Re: [PATCH 17/17] io_uring: defer flushing cached reqs

[Jens Axboe <axboe@kernel.dk>]

On 2/9/21 5:03 PM, Pavel Begunkov wrote:
> Awhile there are requests in the allocation cache -- use them, only if
> those ended go for the stashed memory in comp.free_list. As list
> manipulation are generally heavy and are not good for caches, flush them
> all or as much as can in one go.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 64d3f3e2e93d..17194e0d62ff 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1953,25 +1953,34 @@ static inline void io_req_complete(struct io_kiocb *req, long res)
>  	__io_req_complete(req, 0, res, 0);
>  }
>  
> +static void io_flush_cached_reqs(struct io_submit_state *state)
> +{
> +	do {
> +		struct io_kiocb *req = list_first_entry(&state->comp.free_list,
> +						struct io_kiocb, compl.list);
> +
> +		list_del(&req->compl.list);
> +		state->reqs[state->free_reqs++] = req;
> +		if (state->free_reqs == ARRAY_SIZE(state->reqs))
> +			break;
> +	} while (!list_empty(&state->comp.free_list));
> +}
> +
>  static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
>  {
>  	struct io_submit_state *state = &ctx->submit_state;
>  
>  	BUILD_BUG_ON(IO_REQ_ALLOC_BATCH > ARRAY_SIZE(state->reqs));
>  
> -	if (!list_empty(&state->comp.free_list)) {
> -		struct io_kiocb *req;
> -
> -		req = list_first_entry(&state->comp.free_list, struct io_kiocb,
> -					compl.list);
> -		list_del(&req->compl.list);
> -		return req;
> -	}
> -
>  	if (!state->free_reqs) {
>  		gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
>  		int ret;
>  
> +		if (!list_empty(&state->comp.free_list)) {
> +			io_flush_cached_reqs(state);
> +			goto out;
> +		}

I think that'd be cleaner as:

	if (io_flush_cached_reqs(state))
		goto got_req;

and have io_flush_cached_reqs() return true/false depending on what it did.

-- 
Jens Axboe