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SubjectRe: RFC: Allow block drivers to poll for I/O instead of sleeping
On Mon, Jun 24, 2013 at 09:15:45AM +0200, Jens Axboe wrote:
> Willy, I think the general design is fine, hooking in via the bdi is the
> only way to get back to the right place from where you need to sleep.
> Some thoughts:
>
> - This should be hooked in via blk-iopoll, both of them should call into
> the same driver hook for polling completions.

I actually started working on this, then I realised that it's actually
a bad idea. blk-iopoll's poll function is to poll the single I/O queue
closest to this CPU. The iowait poll function is to poll all queues
that the I/O for this address_space might complete on.

I'm reluctant to ask drivers to define two poll functions, but I'm even
more reluctant to ask them to define one function with two purposes.

> - It needs to be more intelligent in when you want to poll and when you
> want regular irq driven IO.

Oh yeah, absolutely. While the example patch didn't show it, I wouldn't
enable it for all NVMe devices; only ones with sufficiently low latency.
There's also the ability for the driver to look at the number of
outstanding I/Os and return an error (eg -EBUSY) to stop spinning.

> - With the former note, the app either needs to opt in (and hence
> willingly sacrifice CPU cycles of its scheduling slice) or it needs to
> be nicer in when it gives up and goes back to irq driven IO.

Yup. I like the way you framed it. If the task *wants* to spend its
CPU cycles on polling for I/O instead of giving up the remainder of its
time slice, then it should be able to do that. After all, it already can;
it can submit an I/O request via AIO, and then call io_getevents in a
tight loop.

So maybe the right way to do this is with a task flag? If we go that
route, I'd like to further develop this option to allow I/Os to be
designated as "low latency" vs "normal". Taking a page fault would be
"low latency" for all tasks, not just ones that choose to spin for I/O.


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