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block/mq-deadline: Reserve 25% of scheduler tags for synchronous requ…
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…ests

For interactive workloads it is important that synchronous requests are
not delayed. Hence reserve 25% of scheduler tags for synchronous requests.
This patch still allows asynchronous requests to fill the hardware queues
since blk_mq_init_sched() makes sure that the number of scheduler requests
is the double of the hardware queue depth. From blk_mq_init_sched():

	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
				   BLKDEV_MAX_RQ);

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20210618004456.7280-12-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Bart Van Assche authored and Jens Axboe committed Jun 21, 2021
1 parent d6d7f01 commit 0775758
Showing 1 changed file with 55 additions and 0 deletions.
55 changes: 55 additions & 0 deletions block/mq-deadline.c
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,7 @@ struct deadline_data {
int fifo_batch;
int writes_starved;
int front_merges;
u32 async_depth;

spinlock_t lock;
spinlock_t zone_lock;
Expand Down Expand Up @@ -397,6 +398,44 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
return rq;
}

/*
* Called by __blk_mq_alloc_request(). The shallow_depth value set by this
* function is used by __blk_mq_get_tag().
*/
static void dd_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
{
struct deadline_data *dd = data->q->elevator->elevator_data;

/* Do not throttle synchronous reads. */
if (op_is_sync(op) && !op_is_write(op))
return;

/*
* Throttle asynchronous requests and writes such that these requests
* do not block the allocation of synchronous requests.
*/
data->shallow_depth = dd->async_depth;
}

/* Called by blk_mq_update_nr_requests(). */
static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
struct deadline_data *dd = q->elevator->elevator_data;
struct blk_mq_tags *tags = hctx->sched_tags;

dd->async_depth = max(1UL, 3 * q->nr_requests / 4);

sbitmap_queue_min_shallow_depth(tags->bitmap_tags, dd->async_depth);
}

/* Called by blk_mq_init_hctx() and blk_mq_init_sched(). */
static int dd_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
{
dd_depth_updated(hctx);
return 0;
}

static void dd_exit_sched(struct elevator_queue *e)
{
struct deadline_data *dd = e->elevator_data;
Expand Down Expand Up @@ -617,6 +656,7 @@ SHOW_JIFFIES(deadline_read_expire_show, dd->fifo_expire[DD_READ]);
SHOW_JIFFIES(deadline_write_expire_show, dd->fifo_expire[DD_WRITE]);
SHOW_INT(deadline_writes_starved_show, dd->writes_starved);
SHOW_INT(deadline_front_merges_show, dd->front_merges);
SHOW_INT(deadline_async_depth_show, dd->front_merges);
SHOW_INT(deadline_fifo_batch_show, dd->fifo_batch);
#undef SHOW_INT
#undef SHOW_JIFFIES
Expand Down Expand Up @@ -645,6 +685,7 @@ STORE_JIFFIES(deadline_read_expire_store, &dd->fifo_expire[DD_READ], 0, INT_MAX)
STORE_JIFFIES(deadline_write_expire_store, &dd->fifo_expire[DD_WRITE], 0, INT_MAX);
STORE_INT(deadline_writes_starved_store, &dd->writes_starved, INT_MIN, INT_MAX);
STORE_INT(deadline_front_merges_store, &dd->front_merges, 0, 1);
STORE_INT(deadline_async_depth_store, &dd->front_merges, 1, INT_MAX);
STORE_INT(deadline_fifo_batch_store, &dd->fifo_batch, 0, INT_MAX);
#undef STORE_FUNCTION
#undef STORE_INT
Expand All @@ -658,6 +699,7 @@ static struct elv_fs_entry deadline_attrs[] = {
DD_ATTR(write_expire),
DD_ATTR(writes_starved),
DD_ATTR(front_merges),
DD_ATTR(async_depth),
DD_ATTR(fifo_batch),
__ATTR_NULL
};
Expand Down Expand Up @@ -733,6 +775,15 @@ static int deadline_starved_show(void *data, struct seq_file *m)
return 0;
}

static int dd_async_depth_show(void *data, struct seq_file *m)
{
struct request_queue *q = data;
struct deadline_data *dd = q->elevator->elevator_data;

seq_printf(m, "%u\n", dd->async_depth);
return 0;
}

static void *deadline_dispatch_start(struct seq_file *m, loff_t *pos)
__acquires(&dd->lock)
{
Expand Down Expand Up @@ -775,6 +826,7 @@ static const struct blk_mq_debugfs_attr deadline_queue_debugfs_attrs[] = {
DEADLINE_QUEUE_DDIR_ATTRS(write),
{"batching", 0400, deadline_batching_show},
{"starved", 0400, deadline_starved_show},
{"async_depth", 0400, dd_async_depth_show},
{"dispatch", 0400, .seq_ops = &deadline_dispatch_seq_ops},
{},
};
Expand All @@ -783,6 +835,8 @@ static const struct blk_mq_debugfs_attr deadline_queue_debugfs_attrs[] = {

static struct elevator_type mq_deadline = {
.ops = {
.depth_updated = dd_depth_updated,
.limit_depth = dd_limit_depth,
.insert_requests = dd_insert_requests,
.dispatch_request = dd_dispatch_request,
.prepare_request = dd_prepare_request,
Expand All @@ -796,6 +850,7 @@ static struct elevator_type mq_deadline = {
.has_work = dd_has_work,
.init_sched = dd_init_sched,
.exit_sched = dd_exit_sched,
.init_hctx = dd_init_hctx,
},

#ifdef CONFIG_BLK_DEBUG_FS
Expand Down

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