RE: [PATCH V3] scsi: core: only re-run queue in scsi_end_request() if device queue is busy

[Long Li <longli@microsoft.com>]

>Subject: Re: [PATCH V3] scsi: core: only re-run queue in scsi_end_request() if
>device queue is busy
>
>On Thu, Nov 21, 2019 at 11:26:34AM +0800, Ming Lei wrote:
>> Now the request queue is run in scsi_end_request() unconditionally if
>> both target queue and host queue is ready. We should have re-run
>> request queue only after this device queue becomes busy for restarting this
>LUN only.
>>
>> Recently Long Li reported that cost of run queue may be very heavy in
>> case of high queue depth. So improve this situation by only running
>> the request queue when this LUN is busy.
>>
>> Cc: Jens Axboe <axboe@kernel.dk>
>> Cc: Ewan D. Milne <emilne@redhat.com>
>> Cc: Kashyap Desai <kashyap.desai@broadcom.com>
>> Cc: Hannes Reinecke <hare@suse.de>
>> Cc: Bart Van Assche <bvanassche@acm.org>
>> Cc: Damien Le Moal <damien.lemoal@wdc.com>
>> Cc: Long Li <longli@microsoft.com>
>> Reported-by: Long Li <longli@microsoft.com>
>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>> ---
>> V3:
>> 	- add one smp_mb() in scsi_mq_get_budget() and comment
>>
>> V2:
>> 	- commit log change, no any code change
>> 	- add reported-by tag
>>
>>
>>  drivers/scsi/scsi_lib.c    | 43 ++++++++++++++++++++++++++++++++++++-
>-
>>  include/scsi/scsi_device.h |  1 +
>>  2 files changed, 42 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c index
>> 379533ce8661..d3d237a09a78 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -607,12 +607,17 @@ static bool scsi_end_request(struct request *req,
>blk_status_t error,
>>  	 */
>>  	percpu_ref_get(&q->q_usage_counter);
>>
>> +	/*
>> +	 * One smp_mb() is implied by either rq->end_io or
>> +	 * blk_mq_free_request for ordering writing .device_busy in
>> +	 * scsi_device_unbusy() and reading sdev->restart.
>> +	 */
>>  	__blk_mq_end_request(req, error);
>>
>>  	if (scsi_target(sdev)->single_lun ||
>>  	    !list_empty(&sdev->host->starved_list))
>>  		kblockd_schedule_work(&sdev->requeue_work);
>> -	else
>> +	else if (READ_ONCE(sdev->restart))
>>  		blk_mq_run_hw_queues(q, true);
>>
>>  	percpu_ref_put(&q->q_usage_counter);
>> @@ -1632,8 +1637,42 @@ static bool scsi_mq_get_budget(struct
>blk_mq_hw_ctx *hctx)
>>  	struct request_queue *q = hctx->queue;
>>  	struct scsi_device *sdev = q->queuedata;
>>
>> -	if (scsi_dev_queue_ready(q, sdev))
>> +	if (scsi_dev_queue_ready(q, sdev)) {
>> +		WRITE_ONCE(sdev->restart, 0);
>>  		return true;
>> +	}
>> +
>> +	/*
>> +	 * If all in-flight requests originated from this LUN are completed
>> +	 * before setting .restart, sdev->device_busy will be observed as
>> +	 * zero, then blk_mq_delay_run_hw_queue() will dispatch this
>request
>> +	 * soon. Otherwise, completion of one of these request will observe
>> +	 * the .restart flag, and the request queue will be run for handling
>> +	 * this request, see scsi_end_request().
>> +	 *
>> +	 * However, the .restart flag may be cleared from other dispatch code
>> +	 * path after one inflight request is completed, then:
>> +	 *
>> +	 * 1) if this request is dispatched from scheduler queue or sw queue
>one
>> +	 * by one, this request will be handled in that dispatch path too given
>> +	 * the request still stays at scheduler/sw queue when
>calling .get_budget()
>> +	 * callback.
>> +	 *
>> +	 * 2) if this request is dispatched from hctx->dispatch or
>> +	 * blk_mq_flush_busy_ctxs(), this request will be put into hctx-
>>dispatch
>> +	 * list soon, and blk-mq will be responsible for covering it, see
>> +	 * blk_mq_dispatch_rq_list().
>> +	 */
>> +	WRITE_ONCE(sdev->restart, 1);
>> +
>> +	/*
>> +	 * Order writting .restart and reading .device_busy, and make sure
>> +	 * .restart is visible to scsi_end_request(). Its pair is implied by
>> +	 * __blk_mq_end_request() in scsi_end_request() for ordering
>> +	 * writing .device_busy in scsi_device_unbusy() and reading .restart.
>> +	 *
>> +	 */
>> +	smp_mb();
>>
>>  	if (atomic_read(&sdev->device_busy) == 0
>&& !scsi_device_blocked(sdev))
>>  		blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY);
>diff --git
>> a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h index
>> 202f4d6a4342..9d8ca662ae86 100644
>> --- a/include/scsi/scsi_device.h
>> +++ b/include/scsi/scsi_device.h
>> @@ -109,6 +109,7 @@ struct scsi_device {
>>  	atomic_t device_busy;		/* commands actually active on LLDD
>*/
>>  	atomic_t device_blocked;	/* Device returned QUEUE_FULL. */
>>
>> +	unsigned int restart;
>>  	spinlock_t list_lock;
>>  	struct list_head cmd_list;	/* queue of in use SCSI Command
>structures */
>>  	struct list_head starved_entry;
>> --
>> 2.20.1
>>
>
>Ping...

This patch passed stress tests on Linux VM running in Azure.

Tested-by: Long Li <longli@microsoft.com>

>
>Thanks,
>Ming