Re: [PATCH v6 1/4] md: add support for REQ_NOWAIT

[Jens Axboe <axboe@kernel.dk>]

On 12/21/21 1:06 PM, Vishal Verma wrote:
> commit 021a24460dc2 ("block: add QUEUE_FLAG_NOWAIT") added support
> for checking whether a given bdev supports handling of REQ_NOWAIT or not.
> Since then commit 6abc49468eea ("dm: add support for REQ_NOWAIT and enable
> it for linear target") added support for REQ_NOWAIT for dm. This uses
> a similar approach to incorporate REQ_NOWAIT for md based bios.
> 
> This patch was tested using t/io_uring tool within FIO. A nvme drive
> was partitioned into 2 partitions and a simple raid 0 configuration
> /dev/md0 was created.
> 
> md0 : active raid0 nvme4n1p1[1] nvme4n1p2[0]
>       937423872 blocks super 1.2 512k chunks
> 
> Before patch:
> 
> $ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100
> 
> Running top while the above runs:
> 
> $ ps -eL | grep $(pidof io_uring)
> 
>   38396   38396 pts/2    00:00:00 io_uring
>   38396   38397 pts/2    00:00:15 io_uring
>   38396   38398 pts/2    00:00:13 iou-wrk-38397
> 
> We can see iou-wrk-38397 io worker thread created which gets created
> when io_uring sees that the underlying device (/dev/md0 in this case)
> doesn't support nowait.
> 
> After patch:
> 
> $ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100
> 
> Running top while the above runs:
> 
> $ ps -eL | grep $(pidof io_uring)
> 
>   38341   38341 pts/2    00:10:22 io_uring
>   38341   38342 pts/2    00:10:37 io_uring
> 
> After running this patch, we don't see any io worker thread
> being created which indicated that io_uring saw that the
> underlying device does support nowait. This is the exact behaviour
> noticed on a dm device which also supports nowait.
> 
> For all the other raid personalities except raid0, we would need
> to train pieces which involves make_request fn in order for them
> to correctly handle REQ_NOWAIT.

1-4 look fine to me now:

Reviewed-by: Jens Axboe <axboe@kernel.dk>

-- 
Jens Axboe