Re: [PATCH v4 6/6] dm rq: Avoid that request processing stalls sporadically

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

On Fri, 2017-04-14 at 09:13 +0800, Ming Lei wrote:
> On Thu, Apr 13, 2017 at 09:59:57AM -0700, Bart Van Assche wrote:
> > On 04/12/17 19:20, Ming Lei wrote:
> > > On Wed, Apr 12, 2017 at 06:38:07PM +0000, Bart Van Assche wrote:
> > > > If the blk-mq core would always rerun a hardware queue if a block driver
> > > > returns BLK_MQ_RQ_QUEUE_BUSY then that would cause 100% of a single CPU core
> > > 
> > > It won't casue 100% CPU utilization since we restart queue in completion
> > > path and at that time at least one tag is available, then progress can be
> > > made.
> > 
> > Hello Ming,
> > 
> > Sorry but you are wrong. If .queue_rq() returns BLK_MQ_RQ_QUEUE_BUSY
> > then it's likely that calling .queue_rq() again after only a few
> > microseconds will cause it to return BLK_MQ_RQ_QUEUE_BUSY again. If you
> > don't believe me, change "if (!blk_mq_sched_needs_restart(hctx) &&
> > !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state)) blk_mq_run_hw_queue(hctx,
> > true);" into "blk_mq_run_hw_queue(hctx, true);", trigger a busy
> 
> Yes, that can be true, but I mean it is still OK to run the queue again
> with
> 
> 	if (!blk_mq_sched_needs_restart(hctx) &&
> 	    !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
> 			blk_mq_run_hw_queue(hctx, true);
> 
> and restarting queue in __blk_mq_finish_request() when
> BLK_MQ_RQ_QUEUE_BUSY is returned from .queue_rq(). And both are in current
> blk-mq implementation.
> 
> Then why do we need blk_mq_delay_run_hw_queue(hctx, 100/*ms*/) in dm?

Because if dm_mq_queue_rq() returns BLK_MQ_RQ_QUEUE_BUSY that there is no
guarantee that __blk_mq_finish_request() will be called later on for the
same queue. dm_mq_queue_rq() can e.g. return BLK_MQ_RQ_QUEUE_BUSY while no
dm requests are in progress because the SCSI error handler is active for
all underlying paths. See also scsi_lld_busy() and scsi_host_in_recovery().

Bart.