[PATCH 2/2] nvme: use blk-mq polling for uring commands

[Kanchan Joshi]

On Mon, Mar 27, 2023 at 06:48:30PM -0600, Keith Busch wrote:
>On Mon, Mar 27, 2023 at 10:50:47PM +0530, Kanchan Joshi wrote:
>> On Mon, Mar 27, 2023 at 8:59 PM Keith Busch <kbusch at kernel.org> wrote:
>> > > >     rcu_read_lock();
>> > > > -   bio = READ_ONCE(ioucmd->cookie);
>> > > > -   ns = container_of(file_inode(ioucmd->file)->i_cdev,
>> > > > -                   struct nvme_ns, cdev);
>> > > > -   q = ns->queue;
>> > > > -   if (test_bit(QUEUE_FLAG_POLL, &q->queue_flags) && bio && bio->bi_bdev)
>> > > > -           ret = bio_poll(bio, iob, poll_flags);
>> > > > +   req = READ_ONCE(ioucmd->cookie);
>> > > > +   if (req) {
>> > >
>> > > This is risky. We are not sure if the cookie is actually "req" at this
>> > > moment.
>> >
>> > What else could it be? It's either a real request from a polled hctx tag, or
>> > NULL at this point.
>>
>> It can also be a function pointer that gets assigned on irq-driven completion.
>> See the "struct io_uring_cmd" - we are tight on cacheline, so cookie
>> and task_work_cb share the storage.
>>
>> > It's safe to check the cookie like this and rely on its contents.
>> Hence not safe. Please try running this without poll-queues (at nvme
>> level), you'll see failures.
>
>Okay, you have a iouring polling instance used with a file that has poll
>capabilities, but doesn't have any polling hctx's. It would be nice to exclude
>these from io_uring's polling since they're wasting CPU time, but that doesn't
>look easily done.

Do you mean having the ring with IOPOLL set, and yet skip the attempt of
actively reaping the completion for certain IOs?

>This simple patch atop should work though.
>
>---
>diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
>index 369e8519b87a2..e3ff019404816 100644
>--- a/drivers/nvme/host/ioctl.c
>+++ b/drivers/nvme/host/ioctl.c
>@@ -612,6 +612,8 @@ static int nvme_uring_cmd_io(struct nvme_ctrl *ctrl, struct nvme_ns *ns,
>
> 	if (blk_rq_is_poll(req))
> 		WRITE_ONCE(ioucmd->cookie, req);
>+	else if (issue_flags & IO_URING_F_IOPOLL)
>+		ioucmd->flags |= IORING_URING_CMD_NOPOLL;

If IO_URING_F_IOPOLL would have come here as part of "ioucmd->flags", we
could have just cleared that here. That would avoid the need of NOPOLL flag.
That said, I don't feel strongly about new flag too. You decide.