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

[Bart Van Assche]

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
condition for either a SCSI LLD or a dm-rq driver, run top and you will
see that the CPU usage of a kworker thread jumps up to 100%.

Bart.