From 3c395a969acc690f331d5fb91ec99ea8eb5155dd Mon Sep 17 00:00:00 2001 From: Paolo Valente Date: Tue, 1 Dec 2015 11:48:17 +0100 Subject: [PATCH] null_blk: set a separate timer for each command For the Timer IRQ mode (i.e., when command completions are delayed), there is one timer for each CPU. Each of these timers . has a completion queue associated with it, containing all the command completions to be executed when the timer fires; . is set, and a new completion-to-execute is inserted into its completion queue, every time the dispatch code for a new command happens to be executed on the CPU related to the timer. This implies that, if the dispatch of a new command happens to be executed on a CPU whose timer has already been set, but has not yet fired, then the timer is set again, to the completion time of the newly arrived command. When the timer eventually fires, all its queued completions are executed. This way of handling delayed command completions entails the following problem: if more than one command completion is inserted into the queue of a timer before the timer fires, then the expiration time for the timer is moved forward every time each of these completions is enqueued. As a consequence, only the last completion enqueued enjoys a correct execution time, while all previous completions are unjustly delayed until the last completion is executed (and at that time they are executed all together). Specifically, if all the above completions are enqueued almost at the same time, then the problem is negligible. On the opposite end, if every completion is enqueued a while after the previous completion was enqueued (in the extreme case, it is enqueued only right before the timer would have expired), then every enqueued completion, except for the last one, experiences an inflated delay, proportional to the number of completions enqueued after it. In the end, commands, and thus I/O requests, may be completed at an arbitrarily lower rate than the desired one. This commit addresses this issue by replacing per-CPU timers with per-command timers, i.e., by associating an individual timer with each command. Signed-off-by: Paolo Valente Signed-off-by: Arianna Avanzini Signed-off-by: Jens Axboe --- drivers/block/null_blk.c | 79 ++++++++++++---------------------------- 1 file changed, 24 insertions(+), 55 deletions(-) diff --git a/drivers/block/null_blk.c b/drivers/block/null_blk.c index 5c8ba5484d86..08932f5ea9f3 100644 --- a/drivers/block/null_blk.c +++ b/drivers/block/null_blk.c @@ -18,6 +18,7 @@ struct nullb_cmd { struct bio *bio; unsigned int tag; struct nullb_queue *nq; + struct hrtimer timer; }; struct nullb_queue { @@ -49,17 +50,6 @@ static int null_major; static int nullb_indexes; static struct kmem_cache *ppa_cache; -struct completion_queue { - struct llist_head list; - struct hrtimer timer; -}; - -/* - * These are per-cpu for now, they will need to be configured by the - * complete_queues parameter and appropriately mapped. - */ -static DEFINE_PER_CPU(struct completion_queue, completion_queues); - enum { NULL_IRQ_NONE = 0, NULL_IRQ_SOFTIRQ = 1, @@ -180,6 +170,8 @@ static void free_cmd(struct nullb_cmd *cmd) put_tag(cmd->nq, cmd->tag); } +static enum hrtimer_restart null_cmd_timer_expired(struct hrtimer *timer); + static struct nullb_cmd *__alloc_cmd(struct nullb_queue *nq) { struct nullb_cmd *cmd; @@ -190,6 +182,11 @@ static struct nullb_cmd *__alloc_cmd(struct nullb_queue *nq) cmd = &nq->cmds[tag]; cmd->tag = tag; cmd->nq = nq; + if (irqmode == NULL_IRQ_TIMER) { + hrtimer_init(&cmd->timer, CLOCK_MONOTONIC, + HRTIMER_MODE_REL); + cmd->timer.function = null_cmd_timer_expired; + } return cmd; } @@ -238,47 +235,28 @@ static void end_cmd(struct nullb_cmd *cmd) static enum hrtimer_restart null_cmd_timer_expired(struct hrtimer *timer) { - struct completion_queue *cq; - struct llist_node *entry; - struct nullb_cmd *cmd; - - cq = &per_cpu(completion_queues, smp_processor_id()); - - while ((entry = llist_del_all(&cq->list)) != NULL) { - entry = llist_reverse_order(entry); - do { - struct request_queue *q = NULL; + struct nullb_cmd *cmd = container_of(timer, struct nullb_cmd, timer); + struct request_queue *q = NULL; - cmd = container_of(entry, struct nullb_cmd, ll_list); - entry = entry->next; - if (cmd->rq) - q = cmd->rq->q; - end_cmd(cmd); + if (cmd->rq) + q = cmd->rq->q; - if (q && !q->mq_ops && blk_queue_stopped(q)) { - spin_lock(q->queue_lock); - if (blk_queue_stopped(q)) - blk_start_queue(q); - spin_unlock(q->queue_lock); - } - } while (entry); + if (q && !q->mq_ops && blk_queue_stopped(q)) { + spin_lock(q->queue_lock); + if (blk_queue_stopped(q)) + blk_start_queue(q); + spin_unlock(q->queue_lock); } + end_cmd(cmd); return HRTIMER_NORESTART; } static void null_cmd_end_timer(struct nullb_cmd *cmd) { - struct completion_queue *cq = &per_cpu(completion_queues, get_cpu()); - - cmd->ll_list.next = NULL; - if (llist_add(&cmd->ll_list, &cq->list)) { - ktime_t kt = ktime_set(0, completion_nsec); + ktime_t kt = ktime_set(0, completion_nsec); - hrtimer_start(&cq->timer, kt, HRTIMER_MODE_REL_PINNED); - } - - put_cpu(); + hrtimer_start(&cmd->timer, kt, HRTIMER_MODE_REL); } static void null_softirq_done_fn(struct request *rq) @@ -376,6 +354,10 @@ static int null_queue_rq(struct blk_mq_hw_ctx *hctx, { struct nullb_cmd *cmd = blk_mq_rq_to_pdu(bd->rq); + if (irqmode == NULL_IRQ_TIMER) { + hrtimer_init(&cmd->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); + cmd->timer.function = null_cmd_timer_expired; + } cmd->rq = bd->rq; cmd->nq = hctx->driver_data; @@ -813,19 +795,6 @@ static int __init null_init(void) mutex_init(&lock); - /* Initialize a separate list for each CPU for issuing softirqs */ - for_each_possible_cpu(i) { - struct completion_queue *cq = &per_cpu(completion_queues, i); - - init_llist_head(&cq->list); - - if (irqmode != NULL_IRQ_TIMER) - continue; - - hrtimer_init(&cq->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); - cq->timer.function = null_cmd_timer_expired; - } - null_major = register_blkdev(0, "nullb"); if (null_major < 0) return null_major; -- 2.20.1