[dm-devel] [PATCH V3] blk-mq: introduce BLK_STS_DEV_RESOURCE
Bart Van Assche
Bart.VanAssche at wdc.com
Tue Jan 23 16:24:20 UTC 2018
On Wed, 2018-01-24 at 00:16 +0800, Ming Lei wrote:
> @@ -1280,10 +1282,18 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> * - Some but not all block drivers stop a queue before
> * returning BLK_STS_RESOURCE. Two exceptions are scsi-mq
> * and dm-rq.
> + *
> + * If drivers return BLK_STS_RESOURCE and S_SCHED_RESTART
> + * bit is set, run queue after 10ms for avoiding IO hang
> + * because the queue may be idle and the RESTART mechanism
> + * can't work any more.
> */
> - 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, 10);
> }
My opinion about this patch is as follows:
* Changing a blk_mq_delay_run_hw_queue() call followed by return
BLK_STS_DEV_RESOURCE into return BLK_STS_RESOURCE is wrong because it changes
a guaranteed queue rerun into a queue rerun that may or may not happen
depending on whether or not multiple queue runs happen simultaneously.
* This change makes block drivers less readable because anyone who encounters
BLK_STS_DEV_RESOURCE will have to look up its definition to figure out what
it's meaning is.
* We don't need the new status code BLK_STS_DEV_RESOURCE because a delayed
queue run can be implemented easily with the existing block layer API.
Bart.
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