On Tue, Jan 30, 2018 at 09:52:31AM -0800, Bart Van Assche wrote: > 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 Right. > 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 This patch doesn't change anything about that. When BLK_STS_RESOURCE is returned, this request is added to hctx->dispatch, next time, before dispatching this request, SCHED_RESTART is set and the loop is cut. > 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 That way isn't safe, I have explained to you in the following link: https://marc.info/?l=dm-devel&m=151727454815018&w=2 > 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? No such loop as I explained above. > > 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? It does avoid the race I mentioned in the following link: https://marc.info/?l=dm-devel&m=151727454815018&w=2 More importantly, every driver need this change, if you have better idea to fix them all, please post a patch, then we can compare both. Thanks, Ming
