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

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

On 10/29/26 15:03, Andrey Zhadchenko wrote:
> 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.
This commit drops multi-submitter throughput and the next patch brings
it back, which leaves a regression for bisect to land on. Maybe it has sense
to swap the order, so the batching patch goes first and this one follows?
>
> 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,
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