Re: [PATCH 5.9] io_uring: replace rw->task_work with rq->task_work

[Jens Axboe <axboe@kernel.dk>]

On 7/13/20 2:03 AM, Pavel Begunkov wrote:
> On 12/07/2020 23:29, Jens Axboe wrote:
>> On 7/12/20 11:42 AM, Pavel Begunkov wrote:
>>> io_kiocb::task_work was de-unionised, and is not planned to be shared
>>> back, because it's too useful and commonly used. Hence, instead of
>>> keeping a separate task_work in struct io_async_rw just reuse
>>> req->task_work.
>>
>> This is a good idea, req->task_work is a first class citizen these days.
>> Unfortunately it doesn't do much good for io_async_ctx, since it's so
>> huge with the msghdr related bits. It'd be nice to do something about
>> that too, though not a huge priority as allocating async context is
> 
> We can allocate not an entire struct/union io_async_ctx but its particular
> member. Should be a bit better for writes.

Right, we probably just need to turn req->io into a:

	void *async_ctx;

and have it be assigned with the various types that are needed for
async deferral.

> And if we can save another 16B in io_async_rw, it'd be 3 cachelines for
> io_async_rw. E.g. there are two 4B holes in struct wait_page_queue, one is
> from "int bit_nr", the second is inside "wait_queue_entry_t wait".

An easy 8 bytes is just turning nr_segs and size into 32-bit types. The
size will never be more than 2G, and segs is limited at 1k iirc.

-- 
Jens Axboe