Re: [PATCH 2/6] io_uring: add io_file_can_poll() helper

[Jens Axboe <axboe@kernel.dk>]

On 2/6/24 5:57 PM, Pavel Begunkov wrote:
> On 2/6/24 16:22, Jens Axboe wrote:
>> This adds a flag to avoid dipping dereferencing file and then f_op
>> to figure out if the file has a poll handler defined or not. We
>> generally call this at least twice for networked workloads.
> 
> Sends are not using poll every time. For recv, we touch it
> in io_arm_poll_handler(), which is done only once, and so
> ammortised to 0 for multishots.

Correct

> Looking at the patch, the second time we might care about is
> in io_ring_buffer_select(), but I'd argue that it shouldn't
> be there in the first place. It's fragile, and I don't see
> why selected buffers would care specifically about polling
> but not asking more generally "can it go true async"? For
> reads you might want to also test FMODE_BUF_RASYNC.

That is indeed the second case that is hit, and I don't think we can
easily get around that which is the reason for the hint.

> Also note that when called from recv we already know that
> it's pollable, it might be much easier to pass it in as an
> argument.

I did think about that, but I don't see a clean way to do it. We could
potentially do it as an issue flag, but that seems kind of ugly to me.
Open to suggestions!

-- 
Jens Axboe