Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

[Bart Van Assche]

On 01/19/18 07:24, Jens Axboe wrote:
> That's what I thought. So for a low queue depth underlying queue, it's
> quite possible that this situation can happen. Two potential solutions
> I see:
>
> 1) As described earlier in this thread, having a mechanism for being
>     notified when the scarce resource becomes available. It would not
>     be hard to tap into the existing sbitmap wait queue for that.
>
> 2) Have dm set BLK_MQ_F_BLOCKING and just sleep on the resource
>     allocation. I haven't read the dm code to know if this is a
>     possibility or not.
>
> I'd probably prefer #1. It's a classic case of trying to get the
> request, and if it fails, add ourselves to the sbitmap tag wait
> queue head, retry, and bail if that also fails. Connecting the
> scarce resource and the consumer is the only way to really fix
> this, without bogus arbitrary delays.

(replying to an e-mail from ten days ago)

Implementing a notification mechanism for all cases in which 
blk_insert_cloned_request() returns BLK_STS_RESOURCE today would require 
a lot of work. If e.g. a SCSI LLD returns one of the SCSI_MLQUEUE_*_BUSY 
return codes from its .queuecommand() implementation then the SCSI core 
will translate that return code into BLK_STS_RESOURCE. From scsi_queue_rq():

	reason = scsi_dispatch_cmd(cmd);
	if (reason) {
		scsi_set_blocked(cmd, reason);
		ret = BLK_STS_RESOURCE;
		goto out_dec_host_busy;
	}

In other words, implementing a notification mechanism for all cases in 
which blk_insert_cloned_request() can return BLK_STS_RESOURCE would 
require to modify all SCSI LLDs.

Bart.