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@xxxxxxx> 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.
