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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 two 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). For brevity, we call just weight-raising the combination of these two 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 Signed-off-by: Arianna Avanzini --- Documentation/block/bfq-iosched.txt | 9 + block/bfq-iosched.c | 740 ++++++++++++++++++++++++++++++++---- 2 files changed, 675 insertions(+), 74 deletions(-) -- 2.10.0 diff --git a/Documentation/block/bfq-iosched.txt b/Documentation/block/bfq-iosched.txt index 461b27f..1b87df6 100644 --- a/Documentation/block/bfq-iosched.txt +++ b/Documentation/block/bfq-iosched.txt @@ -375,6 +375,11 @@ default, low latency mode is enabled. If enabled, interactive and soft real-time applications are privileged and experience a lower latency, as explained in more detail in the description of how BFQ works. +DO NOT enable this mode if you need full control on bandwidth +distribution. In fact, if it is enabled, then BFQ automatically +increases the bandwidth share of privileged applications, as the main +means to guarantee a lower latency to them. + timeout_sync ------------ @@ -507,6 +512,10 @@ linear mapping between ioprio and weights, described at the beginning of the tunable section, is still valid, but all weights higher than IOPRIO_BE_NR*10 are mapped to ioprio 0. +Recall that, if low-latency is set, then BFQ automatically raises the +weight of the queues associated with interactive and soft real-time +applications. Unset this tunable if you need/want to control weights. + [1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O Scheduler", Proceedings of the First Workshop on Mobile System diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index dce273b..1a32c83 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -339,6 +339,17 @@ struct bfq_queue { /* pid of the process owning the queue, used for logging purposes */ pid_t pid; + + /* current maximum weight-raising time for this queue */ + unsigned long wr_cur_max_time; + /* + * 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. + */ + unsigned long last_wr_start_finish; + /* factor by which the weight of this queue is multiplied */ + unsigned int wr_coeff; }; /** @@ -356,6 +367,11 @@ struct bfq_io_cq { #endif }; +enum bfq_device_speed { + BFQ_BFQD_FAST, + BFQ_BFQD_SLOW, +}; + /** * struct bfq_data - per-device data structure. * @@ -487,6 +503,34 @@ struct bfq_data { */ bool strict_guarantees; + /* if set to true, low-latency heuristics are enabled */ + bool low_latency; + /* + * Maximum factor by which the weight of a weight-raised queue + * is multiplied. + */ + unsigned int bfq_wr_coeff; + /* maximum duration of a weight-raising period (jiffies) */ + unsigned int bfq_wr_max_time; + /* + * Minimum idle period after which weight-raising may be + * reactivated for a queue (in jiffies). + */ + unsigned int bfq_wr_min_idle_time; + /* + * Minimum period between request arrivals after which + * weight-raising may be reactivated for an already busy async + * queue (in jiffies). + */ + unsigned long bfq_wr_min_inter_arr_async; + /* + * Cached value of the product R*T, used for computing the + * maximum duration of weight raising automatically. + */ + u64 RT_prod; + /* device-speed class for the low-latency heuristic */ + enum bfq_device_speed device_speed; + /* fallback dummy bfqq for extreme OOM conditions */ struct bfq_queue oom_bfqq; @@ -515,7 +559,6 @@ enum bfqq_state_flags { BFQQF_fifo_expire, /* FIFO checked in this slice */ BFQQF_idle_window, /* slice idling enabled */ BFQQF_sync, /* synchronous queue */ - BFQQF_budget_new, /* no completion with this budget */ BFQQF_IO_bound, /* * bfqq has timed-out at least once * having consumed at most 2/10 of @@ -543,7 +586,6 @@ BFQ_BFQQ_FNS(non_blocking_wait_rq); BFQ_BFQQ_FNS(fifo_expire); BFQ_BFQQ_FNS(idle_window); BFQ_BFQQ_FNS(sync); -BFQ_BFQQ_FNS(budget_new); BFQ_BFQQ_FNS(IO_bound); #undef BFQ_BFQQ_FNS @@ -637,7 +679,7 @@ struct bfq_group_data { /* must be the first member */ struct blkcg_policy_data pd; - unsigned short weight; + unsigned int weight; }; /** @@ -732,6 +774,8 @@ static void bfq_put_queue(struct bfq_queue *bfqq); static struct bfq_queue *bfq_get_queue(struct bfq_data *bfqd, struct bio *bio, bool is_sync, struct bfq_io_cq *bic); +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); @@ -787,6 +831,56 @@ static 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. The reference + * rates are not the actual peak rates of the devices used as a + * reference, but slightly lower values. The reason for using these + * slightly lower values is that the peak-rate estimator tends to + * yield slightly lower values than the actual peak rate (it can yield + * the actual peak rate only if there is only one process doing I/O, + * and the process does sequential I/O). + * + * Both the reference peak rates and the thresholds are measured in + * sectors/usec, left-shifted by BFQ_RATE_SHIFT. + */ +static int R_slow[2] = {1000, 10700}; +static int R_fast[2] = {14000, 33000}; +/* + * 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 }) @@ -1486,7 +1580,7 @@ __bfq_entity_update_weight_prio(struct bfq_service_tree *old_st, if (entity->prio_changed) { struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity); - unsigned short prev_weight, new_weight; + unsigned int prev_weight, new_weight; struct bfq_data *bfqd = NULL; #ifdef CONFIG_BFQ_GROUP_IOSCHED struct bfq_sched_data *sd; @@ -1535,7 +1629,8 @@ __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; @@ -1630,6 +1725,8 @@ static void bfq_update_fin_time_enqueue(struct bfq_entity *entity, struct bfq_service_tree *st, bool backshifted) { + struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity); + st = __bfq_entity_update_weight_prio(st, entity); bfq_calc_finish(entity, entity->budget); @@ -1659,10 +1756,19 @@ static void bfq_update_fin_time_enqueue(struct bfq_entity *entity, * time. This may introduce a little unfairness among queues * with backshifted timestamps, but it does not break * worst-case fairness guarantees. + * + * As a special case, if bfqq is weight-raised, push up + * timestamps much less, to keep very low the probability that + * this push up causes the backshifted finish timestamps of + * weight-raised queues to become higher than the backshifted + * finish timestamps of non weight-raised queues. */ if (backshifted && bfq_gt(st->vtime, entity->finish)) { unsigned long delta = st->vtime - entity->finish; + if (bfqq) + delta /= bfqq->wr_coeff; + entity->start += delta; entity->finish += delta; } @@ -3070,6 +3176,18 @@ 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)); @@ -3433,6 +3551,11 @@ static void bfq_init_entity(struct bfq_entity *entity, static void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio) {} +static void bfq_end_wr_async(struct bfq_data *bfqd) +{ + bfq_end_wr_async_queues(bfqd, bfqd->root_group); +} + static struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd, struct blkcg *blkcg) { @@ -3613,7 +3736,7 @@ static struct request *bfq_find_next_rq(struct bfq_data *bfqd, static unsigned long bfq_serv_to_charge(struct request *rq, struct bfq_queue *bfqq) { - if (bfq_bfqq_sync(bfqq)) + if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1) return blk_rq_sectors(rq); return blk_rq_sectors(rq) * bfq_async_charge_factor; @@ -3700,12 +3823,12 @@ static void bfq_bfqq_expire(struct bfq_data *bfqd, * whether the in-service queue should be expired, by returning * true. The purpose of expiring the in-service queue is to give bfqq * the chance to possibly preempt the in-service queue, and the reason - * for preempting the in-service queue is to achieve the following - * goal: guarantee to bfqq its reserved bandwidth even if bfqq has - * expired because it has remained idle. + * for preempting the in-service queue is to achieve one of the two + * goals below. * - * In particular, bfqq may have expired for one of the following two - * reasons: + * 1. Guarantee to bfqq its reserved bandwidth even if bfqq has + * expired because it has remained idle. In particular, bfqq may have + * expired for one of the following two reasons: * * - BFQQE_NO_MORE_REQUESTS bfqq did not enjoy any device idling * and did not make it to issue a new request before its last @@ -3769,10 +3892,36 @@ static void bfq_bfqq_expire(struct bfq_data *bfqd, * above-described special way, and signals that the in-service queue * should be expired. Timestamp back-shifting is done later in * __bfq_activate_entity. + * + * 2. Reduce latency. Even if timestamps are not backshifted to let + * the process associated with bfqq recover a service hole, bfqq may + * however happen to have, after being (re)activated, a lower finish + * timestamp than the in-service queue. That is, the next budget of + * bfqq may have to be completed before the one of the in-service + * queue. If this is the case, then preempting the in-service queue + * allows this goal to be achieved, apart from the unpreemptible, + * outstanding requests mentioned above. + * + * Unfortunately, regardless of which of the above two goals one wants + * to achieve, service trees need first to be updated to know whether + * the in-service queue must be preempted. To have service trees + * correctly updated, the in-service queue must be expired and + * rescheduled, and bfqq must be scheduled too. This is one of the + * most costly operations (in future versions, the scheduling + * mechanism may be re-designed in such a way to make it possible to + * know whether preemption is needed without needing to update service + * trees). In addition, queue preemptions almost always cause random + * I/O, and thus loss of throughput. Because of these facts, the next + * function adopts the following simple scheme to avoid both costly + * operations and too frequent preemptions: it requests the expiration + * of the in-service queue (unconditionally) only for queues that need + * to recover a hole, or that either are weight-raised or deserve to + * be weight-raised. */ static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd, struct bfq_queue *bfqq, - bool arrived_in_time) + bool arrived_in_time, + bool wr_or_deserves_wr) { struct bfq_entity *entity = &bfqq->entity; @@ -3807,14 +3956,85 @@ static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd, entity->budget = max_t(unsigned long, bfqq->max_budget, bfq_serv_to_charge(bfqq->next_rq, bfqq)); bfq_clear_bfqq_non_blocking_wait_rq(bfqq); - return false; + return wr_or_deserves_wr; +} + +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); + + /* + * Limit duration between 3 and 13 seconds. Tests show that + * higher values than 13 seconds often yield the opposite of + * the desired result, i.e., worsen responsiveness by letting + * non-interactive and non-soft-real-time applications + * preserve weight raising for a too long time interval. + * + * On the other end, lower values than 3 seconds make it + * difficult for most interactive tasks to complete their jobs + * before weight-raising finishes. + */ + if (dur > msecs_to_jiffies(13000)) + dur = msecs_to_jiffies(13000); + else if (dur < msecs_to_jiffies(3000)) + dur = msecs_to_jiffies(3000); + + return dur; +} + +static void bfq_update_bfqq_wr_on_rq_arrival(struct bfq_data *bfqd, + struct bfq_queue *bfqq, + unsigned int old_wr_coeff, + bool wr_or_deserves_wr, + bool interactive) +{ + if (old_wr_coeff == 1 && wr_or_deserves_wr) { + /* start a weight-raising period */ + bfqq->wr_coeff = bfqd->bfq_wr_coeff; + /* update wr duration */ + bfqq->wr_cur_max_time = bfq_wr_duration(bfqd); + + /* + * If needed, further reduce budget to make sure it is + * close to bfqq's backlog, so as to reduce the + * scheduling-error component due to a too large + * budget. Do not care about throughput consequences, + * but only about latency. Finally, do not assign a + * too small budget either, to avoid increasing + * latency by causing too frequent expirations. + */ + bfqq->entity.budget = min_t(unsigned long, + bfqq->entity.budget, + 2 * bfq_min_budget(bfqd)); + } else if (old_wr_coeff > 1) { + /* update wr duration */ + bfqq->wr_cur_max_time = bfq_wr_duration(bfqd); + } +} + +static bool bfq_bfqq_idle_for_long_time(struct bfq_data *bfqd, + struct bfq_queue *bfqq) +{ + return bfqq->dispatched == 0 && + time_is_before_jiffies( + bfqq->budget_timeout + + bfqd->bfq_wr_min_idle_time); } static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd, struct bfq_queue *bfqq, - struct request *rq) + int old_wr_coeff, + struct request *rq, + bool *interactive) { - bool bfqq_wants_to_preempt, + bool wr_or_deserves_wr, bfqq_wants_to_preempt, + idle_for_long_time = bfq_bfqq_idle_for_long_time(bfqd, bfqq), /* * See the comments on * bfq_bfqq_update_budg_for_activation for @@ -3827,12 +4047,23 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd, bfqg_stats_update_io_add(bfqq_group(RQ_BFQQ(rq)), bfqq, rq->cmd_flags); /* - * Update budget and check whether bfqq may want to preempt - * the in-service queue. + * bfqq deserves to be weight-raised if: + * - it is sync, + * - it has been idle for enough time. + */ + *interactive = idle_for_long_time; + wr_or_deserves_wr = bfqd->low_latency && + (bfqq->wr_coeff > 1 || + (bfq_bfqq_sync(bfqq) && *interactive)); + + /* + * Using the last flag, update budget and check whether bfqq + * may want to preempt the in-service queue. */ bfqq_wants_to_preempt = bfq_bfqq_update_budg_for_activation(bfqd, bfqq, - arrived_in_time); + arrived_in_time, + wr_or_deserves_wr); if (!bfq_bfqq_IO_bound(bfqq)) { if (arrived_in_time) { @@ -3844,6 +4075,16 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd, bfqq->requests_within_timer = 0; } + if (bfqd->low_latency) { + bfq_update_bfqq_wr_on_rq_arrival(bfqd, bfqq, + old_wr_coeff, + wr_or_deserves_wr, + *interactive); + + if (old_wr_coeff != bfqq->wr_coeff) + bfqq->entity.prio_changed = 1; + } + bfq_add_bfqq_busy(bfqd, bfqq); /* @@ -3857,6 +4098,7 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd, * function bfq_bfqq_update_budg_for_activation). */ if (bfqd->in_service_queue && bfqq_wants_to_preempt && + bfqd->in_service_queue->wr_coeff == 1 && next_queue_may_preempt(bfqd)) bfq_bfqq_expire(bfqd, bfqd->in_service_queue, false, BFQQE_PREEMPTED); @@ -3867,6 +4109,8 @@ static void bfq_add_request(struct request *rq) struct bfq_queue *bfqq = RQ_BFQQ(rq); struct bfq_data *bfqd = bfqq->bfqd; struct request *next_rq, *prev; + unsigned int old_wr_coeff = bfqq->wr_coeff; + bool interactive = false; bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq)); bfqq->queued[rq_is_sync(rq)]++; @@ -3882,9 +4126,45 @@ static void bfq_add_request(struct request *rq) bfqq->next_rq = next_rq; if (!bfq_bfqq_busy(bfqq)) /* switching to busy ... */ - bfq_bfqq_handle_idle_busy_switch(bfqd, bfqq, rq); - else if (prev != bfqq->next_rq) - bfq_updated_next_req(bfqd, bfqq); + bfq_bfqq_handle_idle_busy_switch(bfqd, bfqq, old_wr_coeff, + rq, &interactive); + 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); + + bfqq->entity.prio_changed = 1; + } + if (prev != bfqq->next_rq) + bfq_updated_next_req(bfqd, bfqq); + } + + /* + * Assign jiffies to last_wr_start_finish in the following + * cases: + * + * . if bfqq is not going to be weight-raised, because, for + * non weight-raised queues, last_wr_start_finish stores the + * arrival time of the last request; as of now, this piece + * of information is used only for deciding whether to + * weight-raise async queues + * + * . if bfqq is not weight-raised, because, if bfqq is now + * switching to weight-raised, then last_wr_start_finish + * stores the time when weight-raising starts + * + * . if bfqq is interactive, because, regardless of whether + * bfqq is currently weight-raised, the weight-raising + * period must start or restart (this case is considered + * separately because it is not detected by the above + * conditions, if bfqq is already weight-raised) + */ + if (bfqd->low_latency && + (old_wr_coeff == 1 || bfqq->wr_coeff == 1 || interactive)) + bfqq->last_wr_start_finish = jiffies; } static struct request *bfq_find_rq_fmerge(struct bfq_data *bfqd, @@ -4087,6 +4367,46 @@ static void bfq_requests_merged(struct request_queue *q, struct request *rq, bfqg_stats_update_io_merged(bfqq_group(bfqq), next->cmd_flags); } +/* 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 invocation of + * __bfq_entity_update_weight_prio. + */ + 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->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->lock); +} + static bool bfq_allow_bio_merge(struct request_queue *q, struct request *rq, struct bio *bio) { @@ -4110,16 +4430,32 @@ static bool bfq_allow_bio_merge(struct request_queue *q, struct request *rq, return bfqq == RQ_BFQQ(rq); } +/* + * Set the maximum time for the in-service queue to consume its + * budget. This prevents seeky processes from lowering the throughput. + * In practice, a time-slice service scheme is used with seeky + * processes. + */ +static void bfq_set_budget_timeout(struct bfq_data *bfqd, + struct bfq_queue *bfqq) +{ + bfqd->last_budget_start = ktime_get(); + + bfqq->budget_timeout = jiffies + + bfqd->bfq_timeout * + (bfqq->entity.weight / bfqq->entity.orig_weight); +} + static void __bfq_set_in_service_queue(struct bfq_data *bfqd, struct bfq_queue *bfqq) { if (bfqq) { bfqg_stats_update_avg_queue_size(bfqq_group(bfqq)); - bfq_mark_bfqq_budget_new(bfqq); bfq_clear_bfqq_fifo_expire(bfqq); bfqd->budgets_assigned = (bfqd->budgets_assigned * 7 + 256) / 8; + bfq_set_budget_timeout(bfqd, bfqq); bfq_log_bfqq(bfqd, bfqq, "set_in_service_queue, cur-budget = %d", bfqq->entity.budget); @@ -4159,9 +4495,13 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd) */ sl = bfqd->bfq_slice_idle; /* - * Grant only minimum idle time if the queue is seeky. + * Unless the queue is being weight-raised, grant only minimum + * idle time if the queue is 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 (BFQQ_SEEKY(bfqq)) + if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1) sl = min_t(u64, sl, BFQ_MIN_TT); bfqd->last_idling_start = ktime_get(); @@ -4171,27 +4511,6 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd) } /* - * Set the maximum time for the in-service queue to consume its - * budget. This prevents seeky processes from lowering the disk - * throughput (always guaranteed with a time slice scheme as in CFQ). - */ -static void bfq_set_budget_timeout(struct bfq_data *bfqd) -{ - struct bfq_queue *bfqq = bfqd->in_service_queue; - unsigned int timeout_coeff = bfqq->entity.weight / - bfqq->entity.orig_weight; - - bfqd->last_budget_start = ktime_get(); - - bfq_clear_bfqq_budget_new(bfqq); - bfqq->budget_timeout = jiffies + - bfqd->bfq_timeout * timeout_coeff; - - bfq_log_bfqq(bfqd, bfqq, "set budget_timeout %u", - jiffies_to_msecs(bfqd->bfq_timeout * timeout_coeff)); -} - -/* * In autotuning mode, max_budget is dynamically recomputed as the * amount of sectors transferred in timeout at the estimated peak * rate. This enables BFQ to utilize a full timeslice with a full @@ -4204,6 +4523,42 @@ static unsigned long bfq_calc_max_budget(struct bfq_data *bfqd) jiffies_to_msecs(bfqd->bfq_timeout)>>BFQ_RATE_SHIFT; } +/* + * Update parameters related to throughput and responsiveness, as a + * function of the estimated peak rate. See comments on + * bfq_calc_max_budget(), and on T_slow and T_fast arrays. + */ +static void update_thr_responsiveness_params(struct bfq_data *bfqd) +{ + int dev_type = blk_queue_nonrot(bfqd->queue); + + if (bfqd->bfq_user_max_budget == 0) + bfqd->bfq_max_budget = + bfq_calc_max_budget(bfqd); + + 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]; + } + + bfq_log(bfqd, +"dev_type %s dev_speed_class = %s (%llu sects/sec), thresh %llu setcs/sec", + dev_type == 0 ? "ROT" : "NONROT", + bfqd->device_speed == BFQ_BFQD_FAST ? "FAST" : "SLOW", + bfqd->device_speed == BFQ_BFQD_FAST ? + (USEC_PER_SEC*(u64)R_fast[dev_type])>>BFQ_RATE_SHIFT : + (USEC_PER_SEC*(u64)R_slow[dev_type])>>BFQ_RATE_SHIFT, + (USEC_PER_SEC*(u64)device_speed_thresh[dev_type])>> + BFQ_RATE_SHIFT); +} + static void bfq_reset_rate_computation(struct bfq_data *bfqd, struct request *rq) { @@ -4315,9 +4670,7 @@ static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq) rate /= divisor; /* smoothing constant alpha = 1/divisor */ bfqd->peak_rate += rate; - if (bfqd->bfq_user_max_budget == 0) - bfqd->bfq_max_budget = - bfq_calc_max_budget(bfqd); + update_thr_responsiveness_params(bfqd); reset_computation: bfq_reset_rate_computation(bfqd, rq); @@ -4439,9 +4792,18 @@ static void bfq_dispatch_remove(struct request_queue *q, struct request *rq) static void __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq) { - if (RB_EMPTY_ROOT(&bfqq->sort_list)) + if (RB_EMPTY_ROOT(&bfqq->sort_list)) { + if (bfqq->dispatched == 0) + /* + * Overloading budget_timeout field to store + * the time at which the queue remains with no + * backlog and no outstanding request; used by + * the weight-raising mechanism. + */ + bfqq->budget_timeout = jiffies; + bfq_del_bfqq_busy(bfqd, bfqq, true); - else + } else bfq_requeue_bfqq(bfqd, bfqq); /* @@ -4468,9 +4830,18 @@ static void __bfq_bfqq_recalc_budget(struct bfq_data *bfqd, struct request *next_rq; int budget, min_budget; - budget = bfqq->max_budget; min_budget = bfq_min_budget(bfqd); + if (bfqq->wr_coeff == 1) + budget = bfqq->max_budget; + else /* + * Use a constant, low budget for weight-raised queues, + * to help achieve a low latency. Keep it slightly higher + * than the minimum possible budget, to cause a little + * bit fewer expirations. + */ + budget = 2 * min_budget; + bfq_log_bfqq(bfqd, bfqq, "recalc_budg: last budg %d, budg left %d", bfqq->entity.budget, bfq_bfqq_budget_left(bfqq)); bfq_log_bfqq(bfqd, bfqq, "recalc_budg: last max_budg %d, min budg %d", @@ -4478,7 +4849,7 @@ static void __bfq_bfqq_recalc_budget(struct bfq_data *bfqd, bfq_log_bfqq(bfqd, bfqq, "recalc_budg: sync %d, seeky %d", bfq_bfqq_sync(bfqq), BFQQ_SEEKY(bfqd->in_service_queue)); - if (bfq_bfqq_sync(bfqq)) { + if (bfq_bfqq_sync(bfqq) && bfqq->wr_coeff == 1) { switch (reason) { /* * Caveat: in all the following cases we trade latency @@ -4577,7 +4948,7 @@ static void __bfq_bfqq_recalc_budget(struct bfq_data *bfqd, default: return; } - } else { + } else if (!bfq_bfqq_sync(bfqq)) { /* * Async queues get always the maximum possible * budget, as for them we do not care about latency @@ -4766,15 +5137,19 @@ static void bfq_bfqq_expire(struct bfq_data *bfqd, * bandwidth, and not time, distribution with little unlucky * or quasi-sequential processes. */ - if (slow || - (reason == BFQQE_BUDGET_TIMEOUT && - bfq_bfqq_budget_left(bfqq) >= entity->budget / 3)) + if (bfqq->wr_coeff == 1 && + (slow || + (reason == BFQQE_BUDGET_TIMEOUT && + bfq_bfqq_budget_left(bfqq) >= entity->budget / 3))) bfq_bfqq_charge_time(bfqd, bfqq, delta); if (reason == BFQQE_TOO_IDLE && entity->service <= 2 * 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)); @@ -4801,10 +5176,7 @@ static void bfq_bfqq_expire(struct bfq_data *bfqd, */ static bool bfq_bfqq_budget_timeout(struct bfq_queue *bfqq) { - if (bfq_bfqq_budget_new(bfqq) || - time_is_after_jiffies(bfqq->budget_timeout)) - return false; - return true; + return time_is_before_eq_jiffies(bfqq->budget_timeout); } /* @@ -4831,19 +5203,40 @@ 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; if (bfqd->strict_guarantees) return true; /* - * 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 @@ -4857,13 +5250,80 @@ 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 with 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 with bfqq is I/O-bound, then + * idling is already granted with 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); } /* @@ -4986,6 +5446,43 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd) 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 next request from bfqq. */ @@ -5001,6 +5498,19 @@ static struct request *bfq_dispatch_rq_from_bfqq(struct bfq_data *bfqd, bfq_dispatch_remove(bfqd->queue, rq); + /* + * If weight raising has to terminate for bfqq, then next + * function causes an immediate update of bfqq's weight, + * without waiting for next activation. As a consequence, on + * expiration, bfqq will be timestamped as if has never been + * weight-raised during this service slot, even if it has + * received part or even most of the service as a + * weight-raised queue. This inflates bfqq's timestamps, which + * is beneficial, as bfqq is then more willing to leave the + * device immediately to possible other weight-raised queues. + */ + bfq_update_wr_data(bfqd, bfqq); + if (!bfqd->in_service_bic) { atomic_long_inc(&RQ_BIC(rq)->icq.ioc->refcount); bfqd->in_service_bic = RQ_BIC(rq); @@ -5306,6 +5816,9 @@ static void bfq_init_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq, bfqq->max_budget = (2 * bfq_max_budget(bfqd)) / 3; bfqq->budget_timeout = bfq_smallest_from_now(); + bfqq->wr_coeff = 1; + bfqq->last_wr_start_finish = bfq_smallest_from_now(); + /* first request is almost certainly seeky */ bfqq->seek_history = 1; } @@ -5440,7 +5953,8 @@ static void bfq_update_idle_window(struct bfq_data *bfqd, (bfqd->hw_tag && BFQQ_SEEKY(bfqq))) enable_idle = 0; else if (bfq_sample_valid(bfqq->ttime.ttime_samples)) { - if (bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle) + if (bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle && + bfqq->wr_coeff == 1) enable_idle = 0; else enable_idle = 1; @@ -5618,6 +6132,16 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd) bfqd->rq_in_driver--; bfqq->dispatched--; + if (!bfqq->dispatched && !bfq_bfqq_busy(bfqq)) { + /* + * Set budget_timeout (which we overload to store the + * time at which the queue remains with no backlog and + * no outstanding request; used by the weight-raising + * mechanism). + */ + bfqq->budget_timeout = jiffies; + } + now_ns = ktime_get_ns(); bfqq->ttime.last_end_request = now_ns; @@ -5655,10 +6179,7 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd) * or if we want to idle in case it has no pending requests. */ if (bfqd->in_service_queue == bfqq) { - if (bfq_bfqq_budget_new(bfqq)) - bfq_set_budget_timeout(bfqd); - - if (bfq_bfqq_must_idle(bfqq)) { + if (bfqq->dispatched == 0 && bfq_bfqq_must_idle(bfqq)) { bfq_arm_slice_timer(bfqd); return; } else if (bfq_may_expire_for_budg_timeout(bfqq)) @@ -5966,6 +6487,26 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e) bfqd->bfq_requests_within_timer = 120; + bfqd->low_latency = true; + + /* + * Trade-off between responsiveness and fairness. + */ + bfqd->bfq_wr_coeff = 30; + 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 is a + * high-speed one, and that its peak rate is equal to 2/3 of + * 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)] * 2 / 3; + bfqd->device_speed = BFQ_BFQD_FAST; + spin_lock_init(&bfqd->lock); /* @@ -6047,6 +6588,7 @@ SHOW_FUNCTION(bfq_slice_idle_show, bfqd->bfq_slice_idle, 2); SHOW_FUNCTION(bfq_max_budget_show, bfqd->bfq_user_max_budget, 0); SHOW_FUNCTION(bfq_timeout_sync_show, bfqd->bfq_timeout, 1); SHOW_FUNCTION(bfq_strict_guarantees_show, bfqd->strict_guarantees, 0); +SHOW_FUNCTION(bfq_low_latency_show, bfqd->low_latency, 0); #undef SHOW_FUNCTION #define USEC_SHOW_FUNCTION(__FUNC, __VAR) \ @@ -6167,6 +6709,22 @@ static ssize_t bfq_strict_guarantees_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, 0644, bfq_##name##_show, bfq_##name##_store) @@ -6180,6 +6738,7 @@ static struct elv_fs_entry bfq_attrs[] = { BFQ_ATTR(max_budget), BFQ_ATTR(timeout_sync), BFQ_ATTR(strict_guarantees), + BFQ_ATTR(low_latency), __ATTR_NULL }; @@ -6242,6 +6801,39 @@ 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 next two + * arrays). Actually, we use slightly slower values, as the + * estimated peak rate tends to be smaller than the actual + * peak rate. The reason for this last fact is that estimates + * are computed over much shorter time intervals than the long + * intervals typically used for benchmarking. Why? First, to + * adapt more quickly to variations. Second, because an I/O + * scheduler cannot rely on a peak-rate-evaluation workload to + * be run for a long time. + */ + T_slow[0] = msecs_to_jiffies(3500); /* actually 4 sec */ + T_slow[1] = msecs_to_jiffies(6000); /* actually 6.5 sec */ + T_fast[0] = msecs_to_jiffies(7000); /* actually 8 sec */ + T_fast[1] = msecs_to_jiffies(2500); /* actually 3 sec */ + + /* + * Thresholds that determine the switch between speed classes + * (see the comments before the definition of the array + * device_speed_thresh). These thresholds are biased towards + * transitions to the fast class. This is safer than the + * opposite bias. In fact, a wrong transition to the slow + * class results in short weight-raising periods, because the + * speed of the device then tends to be higher that the + * reference peak rate. On the opposite end, a wrong + * transition to the fast class tends to increase + * weight-raising periods, because of the opposite reason. + */ + device_speed_thresh[0] = (4 * R_slow[0]) / 3; + device_speed_thresh[1] = (4 * R_slow[1]) / 3; + ret = elv_register(&iosched_bfq_mq); if (ret) goto err_pol_unreg;