On 11/2/21 8:40 AM, 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.
>
> I also successfully tested this patch on various other
> raid personalities (1, 6 and 10).
This seems incomplete. It looks like it's propagating the nowait
flag based on constituent devices, which is correct, but surely there
are cases off the the make_request path that now need to check for NOWAIT
and return -EAGAIN if they need to block.
>From a quick look, raid0 looks fine as-is. raid1 would need checking
for waiting on a read barrier, for example.
If we just add nowait and don't _actually_ fix the driver, then the
nowait exercise is pointless as it would then just mean that io_uring
would block attempting to submit. That's the broken aio behavior, and
that's certainly not the desired outcome here.
--
Jens Axboe
