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.
