[PATCH RFC 1/2] cfq: request-deadline policy

[Konstantin Khlebnikov]

CFQ is designed for sharing disk bandwidth proportionally between queues and groups
and for reordering requests to reduce disks seek time. Currently it cannot
gurantee or estimate latency for individual requests, even if latencies are low
for almost all requests, some of them can stuck inside scheduler for a long time.
The fair policy is good as long as someone luckless begins to die due to a timeout.

This patch implements fifo requests dispatching with deadline policy: now cfq
obliged to dispatch request if it stuck in the queue for more than deadline.

This way now cfq can try to ensure the expected latency of requests execution.
It is like a safety valve, it should not work all time, but it should keep latency
in sane range when the scheduler is unable to effectively handle flow of requests,
especially in cases when the "noop" or "deadline" shows better performance.

deadline can be tuned via /sys/block/<device>/queue/iosched/deadline_{sync,async}
it by default 2000ms for sync and 4000ms for async requests, use 0 to disable it.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
---
 block/cfq-iosched.c |   93 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 93 insertions(+), 0 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f379943..8e68079 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -22,6 +22,7 @@
 /* max queue in one round of service */
 static const int cfq_quantum = 8;
 static const int cfq_fifo_expire[2] = { HZ / 4, HZ / 8 };
+static const int cfq_deadline[2] = { HZ * 4, HZ * 2 };
 /* maximum backwards seek, in KiB */
 static const int cfq_back_max = 16 * 1024;
 /* penalty of a backwards seek */
@@ -120,6 +121,11 @@ struct cfq_queue {
 	/* fifo list of requests in sort_list */
 	struct list_head fifo;
 
+	/* cached rq_fifo_time() for first rq in fifo */
+	unsigned long fifo_key;
+	/* cfqd->fifo_tree member */
+	struct rb_node fifo_node;
+
 	/* time when queue got scheduled in to dispatch first request. */
 	unsigned long dispatch_start;
 	unsigned int allocated_slice;
@@ -238,6 +244,11 @@ struct cfq_data {
 	 */
 	struct rb_root prio_trees[CFQ_PRIO_LISTS];
 
+	/* queues sorted by cfqq->fifo_key */
+	struct rb_root fifo_tree[2];
+	/* leftmost queue in fifo_tree */
+	struct cfq_queue *fifo_first[2];
+
 	unsigned int busy_queues;
 	unsigned int busy_sync_queues;
 
@@ -280,6 +291,7 @@ struct cfq_data {
 	 */
 	unsigned int cfq_quantum;
 	unsigned int cfq_fifo_expire[2];
+	unsigned int cfq_deadline[2];
 	unsigned int cfq_back_penalty;
 	unsigned int cfq_back_max;
 	unsigned int cfq_slice[2];
@@ -1416,6 +1428,46 @@ static void cfq_prio_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 		cfqq->p_root = NULL;
 }
 
+static void cfq_resort_fifo_tree(struct cfq_data *cfqd, struct cfq_queue *cfqq)
+{
+	struct rb_node **p, *parent;
+	struct cfq_queue *__cfqq;
+	int sync = cfq_cfqq_sync(cfqq);
+	int new_first = 1;
+
+	if (!RB_EMPTY_NODE(&cfqq->fifo_node)) {
+		if (cfqq == cfqd->fifo_first[sync]) {
+			parent = rb_next(&cfqq->fifo_node);
+			__cfqq = rb_entry(parent, struct cfq_queue, fifo_node);
+			cfqd->fifo_first[sync] = parent ? __cfqq : NULL;
+		}
+		rb_erase_init(&cfqq->fifo_node, &cfqd->fifo_tree[sync]);
+	}
+
+	if (list_empty(&cfqq->fifo) || !cfqd->cfq_deadline[sync] ||
+			cfq_class_idle(cfqq))
+		return;
+
+	cfqq->fifo_key = rq_fifo_time(rq_entry_fifo(cfqq->fifo.next));
+
+	parent = NULL;
+	p = &cfqd->fifo_tree[sync].rb_node;
+	while (*p) {
+		parent = *p;
+		__cfqq = rb_entry(parent, struct cfq_queue, fifo_node);
+		if (time_before_eq(__cfqq->fifo_key, cfqq->fifo_key)) {
+			p = &(*p)->rb_right;
+			new_first = 0;
+		} else
+			p = &(*p)->rb_left;
+	}
+	rb_link_node(&cfqq->fifo_node, parent, p);
+	rb_insert_color(&cfqq->fifo_node, &cfqd->fifo_tree[sync]);
+
+	if (new_first)
+		cfqd->fifo_first[sync] = cfqq;
+}
+
 /*
  * Update cfqq's position in the service tree.
  */
@@ -1444,6 +1496,7 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 		cfqd->busy_sync_queues++;
 
 	cfq_resort_rr_list(cfqd, cfqq);
+	cfq_resort_fifo_tree(cfqd, cfqq);
 }
 
 /*
@@ -1588,11 +1641,16 @@ static void cfq_deactivate_request(struct request_queue *q, struct request *rq)
 static void cfq_remove_request(struct request *rq)
 {
 	struct cfq_queue *cfqq = RQ_CFQQ(rq);
+	int fifo_first = rq->queuelist.prev == &cfqq->fifo;
 
 	if (cfqq->next_rq == rq)
 		cfqq->next_rq = cfq_find_next_rq(cfqq->cfqd, cfqq, rq);
 
 	list_del_init(&rq->queuelist);
+
+	if (fifo_first && !RB_EMPTY_NODE(&cfqq->fifo_node))
+		cfq_resort_fifo_tree(cfqq->cfqd, cfqq);
+
 	cfq_del_rq_rb(rq);
 
 	cfqq->cfqd->rq_queued--;
@@ -2573,6 +2631,27 @@ static bool cfq_dispatch_request(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	return true;
 }
 
+static bool cfq_dispatch_deadline(struct cfq_data *cfqd, int sync)
+{
+	struct cfq_queue *cfqq = cfqd->fifo_first[sync];
+	unsigned int deadline = cfqd->cfq_deadline[sync];
+
+	if (!cfqq || !deadline)
+		return false;
+
+	if (time_before(jiffies, cfqq->fifo_key + deadline))
+		return false;
+
+	cfq_log_cfqq(cfqd, cfqq, "dispatch deadline");
+	cfq_dispatch_insert(cfqd->queue, rq_entry_fifo(cfqq->fifo.next));
+
+	/* remove empty queue from service tree and expire its slices */
+	if (RB_EMPTY_ROOT(&cfqq->sort_list))
+		__cfq_slice_expired(cfqd, cfqq, 0);
+
+	return true;
+}
+
 /*
  * Find the cfqq that we need to service and move a request from that to the
  * dispatch list
@@ -2588,6 +2667,9 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
 	if (unlikely(force))
 		return cfq_forced_dispatch(cfqd);
 
+	if (cfq_dispatch_deadline(cfqd, 1) || cfq_dispatch_deadline(cfqd, 0))
+		return 1;
+
 	cfqq = cfq_select_queue(cfqd);
 	if (!cfqq)
 		return 0;
@@ -2924,6 +3006,7 @@ static void cfq_init_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 {
 	RB_CLEAR_NODE(&cfqq->rb_node);
 	RB_CLEAR_NODE(&cfqq->p_node);
+	RB_CLEAR_NODE(&cfqq->fifo_node);
 	INIT_LIST_HEAD(&cfqq->fifo);
 
 	cfqq->ref = 0;
@@ -4033,6 +4116,8 @@ static void *cfq_init_queue(struct request_queue *q)
 	for (i = 0; i < CFQ_PRIO_LISTS; i++)
 		cfqd->prio_trees[i] = RB_ROOT;
 
+	cfqd->fifo_tree[0] = cfqd->fifo_tree[1] = RB_ROOT;
+
 	/*
 	 * Our fallback cfqq if cfq_find_alloc_queue() runs into OOM issues.
 	 * Grab a permanent reference to it, so that the normal code flow
@@ -4055,6 +4140,8 @@ static void *cfq_init_queue(struct request_queue *q)
 	cfqd->cfq_quantum = cfq_quantum;
 	cfqd->cfq_fifo_expire[0] = cfq_fifo_expire[0];
 	cfqd->cfq_fifo_expire[1] = cfq_fifo_expire[1];
+	cfqd->cfq_deadline[0] = cfq_deadline[0];
+	cfqd->cfq_deadline[1] = cfq_deadline[1];
 	cfqd->cfq_back_max = cfq_back_max;
 	cfqd->cfq_back_penalty = cfq_back_penalty;
 	cfqd->cfq_slice[0] = cfq_slice_async;
@@ -4130,6 +4217,8 @@ static ssize_t __FUNC(struct elevator_queue *e, char *page)		\
 SHOW_FUNCTION(cfq_quantum_show, cfqd->cfq_quantum, 0);
 SHOW_FUNCTION(cfq_fifo_expire_sync_show, cfqd->cfq_fifo_expire[1], 1);
 SHOW_FUNCTION(cfq_fifo_expire_async_show, cfqd->cfq_fifo_expire[0], 1);
+SHOW_FUNCTION(cfq_deadline_sync_show, cfqd->cfq_deadline[1], 1);
+SHOW_FUNCTION(cfq_deadline_async_show, cfqd->cfq_deadline[0], 1);
 SHOW_FUNCTION(cfq_back_seek_max_show, cfqd->cfq_back_max, 0);
 SHOW_FUNCTION(cfq_back_seek_penalty_show, cfqd->cfq_back_penalty, 0);
 SHOW_FUNCTION(cfq_slice_idle_show, cfqd->cfq_slice_idle, 1);
@@ -4161,6 +4250,8 @@ STORE_FUNCTION(cfq_fifo_expire_sync_store, &cfqd->cfq_fifo_expire[1], 1,
 		UINT_MAX, 1);
 STORE_FUNCTION(cfq_fifo_expire_async_store, &cfqd->cfq_fifo_expire[0], 1,
 		UINT_MAX, 1);
+STORE_FUNCTION(cfq_deadline_sync_store, &cfqd->cfq_deadline[1], 0, UINT_MAX, 1);
+STORE_FUNCTION(cfq_deadline_async_store, &cfqd->cfq_deadline[0], 0, UINT_MAX, 1);
 STORE_FUNCTION(cfq_back_seek_max_store, &cfqd->cfq_back_max, 0, UINT_MAX, 0);
 STORE_FUNCTION(cfq_back_seek_penalty_store, &cfqd->cfq_back_penalty, 1,
 		UINT_MAX, 0);
@@ -4180,6 +4271,8 @@ static struct elv_fs_entry cfq_attrs[] = {
 	CFQ_ATTR(quantum),
 	CFQ_ATTR(fifo_expire_sync),
 	CFQ_ATTR(fifo_expire_async),
+	CFQ_ATTR(deadline_sync),
+	CFQ_ATTR(deadline_async),
 	CFQ_ATTR(back_seek_max),
 	CFQ_ATTR(back_seek_penalty),
 	CFQ_ATTR(slice_sync),

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