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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

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".


# pahole -C io_async_ctx ./fs/io_uring.o
struct io_async_ctx {
        union {
                struct io_async_rw rw;                   /*     0   208 */
                struct io_async_msghdr msg;              /*     0   368 */
                struct io_async_connect connect;         /*     0   128 */
                struct io_timeout_data timeout __attribute__((__aligned__(8)));
							/*     0    96 */
        } __attribute__((__aligned__(8)));               /*     0   368 */

        /* size: 368, cachelines: 6, members: 1 */
        /* forced alignments: 1 */
        /* last cacheline: 48 bytes */
} __attribute__((__aligned__(8)));


> somewhat of a slow path. Though with the proliferation of task_work,
> it's no longer nearly as expensive as it used to be with the async
> thread offload. Could be argued to be a full-on fast path these days.
> 
> Applied, thanks.
> 

-- 
Pavel Begunkov