[Devel] [PATCH hci-8.1 6/7] drivers/md/dm-qcow2: declare DM_TARGET_NOWAIT

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

dm-qcow2 is a request-based target: REQ_NOWAIT handling is done by blk-mq
(tag allocation) before a request reaches us, and clone_and_map_rq()
never blocks. Still, the target does not declare DM_TARGET_NOWAIT, so
dm_table_set_restrictions() clears BLK_FEAT_NOWAIT on the mapped device
and the block device file gets no FMODE_NOWAIT.

As a result io_uring is unable to issue I/O in nowait mode and punts
*every* request to io-wq worker threads: on 4k randwrite (io_uring,
qd=64, 4 jobs) io-wq threads burn ~24% of all cpus doing nothing but a
thread handoff.

                     before            after
  qd=64 nj=4 IOPS    1034K (11% idle)  892K (30% idle)
  qd=1  nj=1 lat     8us               6us

The lower nj=4 number is because io-wq threads were providing "free"
parallelism for the single-cpu submitters; the next patch gets it back
without burning the cpus.

https://virtuozzo.atlassian.net/browse/VSTOR-95494
Feature: dm-qcow2: block device over QCOW2 files driver
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/md/dm-qcow2-target.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
index be09615eb8e2e..4b9ae86ac9443 100644
--- a/drivers/md/dm-qcow2-target.c
+++ b/drivers/md/dm-qcow2-target.c
@@ -1159,7 +1159,7 @@ static void qcow2_resume(struct dm_target *ti)
 static struct target_type qcow2_target = {
 	.name = "qcow2",
 	.version = {1, 0, 0},
-	.features = DM_TARGET_SINGLETON,
+	.features = DM_TARGET_SINGLETON | DM_TARGET_NOWAIT,
 	.module = THIS_MODULE,
 	.ctr = qcow2_ctr,
 	.dtr = qcow2_dtr,
-- 
2.43.5

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