block, cfq: move icq creation and rq->elv.icq association to block core

Now block layer knows everything necessary to create and associate
icq's with requests.  Move ioc_create_icq() to blk-ioc.c and update
get_request() such that, if elevator_type->icq_size is set, requests
are automatically associated with their matching icq's before
elv_set_request().  io_context reference is also managed by block core
on request alloc/free.

* Only ioprio/cgroup changed handling remains from cfq_get_cic().
  Collapsed into cfq_set_request().

* This removes queue kicking on icq allocation failure (for now).  As
  icq allocation failure is rare and the only effect of queue kicking
  achieved was possibily accelerating queue processing, this change
  shouldn't be noticeable.

  There is a larger underlying problem.  Unlike request allocation,
  icq allocation is not guaranteed to succeed eventually after
  retries.  The number of icq is unbound and thus mempool can't be the
  solution either.  This effectively adds allocation dependency on
  memory free path and thus possibility of deadlock.

  This usually wouldn't happen because icq allocation is not a hot
  path and, even when the condition triggers, it's highly unlikely
  that none of the writeback workers already has icq.

  However, this is still possible especially if elevator is being
  switched under high memory pressure, so we better get it fixed.
  Probably the only solution is just bypassing elevator and appending
  to dispatch queue on any elevator allocation failure.

* Comment added to explain how icq's are managed and synchronized.

This completes cleanup of io_context interface.

Signed-off-by: Tejun Heo <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/blk-core.c

@@ -640,13 +640,18 @@ EXPORT_SYMBOL(blk_get_queue);
 
 static inline void blk_free_request(struct request_queue *q, struct request *rq)
 {
-	if (rq->cmd_flags & REQ_ELVPRIV)
+	if (rq->cmd_flags & REQ_ELVPRIV) {
 		elv_put_request(q, rq);
+		if (rq->elv.icq)
+			put_io_context(rq->elv.icq->ioc, q);
+	}
+
 	mempool_free(rq, q->rq.rq_pool);
 }
 
 static struct request *
-blk_alloc_request(struct request_queue *q, unsigned int flags, gfp_t gfp_mask)
+blk_alloc_request(struct request_queue *q, struct io_cq *icq,
+		  unsigned int flags, gfp_t gfp_mask)
 {
 	struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);
 
@@ -657,10 +662,15 @@ blk_alloc_request(struct request_queue *q, unsigned int flags, gfp_t gfp_mask)
 
 	rq->cmd_flags = flags | REQ_ALLOCED;
 
-	if ((flags & REQ_ELVPRIV) &&
-	    unlikely(elv_set_request(q, rq, gfp_mask))) {
-		mempool_free(rq, q->rq.rq_pool);
-		return NULL;
+	if (flags & REQ_ELVPRIV) {
+		rq->elv.icq = icq;
+		if (unlikely(elv_set_request(q, rq, gfp_mask))) {
+			mempool_free(rq, q->rq.rq_pool);
+			return NULL;
+		}
+		/* @rq->elv.icq holds on to io_context until @rq is freed */
+		if (icq)
+			get_io_context(icq->ioc);
 	}
 
 	return rq;
@@ -772,11 +782,14 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
 {
 	struct request *rq = NULL;
 	struct request_list *rl = &q->rq;
+	struct elevator_type *et;
 	struct io_context *ioc;
+	struct io_cq *icq = NULL;
 	const bool is_sync = rw_is_sync(rw_flags) != 0;
 	bool retried = false;
 	int may_queue;
 retry:
+	et = q->elevator->type;
 	ioc = current->io_context;
 
 	if (unlikely(blk_queue_dead(q)))
@@ -837,17 +850,36 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
 	rl->count[is_sync]++;
 	rl->starved[is_sync] = 0;
 
+	/*
+	 * Decide whether the new request will be managed by elevator.  If
+	 * so, mark @rw_flags and increment elvpriv.  Non-zero elvpriv will
+	 * prevent the current elevator from being destroyed until the new
+	 * request is freed.  This guarantees icq's won't be destroyed and
+	 * makes creating new ones safe.
+	 *
+	 * Also, lookup icq while holding queue_lock.  If it doesn't exist,
+	 * it will be created after releasing queue_lock.
+	 */
 	if (blk_rq_should_init_elevator(bio) &&
 	    !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags)) {
 		rw_flags |= REQ_ELVPRIV;
 		rl->elvpriv++;
+		if (et->icq_cache && ioc)
+			icq = ioc_lookup_icq(ioc, q);
 	}
 
 	if (blk_queue_io_stat(q))
 		rw_flags |= REQ_IO_STAT;
 	spin_unlock_irq(q->queue_lock);
 
-	rq = blk_alloc_request(q, rw_flags, gfp_mask);
+	/* create icq if missing */
+	if (unlikely(et->icq_cache && !icq))
+		icq = ioc_create_icq(q, gfp_mask);
+
+	/* rqs are guaranteed to have icq on elv_set_request() if requested */
+	if (likely(!et->icq_cache || icq))
+		rq = blk_alloc_request(q, icq, rw_flags, gfp_mask);
+
 	if (unlikely(!rq)) {
 		/*
 		 * Allocation failed presumably due to memory. Undo anything

block/blk-ioc.c

@@ -289,7 +289,6 @@ void create_io_context_slowpath(struct task_struct *task, gfp_t gfp_flags,
 		kmem_cache_free(iocontext_cachep, ioc);
 	task_unlock(task);
 }
-EXPORT_SYMBOL(create_io_context_slowpath);
 
 /**
  * get_task_io_context - get io_context of a task
@@ -362,6 +361,65 @@ struct io_cq *ioc_lookup_icq(struct io_context *ioc, struct request_queue *q)
 }
 EXPORT_SYMBOL(ioc_lookup_icq);
 
+/**
+ * ioc_create_icq - create and link io_cq
+ * @q: request_queue of interest
+ * @gfp_mask: allocation mask
+ *
+ * Make sure io_cq linking %current->io_context and @q exists.  If either
+ * io_context and/or icq don't exist, they will be created using @gfp_mask.
+ *
+ * The caller is responsible for ensuring @ioc won't go away and @q is
+ * alive and will stay alive until this function returns.
+ */
+struct io_cq *ioc_create_icq(struct request_queue *q, gfp_t gfp_mask)
+{
+	struct elevator_type *et = q->elevator->type;
+	struct io_context *ioc;
+	struct io_cq *icq;
+
+	/* allocate stuff */
+	ioc = create_io_context(current, gfp_mask, q->node);
+	if (!ioc)
+		return NULL;
+
+	icq = kmem_cache_alloc_node(et->icq_cache, gfp_mask | __GFP_ZERO,
+				    q->node);
+	if (!icq)
+		return NULL;
+
+	if (radix_tree_preload(gfp_mask) < 0) {
+		kmem_cache_free(et->icq_cache, icq);
+		return NULL;
+	}
+
+	icq->ioc = ioc;
+	icq->q = q;
+	INIT_LIST_HEAD(&icq->q_node);
+	INIT_HLIST_NODE(&icq->ioc_node);
+
+	/* lock both q and ioc and try to link @icq */
+	spin_lock_irq(q->queue_lock);
+	spin_lock(&ioc->lock);
+
+	if (likely(!radix_tree_insert(&ioc->icq_tree, q->id, icq))) {
+		hlist_add_head(&icq->ioc_node, &ioc->icq_list);
+		list_add(&icq->q_node, &q->icq_list);
+		if (et->ops.elevator_init_icq_fn)
+			et->ops.elevator_init_icq_fn(icq);
+	} else {
+		kmem_cache_free(et->icq_cache, icq);
+		icq = ioc_lookup_icq(ioc, q);
+		if (!icq)
+			printk(KERN_ERR "cfq: icq link failed!\n");
+	}
+
+	spin_unlock(&ioc->lock);
+	spin_unlock_irq(q->queue_lock);
+	radix_tree_preload_end();
+	return icq;
+}
+
 void ioc_set_changed(struct io_context *ioc, int which)
 {
 	struct io_cq *icq;

block/blk.h

@@ -200,6 +200,7 @@ static inline int blk_do_io_stat(struct request *rq)
  */
 void get_io_context(struct io_context *ioc);
 struct io_cq *ioc_lookup_icq(struct io_context *ioc, struct request_queue *q);
+struct io_cq *ioc_create_icq(struct request_queue *q, gfp_t gfp_mask);
 void ioc_clear_queue(struct request_queue *q);
 
 void create_io_context_slowpath(struct task_struct *task, gfp_t gfp_mask,

block/cfq-iosched.c

@@ -2935,117 +2935,6 @@ cfq_get_queue(struct cfq_data *cfqd, bool is_sync, struct io_context *ioc,
 	return cfqq;
 }
 
-/**
- * ioc_create_icq - create and link io_cq
- * @q: request_queue of interest
- * @gfp_mask: allocation mask
- *
- * Make sure io_cq linking %current->io_context and @q exists.  If either
- * io_context and/or icq don't exist, they will be created using @gfp_mask.
- *
- * The caller is responsible for ensuring @ioc won't go away and @q is
- * alive and will stay alive until this function returns.
- */
-static struct io_cq *ioc_create_icq(struct request_queue *q, gfp_t gfp_mask)
-{
-	struct elevator_type *et = q->elevator->type;
-	struct io_context *ioc;
-	struct io_cq *icq;
-
-	/* allocate stuff */
-	ioc = create_io_context(current, gfp_mask, q->node);
-	if (!ioc)
-		return NULL;
-
-	icq = kmem_cache_alloc_node(et->icq_cache, gfp_mask | __GFP_ZERO,
-				    q->node);
-	if (!icq)
-		return NULL;
-
-	if (radix_tree_preload(gfp_mask) < 0) {
-		kmem_cache_free(et->icq_cache, icq);
-		return NULL;
-	}
-
-	icq->ioc = ioc;
-	icq->q = q;
-	INIT_LIST_HEAD(&icq->q_node);
-	INIT_HLIST_NODE(&icq->ioc_node);
-
-	/* lock both q and ioc and try to link @icq */
-	spin_lock_irq(q->queue_lock);
-	spin_lock(&ioc->lock);
-
-	if (likely(!radix_tree_insert(&ioc->icq_tree, q->id, icq))) {
-		hlist_add_head(&icq->ioc_node, &ioc->icq_list);
-		list_add(&icq->q_node, &q->icq_list);
-		if (et->ops.elevator_init_icq_fn)
-			et->ops.elevator_init_icq_fn(icq);
-	} else {
-		kmem_cache_free(et->icq_cache, icq);
-		icq = ioc_lookup_icq(ioc, q);
-		if (!icq)
-			printk(KERN_ERR "cfq: icq link failed!\n");
-	}
-
-	spin_unlock(&ioc->lock);
-	spin_unlock_irq(q->queue_lock);
-	radix_tree_preload_end();
-	return icq;
-}
-
-/**
- * cfq_get_cic - acquire cfq_io_cq and bump refcnt on io_context
- * @cfqd: cfqd to setup cic for
- * @gfp_mask: allocation mask
- *
- * Return cfq_io_cq associating @cfqd and %current->io_context and
- * bump refcnt on io_context.  If ioc or cic doesn't exist, they're created
- * using @gfp_mask.
- *
- * Must be called under queue_lock which may be released and re-acquired.
- * This function also may sleep depending on @gfp_mask.
- */
-static struct cfq_io_cq *cfq_get_cic(struct cfq_data *cfqd, gfp_t gfp_mask)
-{
-	struct request_queue *q = cfqd->queue;
-	struct cfq_io_cq *cic = NULL;
-	struct io_context *ioc;
-
-	lockdep_assert_held(q->queue_lock);
-
-	while (true) {
-		/* fast path */
-		ioc = current->io_context;
-		if (likely(ioc)) {
-			cic = cfq_cic_lookup(cfqd, ioc);
-			if (likely(cic))
-				break;
-		}
-
-		/* slow path - unlock, create missing ones and retry */
-		spin_unlock_irq(q->queue_lock);
-		cic = icq_to_cic(ioc_create_icq(q, gfp_mask));
-		spin_lock_irq(q->queue_lock);
-		if (!cic)
-			return NULL;
-	}
-
-	/* bump @ioc's refcnt and handle changed notifications */
-	get_io_context(ioc);
-
-	if (unlikely(cic->icq.changed)) {
-		if (test_and_clear_bit(ICQ_IOPRIO_CHANGED, &cic->icq.changed))
-			changed_ioprio(cic);
-#ifdef CONFIG_CFQ_GROUP_IOSCHED
-		if (test_and_clear_bit(ICQ_CGROUP_CHANGED, &cic->icq.changed))
-			changed_cgroup(cic);
-#endif
-	}
-
-	return cic;
-}
-
 static void
 __cfq_update_io_thinktime(struct cfq_ttime *ttime, unsigned long slice_idle)
 {
@@ -3524,8 +3413,6 @@ static void cfq_put_request(struct request *rq)
 		BUG_ON(!cfqq->allocated[rw]);
 		cfqq->allocated[rw]--;
 
-		put_io_context(RQ_CIC(rq)->icq.ioc, cfqq->cfqd->queue);
-
 		/* Put down rq reference on cfqg */
 		cfq_put_cfqg(RQ_CFQG(rq));
 		rq->elv.priv[0] = NULL;
@@ -3574,17 +3461,24 @@ static int
 cfq_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
 {
 	struct cfq_data *cfqd = q->elevator->elevator_data;
-	struct cfq_io_cq *cic;
+	struct cfq_io_cq *cic = icq_to_cic(rq->elv.icq);
 	const int rw = rq_data_dir(rq);
 	const bool is_sync = rq_is_sync(rq);
 	struct cfq_queue *cfqq;
 
 	might_sleep_if(gfp_mask & __GFP_WAIT);
 
 	spin_lock_irq(q->queue_lock);
-	cic = cfq_get_cic(cfqd, gfp_mask);
-	if (!cic)
-		goto queue_fail;
+
+	/* handle changed notifications */
+	if (unlikely(cic->icq.changed)) {
+		if (test_and_clear_bit(ICQ_IOPRIO_CHANGED, &cic->icq.changed))
+			changed_ioprio(cic);
+#ifdef CONFIG_CFQ_GROUP_IOSCHED
+		if (test_and_clear_bit(ICQ_CGROUP_CHANGED, &cic->icq.changed))
+			changed_cgroup(cic);
+#endif
+	}
 
 new_queue:
 	cfqq = cic_to_cfqq(cic, is_sync);
@@ -3615,17 +3509,10 @@ cfq_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
 	cfqq->allocated[rw]++;
 
 	cfqq->ref++;
-	rq->elv.icq = &cic->icq;
 	rq->elv.priv[0] = cfqq;
 	rq->elv.priv[1] = cfq_ref_get_cfqg(cfqq->cfqg);
 	spin_unlock_irq(q->queue_lock);
 	return 0;
-
-queue_fail:
-	cfq_schedule_dispatch(cfqd);
-	spin_unlock_irq(q->queue_lock);
-	cfq_log(cfqd, "set_request fail");
-	return 1;
 }
 
 static void cfq_kick_queue(struct work_struct *work)

include/linux/elevator.h

@@ -60,8 +60,8 @@ struct elevator_ops
 	elevator_request_list_fn *elevator_former_req_fn;
 	elevator_request_list_fn *elevator_latter_req_fn;
 
-	elevator_init_icq_fn *elevator_init_icq_fn;
-	elevator_exit_icq_fn *elevator_exit_icq_fn;
+	elevator_init_icq_fn *elevator_init_icq_fn;	/* see iocontext.h */
+	elevator_exit_icq_fn *elevator_exit_icq_fn;	/* ditto */
 
 	elevator_set_req_fn *elevator_set_req_fn;
 	elevator_put_req_fn *elevator_put_req_fn;
@@ -90,8 +90,8 @@ struct elevator_type
 
 	/* fields provided by elevator implementation */
 	struct elevator_ops ops;
-	size_t icq_size;
-	size_t icq_align;
+	size_t icq_size;	/* see iocontext.h */
+	size_t icq_align;	/* ditto */
 	struct elv_fs_entry *elevator_attrs;
 	char elevator_name[ELV_NAME_MAX];
 	struct module *elevator_owner;