Re: [PATCH V4] scsi: core: only re-run queue in scsi_end_request() if device queue is busy

[Ming Lei <ming.lei@redhat.com>]

On Tue, Sep 01, 2020 at 07:40:54PM -0700, Bart Van Assche wrote:
> On 2020-08-17 03:08, Ming Lei wrote:
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index 7c6dd6f75190..a62c29058d26 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -551,8 +551,27 @@ static void scsi_run_queue_async(struct scsi_device *sdev)
> >  	if (scsi_target(sdev)->single_lun ||
> >  	    !list_empty(&sdev->host->starved_list))
> >  		kblockd_schedule_work(&sdev->requeue_work);
> > -	else
> > -		blk_mq_run_hw_queues(sdev->request_queue, true);
> > +	else {
> 
> Has this patch been verified with checkpatch? Checkpatch should have warned
> about the unbalanced braces.

[linux]$ ./scripts/checkpatch.pl -g HEAD
total: 0 errors, 0 warnings, 71 lines checked

Commit 0cbe51645b54 ("scsi: core: only re-run queue in scsi_end_request() if device queue is busy") has no obvious style problems and is ready for submission.

> 
> > +		/*
> > +		 * smp_mb() implied in either rq->end_io or blk_mq_free_request
> > +		 * is for ordering writing .device_busy in scsi_device_unbusy()
> > +		 * and reading sdev->restarts.
> > +		 */
> 
> Hmm ... I don't see what orders the atomic_dec(&sdev->device_busy) from
> scsi_device_unbusy() and the atomic_read() below? I don't think that the block
> layer guarantees ordering of these two memory accesses since both accesses
> happen in the request completion path.

__blk_mq_end_request() is called between scsi_device_unbusy() and
scsi_run_queue_async(). When __blk_mq_end_request() is called, this
request is actually ended really because SCMD_STATE_COMPLETE is covered
race between timeout and normal completion, so:

1) either __blk_mq_free_request() is called, smp_mb__after_atomic() is
implied in sbitmap_queue_clear() called from blk_mq_put_tag()

2) or rq->end_io() is called. We don't have too many ->end_io()
implemented. Either wake_up_process() or blk_mq_free_request() is called
in ->end_io(), so memory barrier is implied.

> 
> > +		int old = atomic_read(&sdev->restarts);
> > +
> > +		if (old) {
> > +			/*
> > +			 * ->restarts has to be kept as non-zero if there is
> > +			 *  new budget contention comes.
> 
> There are two verbs in the above sentence ("is" and "comes"). Please remove
> "comes" such that the sentence becomes grammatically correct.
> 
> > +			 *
> > +			 *  No need to run queue when either another re-run
> > +			 *  queue wins in updating ->restarts or one new budget
> > +			 *  contention comes.
> > +			 */
> > +			if (atomic_cmpxchg(&sdev->restarts, old, 0) == old)
> > +				blk_mq_run_hw_queues(sdev->request_queue, true);
> > +		}
> > +	}
> 
> Please combine the two if-statements into a single if-statement using "&&"
> to keep the indentation level low.
> 
> > @@ -1611,8 +1630,34 @@ static void scsi_mq_put_budget(struct request_queue *q)
> >  static bool scsi_mq_get_budget(struct request_queue *q)
> >  {
> >  	struct scsi_device *sdev = q->queuedata;
> > +	int ret = scsi_dev_queue_ready(q, sdev);
> > +
> > +	if (ret)
> > +		return true;
> > +
> > +	atomic_inc(&sdev->restarts);
> >  
> > -	return scsi_dev_queue_ready(q, sdev);
> > +	/*
> > +	 * Order writing .restarts and reading .device_busy, and make sure
> > +	 * .restarts 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 .restarts.
> > +	 *
> > +	 */
> > +	smp_mb__after_atomic();
> 
> Barriers do not guarantee "is visible to". Barriers enforce ordering of memory
> accesses performed by a certain CPU core. Did you perhaps mean that
> sdev->restarts must be incremented before the code below reads sdev->device busy?

Right, ->restart has to be incremented before reading sdev->device_busy.


Thanks, 
Ming