diff mbox

[RFC,14/22] block, bfq: improve responsiveness

Message ID 1454364778-25179-15-git-send-email-paolo.valente@linaro.org
State New, archived
Headers show

Commit Message

Paolo Valente Feb. 1, 2016, 10:12 p.m. UTC
This patch introduces a simple heuristic to load applications quickly,
and to perform the I/O requested by interactive applications just as
quickly. To this purpose, both a newly-created queue and a queue
associated with an interactive application (we explain in a moment how
BFQ decides whether the associated application is interactive),
receive the following three special treatments:

1) The weight of the queue is raised.

2) The queue unconditionally enjoys device idling when it empties; in
fact, if the requests of a queue are sync, then performing device
idling for the queue is a necessary condition to guarantee that the
queue receives a fraction of the throughput proportional to its weight
(see [1] for details).

3) The device-idling timeout is larger for the queue. This reduces the
probability that the queue is expired because its next request does
not arrive in time.

For brevity, we call just weight-raising the combination of these
three preferential treatments. For a newly-created queue,
weight-raising starts immediately and lasts for a time interval that:
1) depends on the device speed and type (rotational or
non-rotational), and 2) is equal to the time needed to load (start up)
a large-size application on that device, with cold caches and with no
additional workload.

Finally, as for guaranteeing a fast execution to interactive,
I/O-related tasks (such as opening a file), consider that any
interactive application blocks and waits for user input both after
starting up and after executing some task. After a while, the user may
trigger new operations, after which the application stops again, and
so on. Accordingly, the low-latency heuristic weight-raises again a
queue in case it becomes backlogged after being idle for a
sufficiently long (configurable) time. The weight-raising then lasts
for the same time as for a just-created queue.

According to our experiments, the combination of this low-latency
heuristic and of the improvements described in the previous patch
allows BFQ to guarantee a high application responsiveness.

[1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
    Scheduler", Proceedings of the First Workshop on Mobile System
    Technologies (MST-2015), May 2015.
    http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
---
 block/Kconfig.iosched |   3 +-
 block/bfq.h           |  34 ++++
 block/cfq-iosched.c   | 475 ++++++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 480 insertions(+), 32 deletions(-)
diff mbox

Patch

diff --git a/block/Kconfig.iosched b/block/Kconfig.iosched
index 143d44b..ab2dc5a 100644
--- a/block/Kconfig.iosched
+++ b/block/Kconfig.iosched
@@ -28,7 +28,8 @@  config IOSCHED_CFQ
 	  The CFQ I/O scheduler, now internally replaced by BFQ, tries
 	  to distribute bandwidth among all processes according to
 	  their weights, regardless of the device parameters and with
-	  any workload.
+	  any workload.  It also tries to guarantee a low latency to
+	  interactive applications.
 
 	  This is the default I/O scheduler.
 
diff --git a/block/bfq.h b/block/bfq.h
index c8f4f29..3e1dac4 100644
--- a/block/bfq.h
+++ b/block/bfq.h
@@ -190,6 +190,10 @@  struct bfq_group;
  *                         within an idle time slice; used only if the queue's
  *                         IO_bound has been cleared.
  * @pid: pid of the process owning the queue, used for logging purposes.
+ * @last_wr_start_finish: start time of the current weight-raising period if
+ *                        the @bfq-queue is being weight-raised, otherwise
+ *                        finish time of the last weight-raising period
+ * @wr_cur_max_time: current max raising time for this queue
  *
  * A bfq_queue is a leaf request queue; it can be associated with an
  * io_context or more, if it is async. @cgroup holds a reference to
@@ -231,6 +235,11 @@  struct bfq_queue {
 	unsigned int requests_within_timer;
 
 	pid_t pid;
+
+	/* weight-raising fields */
+	unsigned long wr_cur_max_time;
+	unsigned long last_wr_start_finish;
+	unsigned int wr_coeff;
 };
 
 /**
@@ -319,6 +328,19 @@  enum bfq_device_speed {
  *                             again idling to a queue which was marked as
  *                             non-I/O-bound (see the definition of the
  *                             IO_bound flag for further details).
+ * @low_latency: if set to true, low-latency heuristics are enabled.
+ * @bfq_wr_coeff: maximum factor by which the weight of a weight-raised
+ *                queue is multiplied.
+ * @bfq_wr_max_time: maximum duration of a weight-raising period (jiffies).
+ * @bfq_wr_min_idle_time: minimum idle period after which weight-raising
+ *			  may be reactivated for a queue (in jiffies).
+ * @bfq_wr_min_inter_arr_async: minimum period between request arrivals
+ *				after which weight-raising may be
+ *				reactivated for an already busy queue
+ *				(in jiffies).
+ * @RT_prod: cached value of the product R*T used for computing the maximum
+ *	     duration of the weight raising automatically.
+ * @device_speed: device-speed class for the low-latency heuristic.
  * @oom_bfqq: fallback dummy bfqq for extreme OOM conditions.
  *
  * All the fields are protected by the @queue lock.
@@ -368,6 +390,16 @@  struct bfq_data {
 
 	unsigned int bfq_requests_within_timer;
 
+	bool low_latency;
+
+	/* parameters of the low_latency heuristics */
+	unsigned int bfq_wr_coeff;
+	unsigned int bfq_wr_max_time;
+	unsigned int bfq_wr_min_idle_time;
+	unsigned long bfq_wr_min_inter_arr_async;
+	u64 RT_prod;
+	enum bfq_device_speed device_speed;
+
 	struct bfq_queue oom_bfqq;
 };
 
@@ -637,6 +669,8 @@  static void bfq_dispatch_insert(struct request_queue *q, struct request *rq);
 static struct bfq_queue *bfq_get_queue(struct bfq_data *bfqd,
 				       struct bio *bio, int is_sync,
 				       struct bfq_io_cq *bic, gfp_t gfp_mask);
+static void bfq_end_wr_async_queues(struct bfq_data *bfqd,
+				    struct bfq_group *bfqg);
 static void bfq_put_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg);
 static void bfq_exit_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq);
 
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 25ee367..38e6bea 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -35,6 +35,10 @@ 
  * guarantee a low latency to non-I/O bound processes (the latter
  * often belong to time-sensitive applications).
  *
+ * Even better for latency, BFQ explicitly privileges the I/O of
+ * interactive applications, thereby providing these applications with
+ * a very low latency.
+ *
  * With respect to the version of BFQ presented in [1], and in the
  * papers cited therein, this implementation adds a hierarchical
  * extension based on H-WF2Q+. In this extension, also the service of
@@ -122,6 +126,48 @@  struct kmem_cache *bfq_pool;
 /* Shift used for peak rate fixed precision calculations. */
 #define BFQ_RATE_SHIFT		16
 
+/*
+ * By default, BFQ computes the duration of the weight raising for
+ * interactive applications automatically, using the following formula:
+ * duration = (R / r) * T, where r is the peak rate of the device, and
+ * R and T are two reference parameters.
+ * In particular, R is the peak rate of the reference device (see below),
+ * and T is a reference time: given the systems that are likely to be
+ * installed on the reference device according to its speed class, T is
+ * about the maximum time needed, under BFQ and while reading two files in
+ * parallel, to load typical large applications on these systems.
+ * In practice, the slower/faster the device at hand is, the more/less it
+ * takes to load applications with respect to the reference device.
+ * Accordingly, the longer/shorter BFQ grants weight raising to interactive
+ * applications.
+ *
+ * BFQ uses four different reference pairs (R, T), depending on:
+ * . whether the device is rotational or non-rotational;
+ * . whether the device is slow, such as old or portable HDDs, as well as
+ *   SD cards, or fast, such as newer HDDs and SSDs.
+ *
+ * The device's speed class is dynamically (re)detected in
+ * bfq_update_peak_rate() every time the estimated peak rate is updated.
+ *
+ * In the following definitions, R_slow[0]/R_fast[0] and T_slow[0]/T_fast[0]
+ * are the reference values for a slow/fast rotational device, whereas
+ * R_slow[1]/R_fast[1] and T_slow[1]/T_fast[1] are the reference values for
+ * a slow/fast non-rotational device. Finally, device_speed_thresh are the
+ * thresholds used to switch between speed classes.
+ * Both the reference peak rates and the thresholds are measured in
+ * sectors/usec, left-shifted by BFQ_RATE_SHIFT.
+ */
+static int R_slow[2] = {1536, 10752};
+static int R_fast[2] = {17415, 34791};
+/*
+ * To improve readability, a conversion function is used to initialize the
+ * following arrays, which entails that they can be initialized only in a
+ * function.
+ */
+static int T_slow[2];
+static int T_fast[2];
+static int device_speed_thresh[2];
+
 #define BFQ_SERVICE_TREE_INIT	((struct bfq_service_tree)		\
 				{ RB_ROOT, RB_ROOT, NULL, NULL, 0, 0 })
 
@@ -730,7 +776,8 @@  __bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
 		new_st = bfq_entity_service_tree(entity);
 
 		prev_weight = entity->weight;
-		new_weight = entity->orig_weight;
+		new_weight = entity->orig_weight *
+			     (bfqq ? bfqq->wr_coeff : 1);
 		entity->weight = new_weight;
 
 		new_st->wsum += entity->weight;
@@ -1917,6 +1964,18 @@  static void bfq_pd_offline(struct blkg_policy_data *pd)
 	bfqg_stats_xfer_dead(bfqg);
 }
 
+static void bfq_end_wr_async(struct bfq_data *bfqd)
+{
+	struct blkcg_gq *blkg;
+
+	list_for_each_entry(blkg, &bfqd->queue->blkg_list, q_node) {
+		struct bfq_group *bfqg = blkg_to_bfqg(blkg);
+
+		bfq_end_wr_async_queues(bfqd, bfqg);
+	}
+	bfq_end_wr_async_queues(bfqd, bfqd->root_group);
+}
+
 static int bfq_io_show_weight(struct seq_file *sf, void *v)
 {
 	struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
@@ -2275,6 +2334,11 @@  static void bfq_bfqq_move(struct bfq_data *bfqd,
 {
 }
 
+static void bfq_end_wr_async(struct bfq_data *bfqd)
+{
+	bfq_end_wr_async_queues(bfqd, bfqd->root_group);
+}
+
 static void bfq_disconnect_groups(struct bfq_data *bfqd)
 {
 	bfq_put_async_queues(bfqd, bfqd->root_group);
@@ -2452,7 +2516,8 @@  static unsigned long bfq_serv_to_charge(struct request *rq,
 					struct bfq_queue *bfqq)
 {
 	return blk_rq_sectors(rq) *
-		(1 + ((!bfq_bfqq_sync(bfqq)) * bfq_async_charge_factor));
+		(1 + ((!bfq_bfqq_sync(bfqq)) * (bfqq->wr_coeff == 1) *
+		bfq_async_charge_factor));
 }
 
 /**
@@ -2493,12 +2558,27 @@  static void bfq_updated_next_req(struct bfq_data *bfqd,
 	}
 }
 
+static unsigned int bfq_wr_duration(struct bfq_data *bfqd)
+{
+	u64 dur;
+
+	if (bfqd->bfq_wr_max_time > 0)
+		return bfqd->bfq_wr_max_time;
+
+	dur = bfqd->RT_prod;
+	do_div(dur, bfqd->peak_rate);
+
+	return dur;
+}
+
 static void bfq_add_request(struct request *rq)
 {
 	struct bfq_queue *bfqq = RQ_BFQQ(rq);
 	struct bfq_entity *entity = &bfqq->entity;
 	struct bfq_data *bfqd = bfqq->bfqd;
 	struct request *next_rq, *prev;
+	unsigned long old_wr_coeff = bfqq->wr_coeff;
+	bool idle_for_long_time = false;
 
 	bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
 	bfqq->queued[rq_is_sync(rq)]++;
@@ -2514,10 +2594,16 @@  static void bfq_add_request(struct request *rq)
 	bfqq->next_rq = next_rq;
 
 	if (!bfq_bfqq_busy(bfqq)) {
+		idle_for_long_time =
+			time_is_before_jiffies(
+				bfqq->budget_timeout +
+				bfqd->bfq_wr_min_idle_time);
+
 #ifdef CONFIG_CFQ_GROUP_IOSCHED
 		bfqg_stats_update_io_add(bfqq_group(RQ_BFQQ(rq)), bfqq,
 					 rq->cmd_flags);
 #endif
+
 		entity->budget = max_t(unsigned long, bfqq->max_budget,
 				       bfq_serv_to_charge(next_rq, bfqq));
 
@@ -2533,10 +2619,58 @@  static void bfq_add_request(struct request *rq)
 				bfqq->requests_within_timer = 0;
 		}
 
+		if (!bfqd->low_latency)
+			goto add_bfqq_busy;
+
+		/*
+		 * If the queue is not being boosted and has been idle for
+		 * enough time, start a weight-raising period.
+		 */
+		if (old_wr_coeff == 1 && idle_for_long_time) {
+			bfqq->wr_coeff = bfqd->bfq_wr_coeff;
+			bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
+			bfq_log_bfqq(bfqd, bfqq,
+				     "wrais starting at %lu, rais_max_time %u",
+				     jiffies,
+				     jiffies_to_msecs(bfqq->wr_cur_max_time));
+		} else if (old_wr_coeff > 1) {
+			if (idle_for_long_time)
+				bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
+			else {
+				bfqq->wr_coeff = 1;
+				bfq_log_bfqq(bfqd, bfqq,
+					"wrais ending at %lu, rais_max_time %u",
+					jiffies,
+					jiffies_to_msecs(bfqq->
+						wr_cur_max_time));
+			}
+		}
+		if (old_wr_coeff != bfqq->wr_coeff)
+			entity->prio_changed = 1;
+add_bfqq_busy:
 		bfq_add_bfqq_busy(bfqd, bfqq);
-	} else
+	} else {
+		if (bfqd->low_latency && old_wr_coeff == 1 && !rq_is_sync(rq) &&
+		    time_is_before_jiffies(
+				bfqq->last_wr_start_finish +
+				bfqd->bfq_wr_min_inter_arr_async)) {
+			bfqq->wr_coeff = bfqd->bfq_wr_coeff;
+			bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
+
+			entity->prio_changed = 1;
+			bfq_log_bfqq(bfqd, bfqq,
+			    "non-idle wrais starting at %lu, rais_max_time %u",
+			    jiffies,
+			    jiffies_to_msecs(bfqq->wr_cur_max_time));
+		}
 		if (prev != bfqq->next_rq)
 			bfq_updated_next_req(bfqd, bfqq);
+	}
+
+	if (bfqd->low_latency &&
+		(old_wr_coeff == 1 || bfqq->wr_coeff == 1 ||
+		 idle_for_long_time))
+		bfqq->last_wr_start_finish = jiffies;
 }
 
 static struct request *bfq_find_rq_fmerge(struct bfq_data *bfqd,
@@ -2687,6 +2821,43 @@  static void bfq_merged_requests(struct request_queue *q, struct request *rq,
 #endif
 }
 
+/* Must be called with bfqq != NULL */
+static void bfq_bfqq_end_wr(struct bfq_queue *bfqq)
+{
+	bfqq->wr_coeff = 1;
+	bfqq->wr_cur_max_time = 0;
+	/* Trigger a weight change on the next activation of the queue */
+	bfqq->entity.prio_changed = 1;
+}
+
+static void bfq_end_wr_async_queues(struct bfq_data *bfqd,
+				    struct bfq_group *bfqg)
+{
+	int i, j;
+
+	for (i = 0; i < 2; i++)
+		for (j = 0; j < IOPRIO_BE_NR; j++)
+			if (bfqg->async_bfqq[i][j])
+				bfq_bfqq_end_wr(bfqg->async_bfqq[i][j]);
+	if (bfqg->async_idle_bfqq)
+		bfq_bfqq_end_wr(bfqg->async_idle_bfqq);
+}
+
+static void bfq_end_wr(struct bfq_data *bfqd)
+{
+	struct bfq_queue *bfqq;
+
+	spin_lock_irq(bfqd->queue->queue_lock);
+
+	list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
+		bfq_bfqq_end_wr(bfqq);
+	list_for_each_entry(bfqq, &bfqd->idle_list, bfqq_list)
+		bfq_bfqq_end_wr(bfqq);
+	bfq_end_wr_async(bfqd);
+
+	spin_unlock_irq(bfqd->queue->queue_lock);
+}
+
 static int bfq_allow_merge(struct request_queue *q, struct request *rq,
 			   struct bio *bio)
 {
@@ -2796,16 +2967,20 @@  static void bfq_arm_slice_timer(struct bfq_data *bfqd)
 	 */
 	sl = bfqd->bfq_slice_idle;
 	/*
-	 * Grant only minimum idle time if the queue either has been
-	 * seeky for long enough or has already proved to be
-	 * constantly seeky.
+	 * Unless the queue is being weight-raised, grant only minimum
+	 * idle time if the queue either has been seeky for long
+	 * enough or has already proved to be constantly seeky. A long
+	 * idling is preserved for a weight-raised queue, because it
+	 * is needed for guaranteeing to the queue its reserved share of
+	 * the throughput.
 	 */
 	if (bfq_sample_valid(bfqq->seek_samples) &&
 	    ((BFQQ_SEEKY(bfqq) && bfqq->entity.service >
 				  bfq_max_budget(bfqq->bfqd) / 8) ||
-	      bfq_bfqq_constantly_seeky(bfqq)))
+	      bfq_bfqq_constantly_seeky(bfqq)) && bfqq->wr_coeff == 1)
 		sl = min(sl, msecs_to_jiffies(BFQ_MIN_TT));
-
+	else if (bfqq->wr_coeff > 1)
+		sl = sl * 3;
 	bfqd->last_idling_start = ktime_get();
 	mod_timer(&bfqd->idle_slice_timer, jiffies + sl);
 #ifdef CONFIG_CFQ_GROUP_IOSCHED
@@ -2897,9 +3072,15 @@  static void __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 {
 	__bfq_bfqd_reset_in_service(bfqd);
 
-	if (RB_EMPTY_ROOT(&bfqq->sort_list))
+	if (RB_EMPTY_ROOT(&bfqq->sort_list)) {
+		/*
+		 * Overloading budget_timeout field to store the time
+		 * at which the queue remains with no backlog; used by
+		 * the weight-raising mechanism.
+		 */
+		bfqq->budget_timeout = jiffies;
 		bfq_del_bfqq_busy(bfqd, bfqq, 1);
-	else
+	} else
 		bfq_activate_bfqq(bfqd, bfqq);
 }
 
@@ -3117,12 +3298,27 @@  static bool bfq_update_peak_rate(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 			bfqd->peak_rate_samples++;
 
 		if (bfqd->peak_rate_samples == BFQ_PEAK_RATE_SAMPLES &&
-		    update && bfqd->bfq_user_max_budget == 0) {
-			bfqd->bfq_max_budget =
-				bfq_calc_max_budget(bfqd->peak_rate,
-						    timeout);
-			bfq_log(bfqd, "new max_budget=%d",
-				bfqd->bfq_max_budget);
+		    update) {
+			int dev_type = blk_queue_nonrot(bfqd->queue);
+
+			if (bfqd->bfq_user_max_budget == 0) {
+				bfqd->bfq_max_budget =
+					bfq_calc_max_budget(bfqd->peak_rate,
+							    timeout);
+				bfq_log(bfqd, "new max_budget=%d",
+					bfqd->bfq_max_budget);
+			}
+			if (bfqd->device_speed == BFQ_BFQD_FAST &&
+			    bfqd->peak_rate < device_speed_thresh[dev_type]) {
+				bfqd->device_speed = BFQ_BFQD_SLOW;
+				bfqd->RT_prod = R_slow[dev_type] *
+						T_slow[dev_type];
+			} else if (bfqd->device_speed == BFQ_BFQD_SLOW &&
+			    bfqd->peak_rate > device_speed_thresh[dev_type]) {
+				bfqd->device_speed = BFQ_BFQD_FAST;
+				bfqd->RT_prod = R_fast[dev_type] *
+						T_fast[dev_type];
+			}
 		}
 	}
 
@@ -3222,6 +3418,9 @@  static void bfq_bfqq_expire(struct bfq_data *bfqd,
 	    bfqq->entity.service <= 2 * bfqq->entity.budget / 10)
 		bfq_clear_bfqq_IO_bound(bfqq);
 
+	if (bfqd->low_latency && bfqq->wr_coeff == 1)
+		bfqq->last_wr_start_finish = jiffies;
+
 	bfq_log_bfqq(bfqd, bfqq,
 		"expire (%d, slow %d, num_disp %d, idle_win %d)", reason,
 		slow, bfqq->dispatched, bfq_bfqq_idle_window(bfqq));
@@ -3271,16 +3470,37 @@  static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq)
 
 /*
  * For a queue that becomes empty, device idling is allowed only if
- * this function returns true for the queue. And this function returns
- * true only if idling is beneficial for throughput.
+ * this function returns true for the queue. As a consequence, since
+ * device idling plays a critical role in both throughput boosting and
+ * service guarantees, the return value of this function plays a
+ * critical role in both these aspects as well.
+ *
+ * In a nutshell, this function returns true only if idling is
+ * beneficial for throughput or, even if detrimental for throughput,
+ * idling is however necessary to preserve service guarantees (low
+ * latency, desired throughput distribution, ...). In particular, on
+ * NCQ-capable devices, this function tries to return false, so as to
+ * help keep the drives' internal queues full, whenever this helps the
+ * device boost the throughput without causing any service-guarantee
+ * issue.
+ *
+ * In more detail, the return value of this function is obtained by,
+ * first, computing a number of boolean variables that take into
+ * account throughput and service-guarantee issues, and, then,
+ * combining these variables in a logical expression. Most of the
+ * issues taken into account are not trivial. We discuss these issues
+ * individually while introducing the variables.
  */
 static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 {
 	struct bfq_data *bfqd = bfqq->bfqd;
-	bool idling_boosts_thr;
+	bool idling_boosts_thr, asymmetric_scenario;
 
 	/*
-	 * The value of the next variable is computed considering that
+	 * The next variable takes into account the cases where idling
+	 * boosts the throughput.
+	 *
+	 * The value of the variable is computed considering that
 	 * idling is usually beneficial for the throughput if:
 	 * (a) the device is not NCQ-capable, or
 	 * (b) regardless of the presence of NCQ, the request pattern
@@ -3294,13 +3514,80 @@  static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 	idling_boosts_thr = !bfqd->hw_tag || bfq_bfqq_IO_bound(bfqq);
 
 	/*
-	 * We have now the components we need to compute the return
-	 * value of the function, which is true only if both the
-	 * following conditions hold:
+	 * There is then a case where idling must be performed not for
+	 * throughput concerns, but to preserve service guarantees. To
+	 * introduce it, we can note that allowing the drive to
+	 * enqueue more than one request at a time, and hence
+	 * delegating de facto final scheduling decisions to the
+	 * drive's internal scheduler, causes loss of control on the
+	 * actual request service order. In particular, the critical
+	 * situation is when requests from different processes happens
+	 * to be present, at the same time, in the internal queue(s)
+	 * of the drive. In such a situation, the drive, by deciding
+	 * the service order of the internally-queued requests, does
+	 * determine also the actual throughput distribution among
+	 * these processes. But the drive typically has no notion or
+	 * concern about per-process throughput distribution, and
+	 * makes its decisions only on a per-request basis. Therefore,
+	 * the service distribution enforced by the drive's internal
+	 * scheduler is likely to coincide with the desired
+	 * device-throughput distribution only in a completely
+	 * symmetric scenario where: (i) each of these processes must
+	 * get the same throughput as the others; (ii) all these
+	 * processes have the same I/O pattern (either sequential or
+	 * random).  In fact, in such a scenario, the drive will tend
+	 * to treat the requests of each of these processes in about
+	 * the same way as the requests of the others, and thus to
+	 * provide each of these processes with about the same
+	 * throughput (which is exactly the desired throughput
+	 * distribution). In contrast, in any asymmetric scenario,
+	 * device idling is certainly needed to guarantee that bfqq
+	 * receives its assigned fraction of the device throughput
+	 * (see [1] for details).
+	 *
+	 * As for sub-condition (i), actually we check only whether
+	 * bfqq is being weight-raised. In fact, if bfqq is not being
+	 * weight-raised, we have that:
+	 * - if the process associated to bfqq is not I/O-bound, then
+	 *   it is not either latency- or throughput-critical; therefore
+	 *   idling is not needed for bfqq;
+	 * - if the process asociated to bfqq is I/O-bound, then
+	 *   idling is already granted to bfqq (see the comments on
+	 *   idling_boosts_thr).
+	 *
+	 * We do not check sub-condition (ii) at all, i.e., the next
+	 * variable is true if and only if bfqq is being
+	 * weight-raised. We do not need to control sub-condition (ii)
+	 * for the following reason:
+	 * - if bfqq is being weight-raised, then idling is already
+	 *   guaranteed to bfqq by sub-condition (i);
+	 * - if bfqq is not being weight-raised, then idling is
+	 *   already guaranteed to bfqq (only) if it matters, i.e., if
+	 *   bfqq is associated to a currently I/O-bound process (see
+	 *   the above comment on sub-condition (i)).
+	 *
+	 * As a side note, it is worth considering that the above
+	 * device-idling countermeasures may however fail in the
+	 * following unlucky scenario: if idling is (correctly)
+	 * disabled in a time period during which the symmetry
+	 * sub-condition holds, and hence the device is allowed to
+	 * enqueue many requests, but at some later point in time some
+	 * sub-condition stops to hold, then it may become impossible
+	 * to let requests be served in the desired order until all
+	 * the requests already queued in the device have been served.
+	 */
+	asymmetric_scenario = bfqq->wr_coeff > 1;
+
+	/*
+	 * We have now all the components we need to compute the return
+	 * value of the function, which is true only if both the following
+	 * conditions hold:
 	 * 1) bfqq is sync, because idling make sense only for sync queues;
-	 * 2) idling boosts the throughput.
+	 * 2) idling either boosts the throughput (without issues), or
+	 *    is necessary to preserve service guarantees.
 	 */
-	return bfq_bfqq_sync(bfqq) && idling_boosts_thr;
+	return bfq_bfqq_sync(bfqq) &&
+		(idling_boosts_thr || asymmetric_scenario);
 }
 
 /*
@@ -3405,6 +3692,43 @@  keep_queue:
 	return bfqq;
 }
 
+static void bfq_update_wr_data(struct bfq_data *bfqd, struct bfq_queue *bfqq)
+{
+	struct bfq_entity *entity = &bfqq->entity;
+
+	if (bfqq->wr_coeff > 1) { /* queue is being weight-raised */
+		bfq_log_bfqq(bfqd, bfqq,
+			"raising period dur %u/%u msec, old coeff %u, w %d(%d)",
+			jiffies_to_msecs(jiffies - bfqq->last_wr_start_finish),
+			jiffies_to_msecs(bfqq->wr_cur_max_time),
+			bfqq->wr_coeff,
+			bfqq->entity.weight, bfqq->entity.orig_weight);
+
+		if (entity->prio_changed)
+			bfq_log_bfqq(bfqd, bfqq, "WARN: pending prio change");
+
+		/*
+		 * If too much time has elapsed from the beginning of
+		 * this weight-raising period, then end weight
+		 * raising.
+		 */
+		if (time_is_before_jiffies(bfqq->last_wr_start_finish +
+					   bfqq->wr_cur_max_time)) {
+			bfqq->last_wr_start_finish = jiffies;
+			bfq_log_bfqq(bfqd, bfqq,
+				     "wrais ending at %lu, rais_max_time %u",
+				     bfqq->last_wr_start_finish,
+				     jiffies_to_msecs(bfqq->wr_cur_max_time));
+			bfq_bfqq_end_wr(bfqq);
+		}
+	}
+	/* Update weight both if it must be raised and if it must be lowered */
+	if ((entity->weight > entity->orig_weight) != (bfqq->wr_coeff > 1))
+		__bfq_entity_update_weight_prio(
+			bfq_entity_service_tree(entity),
+			entity);
+}
+
 /*
  * Dispatch one request from bfqq, moving it to the request queue
  * dispatch list.
@@ -3451,6 +3775,8 @@  static int bfq_dispatch_request(struct bfq_data *bfqd,
 	bfq_bfqq_served(bfqq, service_to_charge);
 	bfq_dispatch_insert(bfqd->queue, rq);
 
+	bfq_update_wr_data(bfqd, bfqq);
+
 	bfq_log_bfqq(bfqd, bfqq,
 			"dispatched %u sec req (%llu), budg left %d",
 			blk_rq_sectors(rq),
@@ -3735,6 +4061,9 @@  static void bfq_init_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 	/* Tentative initial value to trade off between thr and lat */
 	bfqq->max_budget = (2 * bfq_max_budget(bfqd)) / 3;
 	bfqq->pid = pid;
+
+	bfqq->wr_coeff = 1;
+	bfqq->last_wr_start_finish = 0;
 }
 
 static struct bfq_queue *bfq_find_alloc_queue(struct bfq_data *bfqd,
@@ -3925,7 +4254,8 @@  static void bfq_update_idle_window(struct bfq_data *bfqd,
 		(bfqd->hw_tag && BFQQ_SEEKY(bfqq)))
 		enable_idle = 0;
 	else if (bfq_sample_valid(bic->ttime.ttime_samples)) {
-		if (bic->ttime.ttime_mean > bfqd->bfq_slice_idle)
+		if (bic->ttime.ttime_mean > bfqd->bfq_slice_idle &&
+			bfqq->wr_coeff == 1)
 			enable_idle = 0;
 		else
 			enable_idle = 1;
@@ -4431,6 +4761,22 @@  static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
 
 	bfqd->bfq_requests_within_timer = 120;
 
+	bfqd->low_latency = true;
+
+	bfqd->bfq_wr_coeff = 20;
+	bfqd->bfq_wr_max_time = 0;
+	bfqd->bfq_wr_min_idle_time = msecs_to_jiffies(2000);
+	bfqd->bfq_wr_min_inter_arr_async = msecs_to_jiffies(500);
+
+	/*
+	 * Begin by assuming, optimistically, that the device peak rate is
+	 * equal to the highest reference rate.
+	 */
+	bfqd->RT_prod = R_fast[blk_queue_nonrot(bfqd->queue)] *
+			T_fast[blk_queue_nonrot(bfqd->queue)];
+	bfqd->peak_rate = R_fast[blk_queue_nonrot(bfqd->queue)];
+	bfqd->device_speed = BFQ_BFQD_FAST;
+
 	return 0;
 
 out_free:
@@ -4469,6 +4815,15 @@  static ssize_t bfq_var_store(unsigned long *var, const char *page,
 	return count;
 }
 
+static ssize_t bfq_wr_max_time_show(struct elevator_queue *e, char *page)
+{
+	struct bfq_data *bfqd = e->elevator_data;
+
+	return sprintf(page, "%d\n", bfqd->bfq_wr_max_time > 0 ?
+		       jiffies_to_msecs(bfqd->bfq_wr_max_time) :
+		       jiffies_to_msecs(bfq_wr_duration(bfqd)));
+}
+
 static ssize_t bfq_weights_show(struct elevator_queue *e, char *page)
 {
 	struct bfq_queue *bfqq;
@@ -4483,19 +4838,29 @@  static ssize_t bfq_weights_show(struct elevator_queue *e, char *page)
 	num_char += sprintf(page + num_char, "Active:\n");
 	list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list) {
 		num_char += sprintf(page + num_char,
-				    "pid%d: weight %hu, nr_queued %d %d\n",
+				    "pid%d: weight %hu, nr_queued %d %d, ",
 				    bfqq->pid,
 				    bfqq->entity.weight,
 				    bfqq->queued[0],
 				    bfqq->queued[1]);
+		num_char += sprintf(page + num_char,
+				    "dur %d/%u\n",
+				    jiffies_to_msecs(
+					    jiffies -
+					    bfqq->last_wr_start_finish),
+				    jiffies_to_msecs(bfqq->wr_cur_max_time));
 	}
 
 	num_char += sprintf(page + num_char, "Idle:\n");
 	list_for_each_entry(bfqq, &bfqd->idle_list, bfqq_list) {
 		num_char += sprintf(page + num_char,
-				    "pid%d: weight %hu\n",
+				    "pid%d: weight %hu, dur %d/%u\n",
 				    bfqq->pid,
-				    bfqq->entity.weight);
+				    bfqq->entity.weight,
+				    jiffies_to_msecs(
+					    jiffies -
+					    bfqq->last_wr_start_finish),
+				    jiffies_to_msecs(bfqq->wr_cur_max_time));
 	}
 
 	spin_unlock_irq(bfqd->queue->queue_lock);
@@ -4522,6 +4887,11 @@  SHOW_FUNCTION(bfq_max_budget_async_rq_show,
 	      bfqd->bfq_max_budget_async_rq, 0);
 SHOW_FUNCTION(bfq_timeout_sync_show, bfqd->bfq_timeout[BLK_RW_SYNC], 1);
 SHOW_FUNCTION(bfq_timeout_async_show, bfqd->bfq_timeout[BLK_RW_ASYNC], 1);
+SHOW_FUNCTION(bfq_low_latency_show, bfqd->low_latency, 0);
+SHOW_FUNCTION(bfq_wr_coeff_show, bfqd->bfq_wr_coeff, 0);
+SHOW_FUNCTION(bfq_wr_min_idle_time_show, bfqd->bfq_wr_min_idle_time, 1);
+SHOW_FUNCTION(bfq_wr_min_inter_arr_async_show, bfqd->bfq_wr_min_inter_arr_async,
+	1);
 #undef SHOW_FUNCTION
 
 #define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV)			\
@@ -4553,6 +4923,12 @@  STORE_FUNCTION(bfq_max_budget_async_rq_store, &bfqd->bfq_max_budget_async_rq,
 		1, INT_MAX, 0);
 STORE_FUNCTION(bfq_timeout_async_store, &bfqd->bfq_timeout[BLK_RW_ASYNC], 0,
 		INT_MAX, 1);
+STORE_FUNCTION(bfq_wr_coeff_store, &bfqd->bfq_wr_coeff, 1, INT_MAX, 0);
+STORE_FUNCTION(bfq_wr_max_time_store, &bfqd->bfq_wr_max_time, 0, INT_MAX, 1);
+STORE_FUNCTION(bfq_wr_min_idle_time_store, &bfqd->bfq_wr_min_idle_time, 0,
+		INT_MAX, 1);
+STORE_FUNCTION(bfq_wr_min_inter_arr_async_store,
+		&bfqd->bfq_wr_min_inter_arr_async, 0, INT_MAX, 1);
 #undef STORE_FUNCTION
 
 static ssize_t bfq_fake_lat_show(struct elevator_queue *e, char *page)
@@ -4624,6 +5000,22 @@  static ssize_t bfq_timeout_sync_store(struct elevator_queue *e,
 	return ret;
 }
 
+static ssize_t bfq_low_latency_store(struct elevator_queue *e,
+				     const char *page, size_t count)
+{
+	struct bfq_data *bfqd = e->elevator_data;
+	unsigned long uninitialized_var(__data);
+	int ret = bfq_var_store(&__data, (page), count);
+
+	if (__data > 1)
+		__data = 1;
+	if (__data == 0 && bfqd->low_latency != 0)
+		bfq_end_wr(bfqd);
+	bfqd->low_latency = __data;
+
+	return ret;
+}
+
 #define BFQ_ATTR(name) \
 	__ATTR(name, S_IRUGO|S_IWUSR, bfq_##name##_show, bfq_##name##_store)
 
@@ -4640,8 +5032,12 @@  static struct elv_fs_entry bfq_attrs[] = {
 	BFQ_ATTR(max_budget_async_rq),
 	BFQ_ATTR(timeout_sync),
 	BFQ_ATTR(timeout_async),
+	BFQ_ATTR(low_latency),
+	BFQ_ATTR(wr_coeff),
+	BFQ_ATTR(wr_max_time),
+	BFQ_ATTR(wr_min_idle_time),
+	BFQ_ATTR(wr_min_inter_arr_async),
 	BFQ_ATTR(weights),
-	BFQ_FAKE_LAT_ATTR(low_latency),
 	BFQ_FAKE_LAT_ATTR(target_latency),
 	__ATTR_NULL
 };
@@ -4718,11 +5114,28 @@  static int __init bfq_init(void)
 	if (bfq_slab_setup())
 		goto err_pol_unreg;
 
+	/*
+	 * Times to load large popular applications for the typical systems
+	 * installed on the reference devices (see the comments before the
+	 * definitions of the two arrays).
+	 */
+	T_slow[0] = msecs_to_jiffies(2600);
+	T_slow[1] = msecs_to_jiffies(1000);
+	T_fast[0] = msecs_to_jiffies(5500);
+	T_fast[1] = msecs_to_jiffies(2000);
+
+	/*
+	 * Thresholds that determine the switch between speed classes (see
+	 * the comments before the definition of the array).
+	 */
+	device_speed_thresh[0] = (R_fast[0] + R_slow[0]) / 2;
+	device_speed_thresh[1] = (R_fast[1] + R_slow[1]) / 2;
+
 	ret = elv_register(&iosched_bfq);
 	if (ret)
 		goto err_pol_unreg;
 
-	pr_info("BFQ I/O-scheduler: v0");
+	pr_info("BFQ I/O-scheduler: v1");
 
 	return 0;