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

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

Hi Damine,

On Mon, Nov 18, 2019 at 12:18:48AM +0000, Damien Le Moal wrote:
> On 2019/11/17 17:08, Ming Lei wrote:
> > Now the requeue 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
> > requesut queue when this LUN is busy.
> 
> s/requesut/request
> 
> Also, shouldn't this patch have the tag:
> 
> Reported-by: Long Li <longli@microsoft.com>
> 
> ?

Will do that in V2.

> 
> Another remark is that Long's approach is generic to the block layer
> while your patch here is scsi specific. I wonder if the same problem
> cannot happen with other drivers too ?

Not sure what you meant about the same problem.

It is definitely bad to unconditionally call blk_mq_run_hw_queues()
in driver's IO completion handler from performance viewpoint.

This patch simply addresses this SCSI specific issue, since blk_mq_run_hw_queues()
shouldn't be called from scsi_end_request() when the device queue isn't busy.

If other driver has such kind of issue, I believe it should have been fixed in
driver too.

So my patch isn't contradictory with Long's patch which improves generic blk-mq's
run queue.

Thanks,
Ming