linux-kernel - Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and queue is idle

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Date:   Tue, 5 Dec 2017 16:08:20 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "axboe@...com" <axboe@...com>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hare@...e.com" <hare@...e.com>,
        "holger@...lied-asynchrony.com" <holger@...lied-asynchrony.com>
Subject: Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and
 queue is idle

On Tue, 2017-12-05 at 15:52 +0800, Ming Lei wrote:
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index db9556662e27..1816dd8259b3 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1967,6 +1967,8 @@ static bool scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
>  out_put_device:
>  	put_device(&sdev->sdev_gendev);
>  out:
> +	if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev))
> +		blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY);
>  	return false;
>  }

This cannot work since multiple threads can call scsi_mq_get_budget()
concurrently and hence it can happen that none of them sees
atomic_read(&sdev->device_busy) == 0. BTW, the above patch is something I
consider as a workaround. Have you considered to fix the root cause and to
add blk_mq_sched_mark_restart_hctx() where these are missing, e.g. in 
blk_mq_sched_dispatch_requests()? The patch below is not a full solution
but resulted in a significant improvement in my tests:

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 69e3226e66ca..9d86876ec503 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -226,6 +226,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 		 * TODO: get more budgets, and dequeue more requests in
 		 * one time.
 		 */
+		blk_mq_sched_mark_restart_hctx(hctx);
 		blk_mq_do_dispatch_ctx(hctx);
 	} else {
 		blk_mq_flush_busy_ctxs(hctx, &rq_list);

Bart.