Re: [PATCH V5] scsi: core: only re-run queue in scsi_end_request() if device queue is busy

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

On Mon, Sep 07, 2020 at 09:52:42AM -0700, Bart Van Assche wrote:
> On 2020-09-07 00:10, Ming Lei wrote:
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index 7affaaf8b98e..a05e431ee62a 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -551,8 +551,25 @@ 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 {
> 
> Please follow the Linux kernel coding style and balance braces.

Could you provide one document about such style? The patch does pass
checkpatch, or I am happy to follow your suggestion if checkpatch is
updated to this way.

> 
> > +		/*
> > +		 * 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.
> > +		 */
> > +		int old = atomic_read(&sdev->restarts);
> 
> scsi_run_queue_async() has two callers: scsi_end_request() and scsi_queue_rq().
> I don't see how ordering between scsi_device_unbusy() and the above atomic_read()
> could be guaranteed if this function is called from scsi_queue_rq()?
> 
> Regarding the I/O completion path, my understanding is that the I/O completion
> path is as follows if rq->end_io == NULL:
> 
> scsi_mq_done()
>   blk_mq_complete_request()
>     rq->q->mq_ops->complete(rq) = scsi_softirq_done
>       scsi_finish_command()
>         scsi_device_unbusy()

scsi_device_unbusy()
	atomic_dec(&sdev->device_busy);

>         scsi_cmd_to_driver(cmd)->done(cmd)
>         scsi_io_completion()
>           scsi_end_request()
>             blk_update_request()
>             scsi_mq_uninit_cmd()
>             __blk_mq_end_request()
>               blk_mq_free_request()
>                 __blk_mq_free_request()

__blk_mq_free_request()
	blk_mq_put_tag
		smp_mb__after_atomic()

>                   blk_queue_exit()
>             scsi_run_queue_async()
> 
> I haven't found any store memory barrier between the .device_busy change in
> scsi_device_unbusy() and the scsi_run_queue_async() call? Did I perhaps overlook
> something?
> 
> > +		/*
> > +		 * ->restarts has to be kept as non-zero if there new budget
> > +		 *  contention comes.
> 
> Please fix the grammar in the above sentence.

OK.

> 
> > +	/*
> > +	 * Order writing .restarts and reading .device_busy. 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();
> 
> What does "its pair is implied" mean? Please make the above comment
> unambiguous.

See comment in scsi_run_queue_async().

> 
> > +	/*
> > +	 * If all in-flight requests originated from this LUN are completed
> > +	 * before setting .restarts, sdev->device_busy will be observed as
> > +	 * zero, then blk_mq_delay_run_hw_queues() will dispatch this request
> > +	 * soon. Otherwise, completion of one of these request will observe
> > +	 * the .restarts flag, and the request queue will be run for handling
> > +	 * this request, see scsi_end_request().
> > +	 */
> > +	if (unlikely(atomic_read(&sdev->device_busy) == 0 &&
> > +				!scsi_device_blocked(sdev)))
> > +		blk_mq_delay_run_hw_queues(sdev->request_queue, SCSI_QUEUE_DELAY);
> > +	return false;
> 
> What will happen if all in-flight requests complete after
> scsi_run_queue_async() has read .restarts and before it executes
> atomic_cmpxchg()?

One of these completions will run atomic_cmpxchg() successfully, and the
queue is re-run immediately from scsi_run_queue_async().

> Will that cause the queue to be run after a delay
> although it should be run immediately?

Yeah, blk_mq_delay_run_hw_queues() will be called, however:

If scsi_run_queue_async() has scheduled run queue already, this code path
won't queue a dwork successfully. On the other hand, if
blk_mq_delay_run_hw_queues(SCSI_QUEUE_DELAY) has queued a dwork,
scsi_run_queue_async() still can queue the dwork successfully, since the delay
timer can be deactivated easily, see try_to_grab_pending(). In short, the case
you described is an extremely unlikely event. Even though it happens,
forward progress is still guaranteed.


Thanks,
Ming