linux-kernel - Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

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Date:	Thu, 15 Aug 2013 18:23:16 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Mike Christie <mchristie@...ionio.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	Jeff Garzik <jgarzik@...ox.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

On Fri, Aug 09, 2013 at 01:17:37PM -0700, Nicholas A. Bellinger wrote:
> On Fri, 2013-08-09 at 21:15 +0200, Alexander Gordeev wrote:
> Mmmm, I'm able to reproduce over here with ahci + scsi-mq, and it
> appears to be a bug related with using sdev->sdev_md_req.queue_depth=1,
> that ends up causing the blkdev_issue_flush() to wait forever because
> blk_mq_wait_for_tags() never ends up getting the single tag back for the
> WRITE_FLUSH bio -> SYNCHRONIZE_CACHE cdb.

It turns out this way - blkdev_issue_flush() claims the only tag, submits
the bio and waits for the completion. But because blk_mq_make_request()
does not mark any context in blk_mq_hw_ctx::ctx_map (nor enslists the request
into blk_mq_ctx::rq_list) it never gets processed from blk_mq_work_fn->
__blk_mq_run_hw_queue() and blkdev_issue_flush() waits endlessly. All other
requests are just waiting for the tag availability as result.

[...]

> Bumping queue_depth=2 seems to work-around the issue, but AFAICT it's a
> genuine tag starvation bug with queue_depth=1 and WRITE_FLUSH..

If I try to hack and force __blk_mq_run_hw_queue() to process the request...

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 6fc1df3..c22b6f66 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -889,9 +962,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
 	hctx->queued++;
 
 	if (unlikely(is_flush_fua)) {
+		list_add(&rq->queuelist, &hctx->dispatch);
 		blk_mq_bio_to_request(q, rq, bio);
 		blk_mq_put_ctx(ctx);
 		blk_insert_flush(rq);
 		goto run_queue;
 	}

... I get a kernel BUG at drivers/scsi/scsi_lib.c:1233

	BUG_ON(!req->nr_phys_segments);

IOW I am not sure how to proceed.

> --nab
> 

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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