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

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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>
This commit is contained in:
Kashyap Desai 2020-08-19 23:20:28 +08:00 • committed by Jens Axboe
parent f1b49fdc1c
commit b445547ec1
4 changed files with 16 additions and 0 deletions
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4
include/linux/blk-mq.h
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@ -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;