On Tue, Jan 23 2018 at  7:17am -0500,
Ming Lei [off-list ref] wrote:

On Tue, Jan 23, 2018 at 8:15 PM, Mike Snitzer [off-list ref] wrote:

quoted

On Tue, Jan 23 2018 at  5:53am -0500,
Ming Lei [off-list ref] wrote:

quoted

Hi Mike,

On Tue, Jan 23, 2018 at 10:22:04AM +0100, Mike Snitzer wrote:

quoted

From: Mike Snitzer <redacted>
Date: Tue, 23 Jan 2018 09:40:22 +0100
Subject: [PATCH] block: neutralize blk_insert_cloned_request IO stall regression

The series of blk-mq changes intended to improve sequential IO
performace (through improved merging with dm-mapth blk-mq stacked on
underlying blk-mq device).  Unfortunately these changes have caused
dm-mpath blk-mq IO stalls when blk_mq_request_issue_directly()'s call to
q->mq_ops->queue_rq() fails (due to device-specific resource
unavailability).

Fix this by reverting back to how blk_insert_cloned_request() functioned
prior to commit 396eaf21ee -- by using blk_mq_request_bypass_insert()
instead of blk_mq_request_issue_directly().

In the future, this commit should be reverted as the first change in a
followup series of changes that implements a comprehensive solution to
allowing an underlying blk-mq queue's resource limitation to trigger the
upper blk-mq queue to run once that underlying limited resource is
replenished.

Fixes: 396eaf21ee ("blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback")
Signed-off-by: Mike Snitzer <redacted>
---
 block/blk-core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index cdae69be68e9..a224f282b4a6 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -2520,7 +2520,8 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
             * bypass a potential scheduler on the bottom device for
             * insert.
             */
-           return blk_mq_request_issue_directly(rq);
+           blk_mq_request_bypass_insert(rq, true);
+           return BLK_STS_OK;
    }

If this patch is for fixing IO stall on DM, it isn't needed, and actually
it can't fix the IO stall issue.

It should "fix it" in conjunction with this:

https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.16&id=87b7e73546b55f4a3a37d4726daedd9a17a20509

(Bart already said as much, I just effectively enabled the equivalent of
his reverts)

If the generic solution is take, Bart's revert isn't needed.

Yes, we need Bart to verify your v2 patch:
https://patchwork.kernel.org/patch/10179945/

Bart please test this tree:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=for-next

Please report back with your results (and include details on any changes
you make to the tree, hopefully no changes are needed).

Thanks,
Mike