Re: fuse-io-uring: We need to keep the tag/index

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, 3 Oct 2024 at 12:10, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:

> What I mean is that you wanted to get rid of the 'tag' - using any kind of
> search means we still need it. I.e. we cannot just take last list head
> or tail and use that.
> The array is only dynamic at initialization time. And why spending O(logN)
> to search instead of O(1)?

Because for sane queue depths they are essentially the same.  This is
not where we can gain or lose any significant performance.

> And I know that it is an implementation detail, I just would like to avoid
> many rebasing rounds on these details.

I think the logical interface would be:

 - pass a userspace buffer to FETCH (you told me, but I don't remember
why sqe->addr isn't suitable)

 - set sqe->user_data to an implementation dependent value, this could
be just the userspace buffer, but it could be a request object

 - kernel allocates an idle request and queues it.

 - request comes in, kernel takes a request from the idle queue and fills it

 - cqe->user_data is returned with the original sqe->user_data, which
should be sufficient for the server to identify the request

 - process request, send COMMIT_AND_FETCH with the userspace buffer
and user data

 - the kernel reads the header from the userspace buffer, finds
outh->unique, finds and completes the request

 - then queues the request on the idle queue

...

What's wrong with that?

Thanks,
Miklos