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

[Ming Lei]

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?

Thanks,
Ming