workqueue_struct {
 
 static struct kmem_cache *pwq_cache;
 
+/* hash of all unbound pools keyed by pool->attrs */
+static DEFINE_HASHTABLE(unbound_pool_hash, UNBOUND_POOL_HASH_ORDER);
+
+static struct workqueue_attrs *unbound_std_wq_attrs[NR_STD_WORKER_POOLS];
+
 struct workqueue_struct *system_wq __read_mostly;
 EXPORT_SYMBOL_GPL(system_wq);
 struct workqueue_struct *system_highpri_wq __read_mostly;
@@ -1742,7 +1757,7 @@ static struct worker *create_worker(struct worker_pool *pool)
         worker->pool = pool;
         worker->id = id;
 
-        if (pool->cpu != WORK_CPU_UNBOUND)
+        if (pool->cpu >= 0)
                 worker->task = kthread_create_on_node(worker_thread,
                                         worker, cpu_to_node(pool->cpu),
                                         "kworker/%d:%d%s", pool->cpu, id, pri);
@@ -3161,16 +3176,68 @@ fail:
         return NULL;
 }
 
+static void copy_workqueue_attrs(struct workqueue_attrs *to,
+                                 const struct workqueue_attrs *from)
+{
+        to->nice = from->nice;
+        cpumask_copy(to->cpumask, from->cpumask);
+}
+
+/*
+ * Hacky implementation of jhash of bitmaps which only considers the
+ * specified number of bits.  We probably want a proper implementation in
+ * include/linux/jhash.h.
+ */
+static u32 jhash_bitmap(const unsigned long *bitmap, int bits, u32 hash)
+{
+        int nr_longs = bits / BITS_PER_LONG;
+        int nr_leftover = bits % BITS_PER_LONG;
+        unsigned long leftover = 0;
+
+        if (nr_longs)
+                hash = jhash(bitmap, nr_longs * sizeof(long), hash);
+        if (nr_leftover) {
+                bitmap_copy(&leftover, bitmap + nr_longs, nr_leftover);
+                hash = jhash(&leftover, sizeof(long), hash);
+        }
+        return hash;
+}
+
+/* hash value of the content of @attr */
+static u32 wqattrs_hash(const struct workqueue_attrs *attrs)
+{
+        u32 hash = 0;
+
+        hash = jhash_1word(attrs->nice, hash);
+        hash = jhash_bitmap(cpumask_bits(attrs->cpumask), nr_cpu_ids, hash);
+        return hash;
+}
+
+/* content equality test */
+static bool wqattrs_equal(const struct workqueue_attrs *a,
+                          const struct workqueue_attrs *b)
+{
+        if (a->nice != b->nice)
+                return false;
+        if (!cpumask_equal(a->cpumask, b->cpumask))
+                return false;
+        return true;
+}
+
 /**
  * init_worker_pool - initialize a newly zalloc'd worker_pool
  * @pool: worker_pool to initialize
  *
  * Initiailize a newly zalloc'd @pool.  It also allocates @pool->attrs.
- * Returns 0 on success, -errno on failure.
+ * Returns 0 on success, -errno on failure.  Even on failure, all fields
+ * inside @pool proper are initialized and put_unbound_pool() can be called
+ * on @pool safely to release it.
  */
 static int init_worker_pool(struct worker_pool *pool)
 {
         spin_lock_init(&pool->lock);
+        pool->id = -1;
+        pool->cpu = -1;
         pool->flags |= POOL_DISASSOCIATED;
         INIT_LIST_HEAD(&pool->worklist);
         INIT_LIST_HEAD(&pool->idle_list);
@@ -3187,12 +3254,136 @@ static int init_worker_pool(struct worker_pool *pool)
         mutex_init(&pool->assoc_mutex);
         ida_init(&pool->worker_ida);
 
+        INIT_HLIST_NODE(&pool->hash_node);
+        pool->refcnt = 1;
+
+        /* shouldn't fail above this point */
         pool->attrs = alloc_workqueue_attrs(GFP_KERNEL);
         if (!pool->attrs)
                 return -ENOMEM;
         return 0;
 }
 
+static void rcu_free_pool(struct rcu_head *rcu)
+{
+        struct worker_pool *pool = container_of(rcu, struct worker_pool, rcu);
+
+        ida_destroy(&pool->worker_ida);
+        free_workqueue_attrs(pool->attrs);
+        kfree(pool);
+}
+
+/**
+ * put_unbound_pool - put a worker_pool
+ * @pool: worker_pool to put
+ *
+ * Put @pool.  If its refcnt reaches zero, it gets destroyed in sched-RCU
+ * safe manner.
+ */
+static void put_unbound_pool(struct worker_pool *pool)
+{
+        struct worker *worker;
+
+        spin_lock_irq(&workqueue_lock);
+        if (--pool->refcnt) {
+                spin_unlock_irq(&workqueue_lock);
+                return;
+        }
+
+        /* sanity checks */
+        if (WARN_ON(!(pool->flags & POOL_DISASSOCIATED)) ||
+            WARN_ON(!list_empty(&pool->worklist))) {
+                spin_unlock_irq(&workqueue_lock);
+                return;
+        }
+
+        /* release id and unhash */
+        if (pool->id >= 0)
+                idr_remove(&worker_pool_idr, pool->id);
+        hash_del(&pool->hash_node);
+
+        spin_unlock_irq(&workqueue_lock);
+
+        /* lock out manager and destroy all workers */
+        mutex_lock(&pool->manager_arb);
+        spin_lock_irq(&pool->lock);
+
+        while ((worker = first_worker(pool)))
+                destroy_worker(worker);
+        WARN_ON(pool->nr_workers || pool->nr_idle);
+
+        spin_unlock_irq(&pool->lock);
+        mutex_unlock(&pool->manager_arb);
+
+        /* shut down the timers */
+        del_timer_sync(&pool->idle_timer);
+        del_timer_sync(&pool->mayday_timer);
+
+        /* sched-RCU protected to allow dereferences from get_work_pool() */
+        call_rcu_sched(&pool->rcu, rcu_free_pool);
+}
+
+/**
+ * get_unbound_pool - get a worker_pool with the specified attributes
+ * @attrs: the attributes of the worker_pool to get
+ *
+ * Obtain a worker_pool which has the same attributes as @attrs, bump the
+ * reference count and return it.  If there already is a matching
+ * worker_pool, it will be used; otherwise, this function attempts to
+ * create a new one.  On failure, returns NULL.
+ */
+static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
+{
+        static DEFINE_MUTEX(create_mutex);
+        u32 hash = wqattrs_hash(attrs);
+        struct worker_pool *pool;
+        struct worker *worker;
+
+        mutex_lock(&create_mutex);
+
+        /* do we already have a matching pool? */
+        spin_lock_irq(&workqueue_lock);
+        hash_for_each_possible(unbound_pool_hash, pool, hash_node, hash) {
+                if (wqattrs_equal(pool->attrs, attrs)) {
+                        pool->refcnt++;
+                        goto out_unlock;
+                }
+        }
+        spin_unlock_irq(&workqueue_lock);
+
+        /* nope, create a new one */
+        pool = kzalloc(sizeof(*pool), GFP_KERNEL);
+        if (!pool || init_worker_pool(pool) < 0)
+                goto fail;
+
+        copy_workqueue_attrs(pool->attrs, attrs);
+
+        if (worker_pool_assign_id(pool) < 0)
+                goto fail;
+
+        /* create and start the initial worker */
+        worker = create_worker(pool);
+        if (!worker)
+                goto fail;
+
+        spin_lock_irq(&pool->lock);
+        start_worker(worker);
+        spin_unlock_irq(&pool->lock);
+
+        /* install */
+        spin_lock_irq(&workqueue_lock);
+        hash_add(unbound_pool_hash, &pool->hash_node, hash);
+out_unlock:
+        spin_unlock_irq(&workqueue_lock);
+        mutex_unlock(&create_mutex);
+        return pool;
+fail:
+        mutex_unlock(&create_mutex);
+        if (pool)
+                put_unbound_pool(pool);
+        return NULL;
+}
+
 static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 {
         bool highpri = wq->flags & WQ_HIGHPRI;
@@ -3217,7 +3408,12 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
                 if (!pwq)
                         return -ENOMEM;
 
-                pwq->pool = get_std_worker_pool(WORK_CPU_UNBOUND, highpri);
+                pwq->pool = get_unbound_pool(unbound_std_wq_attrs[highpri]);
+                if (!pwq->pool) {
+                        kmem_cache_free(pwq_cache, pwq);
+                        return -ENOMEM;
+                }
+
                 list_add_tail_rcu(&pwq->pwqs_node, &wq->pwqs);
         }
 
@@ -3395,6 +3591,15 @@ void destroy_workqueue(struct workqueue_struct *wq)
                 kfree(wq->rescuer);
         }
 
+        /*
+         * We're the sole accessor of @wq at this point.  Directly access
+         * the first pwq and put its pool.
+         */
+        if (wq->flags & WQ_UNBOUND) {
+                pwq = list_first_entry(&wq->pwqs, struct pool_workqueue,
+                                       pwqs_node);
+                put_unbound_pool(pwq->pool);
+        }
         free_pwqs(wq);
         kfree(wq);
 }
@@ -3857,19 +4062,14 @@ static int __init init_workqueues(void)
         hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
 
         /* initialize CPU pools */
-        for_each_wq_cpu(cpu) {
+        for_each_possible_cpu(cpu) {
                 struct worker_pool *pool;
 
                 i = 0;
                 for_each_std_worker_pool(pool, cpu) {
                         BUG_ON(init_worker_pool(pool));
                         pool->cpu = cpu;
-
-                        if (cpu != WORK_CPU_UNBOUND)
-                                cpumask_copy(pool->attrs->cpumask, cpumask_of(cpu));
-                        else
-                                cpumask_setall(pool->attrs->cpumask);
-
+                        cpumask_copy(pool->attrs->cpumask, cpumask_of(cpu));
                         pool->attrs->nice = std_nice[i++];
 
                         /* alloc pool ID */
@@ -3878,14 +4078,13 @@ static int __init init_workqueues(void)
         }
 
         /* create the initial worker */
-        for_each_online_wq_cpu(cpu) {
+        for_each_online_cpu(cpu) {
                 struct worker_pool *pool;
 
                 for_each_std_worker_pool(pool, cpu) {
                         struct worker *worker;
 
-                        if (cpu != WORK_CPU_UNBOUND)
-                                pool->flags &= ~POOL_DISASSOCIATED;
+                        pool->flags &= ~POOL_DISASSOCIATED;
 
                         worker = create_worker(pool);
                         BUG_ON(!worker);
@@ -3895,6 +4094,18 @@ static int __init init_workqueues(void)
                 }
         }
 
+        /* create default unbound wq attrs */
+        for (i = 0; i < NR_STD_WORKER_POOLS; i++) {
+                struct workqueue_attrs *attrs;
+
+                BUG_ON(!(attrs = alloc_workqueue_attrs(GFP_KERNEL)));
+
+                attrs->nice = std_nice[i];
+                cpumask_setall(attrs->cpumask);
+
+                unbound_std_wq_attrs[i] = attrs;
+        }
+
         system_wq = alloc_workqueue("events", 0, 0);
         system_highpri_wq = alloc_workqueue("events_highpri", WQ_HIGHPRI, 0);
         system_long_wq = alloc_workqueue("events_long", 0, 0);