On 01/30/18 06:24, Mike Snitzer wrote:
+ * + * If driver returns BLK_STS_RESOURCE and SCHED_RESTART + * bit is set, run queue after a delay to avoid IO stalls + * that could otherwise occur if the queue is idle. */ - if (!blk_mq_sched_needs_restart(hctx) || + needs_restart = blk_mq_sched_needs_restart(hctx); + if (!needs_restart || (no_tag && list_empty_careful(&hctx->dispatch_wait.entry))) blk_mq_run_hw_queue(hctx, true); + else if (needs_restart && (ret == BLK_STS_RESOURCE)) + blk_mq_delay_run_hw_queue(hctx, BLK_MQ_QUEUE_DELAY); }
If a request completes concurrently with execution of the above code then the request completion will trigger a call of blk_mq_sched_restart_hctx() and that call will clear the BLK_MQ_S_SCHED_RESTART bit. If that bit is cleared before the above code tests it then the above code will schedule an asynchronous call of __blk_mq_run_hw_queue(). If the .queue_rq() call triggered by the new queue run returns again BLK_STS_RESOURCE then the above code will be executed again. In other words, a loop occurs. That loop will repeat as long as the described race occurs. The current (kernel v4.15) block layer behavior is simpler: only block drivers call blk_mq_delay_run_hw_queue() and the block layer core never calls that function. Hence that loop cannot occur with the v4.15 block layer core and block drivers. A motivation of why that loop is preferred compared to the current behavior (no loop) is missing. Does this mean that that loop is a needless complication of the block layer core?
Sorry but I still prefer the v4.15 block layer approach because this patch has in my view the following disadvantages:
- It involves a blk-mq API change. API changes are always risky and need some time to stabilize. - The delay after which to rerun the queue is moved from block layer drivers into the block layer core. I think that's wrong because only the block driver authors can make a good choice for this constant. - This patch makes block drivers harder to understand. Anyone who sees return BLK_STS_RESOURCE / return BLK_STS_DEV_RESOURCE for the first time will have to look up the meaning of these constants. An explicit blk_mq_delay_run_hw_queue() call is easier to understand. - This patch makes the blk-mq core harder to understand because of the loop mentioned above. - This patch does not fix any bugs nor makes block drivers easier to read or to implement. So why is this patch considered useful? Thanks, Bart.
