Re: [PATCH 1/5] block: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Wed, 2017-09-20 at 00:55 +0800, Ming Lei wrote:
> On Wed, Sep 20, 2017 at 12:49 AM, Bart Van Assche
> <Bart.VanAssche@wdc.com> wrote:
> > On Wed, 2017-09-20 at 00:04 +0800, Ming Lei wrote:
> > > Run queue at end_io is definitely wrong, because blk-mq has SCHED_RESTART
> > > to do that already.
> > 
> > Sorry but I disagree. If SCHED_RESTART is set that causes the blk-mq core to
> > reexamine the software queues and the hctx dispatch list but not the requeue
> > list. If a block driver returns BLK_STS_RESOURCE then requests end up on the
> > requeue list. Hence the following code in scsi_end_request():
> 
> That doesn't need SCHED_RESTART, because it is requeue's
> responsibility to do that,
> see blk_mq_requeue_work(), which will run hw queue at the end of this func.

That's not what I was trying to explain. What I was trying to explain is that
every block driver that can cause a request to end up on the requeue list is
responsible for kicking the requeue list at a later time. Hence the
kblockd_schedule_work(&sdev->requeue_work) call in the SCSI core and the
blk_mq_kick_requeue_list() and blk_mq_delay_kick_requeue_list() calls in the
dm code. What I would like to see is measurement results for dm-mpath without
this patch series and a call to kick the requeue list added to the dm-mpath
end_io code.

Bart.