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[dm-devel] [PATCH 25/23] io-controller: fix queue vs group fairness



o I found an issue during test and that is if there is a mix of queue and group
  at same level, there can be fairness issue. For example, consider following
  case.

			root
			/ \
		       T1  G1
			    |
			    T2

 T1 and T2 are two tasks with prio 0 and 7 respectively and G1 is the group
 with weight 900. 

 Task T1 prio 0 is mapped to weight 900 and it will get slice length of 180ms
 and then queue will expire and will be put after G1. (Note, in case of reader
 most liekly next request will come after queue expiry hence queue will be
 deleted and once the request comes in again, it will added to tree fresh. A
 fresh queue is added at the end of the tree. So it will be put after G1.).

 Now G1 will get to run (effectivly T2 will run), T2 has prio 7, which will
 map to weight 200 and get slice length of 40ms and will expire after that. Now
 G1 will a new vtime which is effectively charge of 40ms.

 Now to get fairness G1 should run more but instead T1 will be running as we
 gave it a vtime, same as G1. 

 The core issue here is that for readers, when slice expires, queue is empty
 and not backlogged hence it gets deleted from the tree. Because CFQ only
 operates in flat mode, it did a smart thing and did not keep a track of
 history. Instead it provides slice lenghts according to prio and if in one
 round of dispatch one gets fairness it is fine, otherwise upon queue expiry
 you will be placed at the end of service tree.

 This does not work in hierarchical setups where group's slice lenght is
 determined not by group' weight but by the weight of the queue which will
 run under the group.

 Hence we need to keep track of histroy and assign a new vtime based on disk
 time used by the current queue at the time of expiry.

 But here io scheduler is little different from CFS that at the time of expiry
 most of the time reader's queue is empty. So one will end up deleting it from
 the service tree and next request comes with-in 1 ms and it gets into the tree
 again like a new process.

 So we need to keep track of process io queue's vdisktime, even it after got
 deleted from io scheduler's service tree and use that same vdisktime if that
 queue gets backlogged again. But trusting a ioq's vdisktime is bad because
 it can lead to issues if a service tree min_vtime wrap around takes place 
 between two requests of the queue. (Agreed that it can be not that easy to
 hit but it is possible).

 Hence, keep a cache of io queues serviced recently and when a queue gets
 backlogged, if it is found in cache, use that vdisktime otherwise assign
 a new vdisktime. This cache of io queues (idle tree), is basically the idea
 implemented by BFQ guys. I had gotten rid of idle trees in V9 and now I am
 bringing it back. (Now I understand it better. :-)).

 There is one good side affect of keeping the cache of recently service io
 queues. Now CFQ can differentiate between streaming readers and new processes
 doing IO. Now for a new queue (which is not in the cache), we can assign a
 lower vdisktime and for a streaming reader, we assign vdisktime based on disk
 time used. This way small file readers or the processes doing small amount
 of IO will have reduced latencies at the cost of little reduced throughput of
 streaming readers.

Signed-off-by: Vivek Goyal <vgoyal redhat com>
---
 block/cfq-iosched.c |    2 
 block/elevator-fq.c |  252 ++++++++++++++++++++++++++++++++++++++++++++++++----
 block/elevator-fq.h |    9 +
 3 files changed, 246 insertions(+), 17 deletions(-)

Index: linux16/block/elevator-fq.c
===================================================================
--- linux16.orig/block/elevator-fq.c	2009-09-08 15:44:21.000000000 -0400
+++ linux16/block/elevator-fq.c	2009-09-08 15:47:45.000000000 -0400
@@ -52,6 +52,8 @@ static struct kmem_cache *elv_ioq_pool;
 #define elv_log_entity(entity, fmt, args...)
 #endif
 
+static void check_idle_tree_release(struct io_service_tree *st);
+
 static inline struct io_queue *ioq_of(struct io_entity *entity)
 {
 	if (entity->my_sd == NULL)
@@ -109,6 +111,11 @@ elv_prio_to_slice(struct elv_fq_data *ef
 	return elv_weight_slice(efqd, elv_ioq_sync(ioq), ioq->entity.weight);
 }
 
+static inline int vdisktime_gt(u64 a, u64 b)
+{
+	return (s64)(a - b) > 0;
+}
+
 static inline u64 max_vdisktime(u64 min_vdisktime, u64 vdisktime)
 {
 	s64 delta = (s64)(vdisktime - min_vdisktime);
@@ -145,6 +152,7 @@ static void update_min_vdisktime(struct 
 	}
 
 	st->min_vdisktime = max_vdisktime(st->min_vdisktime, vdisktime);
+	check_idle_tree_release(st);
 }
 
 static inline struct io_entity *parent_entity(struct io_entity *entity)
@@ -411,27 +419,46 @@ static void place_entity(struct io_servi
 	struct rb_node *parent;
 	struct io_entity *entry;
 	int nr_active = st->nr_active - 1;
+	struct io_queue *ioq = ioq_of(entity);
+	int sync = 1;
+
+	if (ioq)
+		sync = elv_ioq_sync(ioq);
+
+	if (add_front || !nr_active) {
+		vdisktime = st->min_vdisktime;
+		goto done;
+	}
+
+	if (sync && entity->vdisktime
+	    && vdisktime_gt(entity->vdisktime, st->min_vdisktime)) {
+		/* vdisktime still in future. Use old vdisktime */
+		vdisktime = entity->vdisktime;
+		goto done;
+	}
 
 	/*
-	 * Currently put entity at the end of last entity. This probably will
-	 * require adjustments as we move along
+	 * Effectively a new queue. Assign sync queue a lower vdisktime so
+	 * we can achieve better latencies for small file readers. For async
+	 * queues, put them at the end of the existing queue.
+	 * Group entities are always considered sync.
 	 */
-	if (io_entity_class_idle(entity)) {
-		vdisktime = elv_delta_fair(ELV_IDLE_DELAY, entity);
-		parent = rb_last(&st->active);
-		if (parent) {
-			entry = rb_entry(parent, struct io_entity, rb_node);
-			vdisktime += entry->vdisktime;
-		}
-	} else if (!add_front && nr_active) {
-		parent = rb_last(&st->active);
-		if (parent) {
-			entry = rb_entry(parent, struct io_entity, rb_node);
-			vdisktime = entry->vdisktime;
-		}
-	} else
+	if (sync) {
 		vdisktime = st->min_vdisktime;
+		goto done;
+	}
 
+	/*
+	 * Put entity at the end of the tree. Effectively async queues use
+	 * this path.
+	 */
+	parent = rb_last(&st->active);
+	if (parent) {
+		entry = rb_entry(parent, struct io_entity, rb_node);
+		vdisktime = entry->vdisktime;
+	} else
+		vdisktime = st->min_vdisktime;
+done:
 	entity->vdisktime = max_vdisktime(st->min_vdisktime, vdisktime);
 	elv_log_entity(entity, "place_entity: vdisktime=%llu"
 			" min_vdisktime=%llu", entity->vdisktime,
@@ -447,6 +474,122 @@ static inline void io_entity_update_prio
 		 */
 		init_io_entity_service_tree(entity, parent_entity(entity));
 		entity->ioprio_changed = 0;
+
+		/*
+		 * Assign this entity a fresh vdisktime instead of using
+		 * previous one as prio class will lead to service tree
+		 * change and this vdisktime will not be valid on new
+		 * service tree.
+		 *
+		 * TODO: Handle the case of only prio change.
+		 */
+		entity->vdisktime = 0;
+	}
+}
+
+static void
+__dequeue_io_entity_idle(struct io_service_tree *st, struct io_entity *entity)
+{
+	if (st->rb_leftmost_idle == &entity->rb_node) {
+		struct rb_node *next_node;
+
+		next_node = rb_next(&entity->rb_node);
+		st->rb_leftmost_idle = next_node;
+	}
+
+	rb_erase(&entity->rb_node, &st->idle);
+	RB_CLEAR_NODE(&entity->rb_node);
+}
+
+static void dequeue_io_entity_idle(struct io_entity *entity)
+{
+	struct io_queue *ioq = ioq_of(entity);
+
+	__dequeue_io_entity_idle(entity->st, entity);
+	entity->on_idle_st = 0;
+	if (ioq)
+		elv_put_ioq(ioq);
+}
+
+static void
+__enqueue_io_entity_idle(struct io_service_tree *st, struct io_entity *entity)
+{
+	struct rb_node **node = &st->idle.rb_node;
+	struct rb_node *parent = NULL;
+	struct io_entity *entry;
+	int leftmost = 1;
+
+	while (*node != NULL) {
+		parent = *node;
+		entry = rb_entry(parent, struct io_entity, rb_node);
+
+		if (vdisktime_gt(entry->vdisktime, entity->vdisktime))
+			node = &parent->rb_left;
+		else {
+			node = &parent->rb_right;
+			leftmost = 0;
+		}
+	}
+
+	/*
+	 * Maintain a cache of leftmost tree entries (it is frequently
+	 * used)
+	 */
+	if (leftmost)
+		st->rb_leftmost_idle = &entity->rb_node;
+
+	rb_link_node(&entity->rb_node, parent, node);
+	rb_insert_color(&entity->rb_node, &st->idle);
+}
+
+static void enqueue_io_entity_idle(struct io_entity *entity)
+{
+	struct io_queue *ioq = ioq_of(entity);
+	struct io_group *parent_iog;
+
+	/*
+	 * Don't put an entity on idle tree if it has been marked for deletion.
+	 * We are not expecting more io from this entity. No need to cache it
+	 */
+
+	if (entity->exiting)
+		return;
+
+	/*
+	 * If parent group is exiting, don't put on idle tree. May be task got
+	 * moved to a different cgroup and original cgroup got deleted
+	 */
+	parent_iog = iog_of(parent_entity(entity));
+	if (parent_iog->entity.exiting)
+		return;
+
+	if (ioq)
+		elv_get_ioq(ioq);
+	__enqueue_io_entity_idle(entity->st, entity);
+	entity->on_idle_st = 1;
+}
+
+static void check_idle_tree_release(struct io_service_tree *st)
+{
+	struct io_entity *leftmost;
+
+	if (!st->rb_leftmost_idle)
+		return;
+
+	leftmost = rb_entry(st->rb_leftmost_idle, struct io_entity, rb_node);
+
+	if (vdisktime_gt(st->min_vdisktime, leftmost->vdisktime))
+		dequeue_io_entity_idle(leftmost);
+}
+
+static void flush_idle_tree(struct io_service_tree *st)
+{
+	struct io_entity *entity;
+
+	while(st->rb_leftmost_idle) {
+		entity = rb_entry(st->rb_leftmost_idle, struct io_entity,
+					rb_node);
+		dequeue_io_entity_idle(entity);
 	}
 }
 
@@ -483,6 +626,9 @@ static void dequeue_io_entity(struct io_
 	st->nr_active--;
 	sd->nr_active--;
 	debug_update_stats_dequeue(entity);
+
+	if (vdisktime_gt(entity->vdisktime, st->min_vdisktime))
+		enqueue_io_entity_idle(entity);
 }
 
 static void
@@ -524,6 +670,16 @@ static void enqueue_io_entity(struct io_
 	struct io_service_tree *st;
 	struct io_sched_data *sd = io_entity_sched_data(entity);
 
+	if (entity->on_idle_st)
+		dequeue_io_entity_idle(entity);
+	else
+		/*
+		 * This entity was not in idle tree cache. Zero out vdisktime
+		 * so that we don't rely on old vdisktime instead assign a
+		 * fresh one.
+		 */
+		entity->vdisktime = 0;
+
 	io_entity_update_prio(entity);
 	st = entity->st;
 	st->nr_active++;
@@ -574,6 +730,8 @@ static void requeue_io_entity(struct io_
 	struct io_service_tree *st = entity->st;
 	struct io_entity *next_entity;
 
+	entity->vdisktime = 0;
+
 	if (add_front) {
 		next_entity = __lookup_next_io_entity(st);
 
@@ -1937,11 +2095,18 @@ static void io_free_root_group(struct el
 {
 	struct io_group *iog = e->efqd->root_group;
 	struct io_cgroup *iocg = &io_root_cgroup;
+	struct io_service_tree *st;
+	int i;
 
 	spin_lock_irq(&iocg->lock);
 	hlist_del_rcu(&iog->group_node);
 	spin_unlock_irq(&iocg->lock);
 
+	for (i = 0; i < IO_IOPRIO_CLASSES; i++) {
+		st = iog->sched_data.service_tree + i;
+		flush_idle_tree(st);
+	}
+
 	put_io_group_queues(e, iog);
 	elv_put_iog(iog);
 }
@@ -2039,9 +2204,29 @@ EXPORT_SYMBOL(elv_put_iog);
  */
 static void __io_destroy_group(struct elv_fq_data *efqd, struct io_group *iog)
 {
+	struct io_service_tree *st;
+	int i;
+	struct io_entity *entity = &iog->entity;
+
+	/*
+	 * Mark io group for deletion so that no new entry goes in
+	 * idle tree. Any active queue which is removed from active
+	 * tree will not be put in to idle tree.
+	 */
+	entity->exiting = 1;
+
+	/* We flush idle tree now, and don't put things in there any more. */
+	for (i = 0; i < IO_IOPRIO_CLASSES; i++) {
+		st = iog->sched_data.service_tree + i;
+		flush_idle_tree(st);
+	}
+
 	hlist_del(&iog->elv_data_node);
 	put_io_group_queues(efqd->eq, iog);
 
+	if (entity->on_idle_st)
+		dequeue_io_entity_idle(entity);
+
 	/*
 	 * Put the reference taken at the time of creation so that when all
 	 * queues are gone, group can be destroyed.
@@ -2374,7 +2559,13 @@ static struct io_group *io_alloc_root_gr
 static void io_free_root_group(struct elevator_queue *e)
 {
 	struct io_group *iog = e->efqd->root_group;
+	struct io_service_tree *st;
+	int i;
 
+	for (i = 0; i < IO_IOPRIO_CLASSES; i++) {
+		st = iog->sched_data.service_tree + i;
+		flush_idle_tree(st);
+	}
 	put_io_group_queues(e, iog);
 	kfree(iog);
 }
@@ -3257,6 +3448,35 @@ done:
 		elv_schedule_dispatch(q);
 }
 
+/*
+ * The process associted with ioq (in case of cfq), is going away. Mark it
+ * for deletion.
+ */
+void elv_exit_ioq(struct io_queue *ioq)
+{
+	struct io_entity *entity = &ioq->entity;
+
+	/*
+	 * Async ioq's belong to io group and are cleaned up once group is
+	 * being deleted. Not need to do any cleanup here even if cfq has
+	 * dropped the reference to the queue
+	 */
+	if (!elv_ioq_sync(ioq))
+		return;
+
+	/*
+ 	 * This queue is still under service. Just mark it so that once all
+	 * the IO from queue is done, it is not put back in idle tree.
+	 */
+	if (entity->on_st) {
+		entity->exiting = 1;
+		return;
+	} else if(entity->on_idle_st) {
+		/* Remove ioq from idle tree */
+		dequeue_io_entity_idle(entity);
+	}
+}
+EXPORT_SYMBOL(elv_exit_ioq);
 static void elv_slab_kill(void)
 {
 	/*
Index: linux16/block/cfq-iosched.c
===================================================================
--- linux16.orig/block/cfq-iosched.c	2009-09-08 15:43:36.000000000 -0400
+++ linux16/block/cfq-iosched.c	2009-09-08 15:47:45.000000000 -0400
@@ -1138,6 +1138,7 @@ static void cfq_exit_cfqq(struct cfq_dat
 		elv_schedule_dispatch(cfqd->queue);
 	}
 
+	elv_exit_ioq(cfqq->ioq);
 	cfq_put_queue(cfqq);
 }
 
@@ -1373,6 +1374,7 @@ static void changed_cgroup(struct io_con
 		 */
 		if (iog != __iog) {
 			cic_set_cfqq(cic, NULL, 1);
+			elv_exit_ioq(sync_cfqq->ioq);
 			cfq_put_queue(sync_cfqq);
 		}
 	}
Index: linux16/block/elevator-fq.h
===================================================================
--- linux16.orig/block/elevator-fq.h	2009-09-08 15:43:36.000000000 -0400
+++ linux16/block/elevator-fq.h	2009-09-08 15:47:45.000000000 -0400
@@ -33,6 +33,10 @@ struct io_service_tree {
 	u64 min_vdisktime;
 	struct rb_node *rb_leftmost;
 	unsigned int nr_active;
+
+        /* A cache of io entities which were served and expired */
+        struct rb_root idle;
+        struct rb_node *rb_leftmost_idle;
 };
 
 struct io_sched_data {
@@ -44,9 +48,12 @@ struct io_sched_data {
 struct io_entity {
 	struct rb_node rb_node;
 	int on_st;
+	int on_idle_st;
 	u64 vdisktime;
 	unsigned int weight;
 	struct io_entity *parent;
+	/* This io entity (queue or group) has been marked for deletion */
+	unsigned int exiting;
 
 	struct io_sched_data *my_sd;
 	struct io_service_tree *st;
@@ -572,7 +579,7 @@ extern struct io_queue *elv_alloc_ioq(st
 extern void elv_free_ioq(struct io_queue *ioq);
 extern struct io_group *ioq_to_io_group(struct io_queue *ioq);
 extern int elv_iog_should_idle(struct io_queue *ioq);
-
+extern void elv_exit_ioq(struct io_queue *ioq);
 #else /* CONFIG_ELV_FAIR_QUEUING */
 static inline struct elv_fq_data *
 elv_alloc_fq_data(struct request_queue *q, struct elevator_queue *e)


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