[PATCH v8 10/18] blk-mq, elevator: Count requests per hctx to improve performance

John Garry
2020-08-19T15:27:25Z

From: Kashyap Desai <kashyap.desai@broadcom.com>

High CPU utilization on "native_queued_spin_lock_slowpath" due to lock
contention is possible for mq-deadline and bfq IO schedulers
when nr_hw_queues is more than one.

It is because kblockd work queue can submit IO from all online CPUs
(through blk_mq_run_hw_queues()) even though only one hctx has pending
commands.

The elevator callback .has_work for mq-deadline and bfq scheduler considers
pending work if there are any IOs on request queue but it does not account
hctx context.

Add a per-hctx 'elevator_queued' count to the hctx to avoid triggering
the elevator even though there are no requests queued.

Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
[jpg: Relocated atomic_dec() in dd_dispatch_request(), update commit message per Kashyap]
Signed-off-by: John Garry <john.garry@huawei.com>
---
block/bfq-iosched.c | 5 +++++
block/blk-mq.c | 1 +
block/mq-deadline.c | 6 ++++++
include/linux/blk-mq.h | 4 ++++
4 files changed, 16 insertions(+)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 88f0dfa545d7..4650012f1e55 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -4640,6 +4640,9 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
{
struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;

+ if (!atomic_read(&hctx->elevator_queued))
+ return false;
+
/*
* Avoiding lock: a race on bfqd->busy_queues should cause at
* most a call to dispatch for nothing
@@ -5554,6 +5557,7 @@ static void bfq_insert_requests(struct blk_mq_hw_ctx *hctx,
rq = list_first_entry(list, struct request, queuelist);
list_del_init(&rq->queuelist);
bfq_insert_request(hctx, rq, at_head);
+ atomic_inc(&hctx->elevator_queued);
}
}

@@ -5933,6 +5937,7 @@ static void bfq_finish_requeue_request(struct request *rq)

bfq_completed_request(bfqq, bfqd);
bfq_finish_requeue_request_body(bfqq);
+ atomic_dec(&rq->mq_hctx->elevator_queued);

spin_unlock_irqrestore(&bfqd->lock, flags);
} else {
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 457b43829a4f..361fb9fe1dc5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2649,6 +2649,7 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set,
goto free_hctx;

atomic_set(&hctx->nr_active, 0);
+ atomic_set(&hctx->elevator_queued, 0);
if (node == NUMA_NO_NODE)
node = set->numa_node;
hctx->numa_node = node;
diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index b57470e154c8..800ac902809b 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -386,6 +386,8 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
spin_lock(&dd->lock);
rq = __dd_dispatch_request(dd);
spin_unlock(&dd->lock);
+ if (rq)
+ atomic_dec(&rq->mq_hctx->elevator_queued);

return rq;
}
@@ -533,6 +535,7 @@ static void dd_insert_requests(struct blk_mq_hw_ctx *hctx,
rq = list_first_entry(list, struct request, queuelist);
list_del_init(&rq->queuelist);
dd_insert_request(hctx, rq, at_head);
+ atomic_inc(&hctx->elevator_queued);
}
spin_unlock(&dd->lock);
}
@@ -579,6 +582,9 @@ static bool dd_has_work(struct blk_mq_hw_ctx *hctx)
{
struct deadline_data *dd = hctx->queue->elevator->elevator_data;

+ if (!atomic_read(&hctx->elevator_queued))
+ return false;
+
return !list_empty_careful(&dd->dispatch) ||
!list_empty_careful(&dd->fifo_list[0]) ||
!list_empty_careful(&dd->fifo_list[1]);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index a4b35ec60faf..2f3ba31a1658 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -139,6 +139,10 @@ struct blk_mq_hw_ctx {
* shared across request queues.
*/
atomic_t nr_active;
+ /**
+ * @elevator_queued: Number of queued requests on hctx.
+ */
+ atomic_t elevator_queued;

/** @cpuhp_online: List to store request if CPU is going to die */
struct hlist_node cpuhp_online;
--
2.26.2