blk-mq, elevator: Count requests per hctx to improve performance

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.

[jpg: Relocated atomic_dec() in dd_dispatch_request(), update commit message per Kashyap]

Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: John Garry <john.garry@huawei.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 418ffd4..a251b80 100644 (file)
--- 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 eff9d98..4abb714 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2660,6 +2660,7 @@ 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 b57470e..800ac90 100644 (file)
--- 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 8279c80..b23eeca 100644 (file)
--- 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;