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

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