>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@xxxxxxxxx>
>> Cc: Ewan D. Milne <emilne@xxxxxxxxxx>
>> Cc: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
>> Cc: Hannes Reinecke <hare@xxxxxxx>
>> Cc: Bart Van Assche <bvanassche@xxxxxxx>
>> Cc: Damien Le Moal <damien.lemoal@xxxxxxx>
>> Cc: Long Li <longli@xxxxxxxxxxxxx>
>> Reported-by: Long Li <longli@xxxxxxxxxxxxx>
>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
>> ---
>> 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@xxxxxxxxxxxxx>
>
>Thanks,
>Ming
