Re: [PATCH v3 2/2] io_uring: batch getting pcpu references

Pavel Begunkov
2019-12-21T21:57:14Z

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On 21/12/2019 23:12, Pavel Begunkov wrote:
> percpu_ref_tryget() has its own overhead. Instead getting a reference
> for each request, grab a bunch once per io_submit_sqes().
>
> basic benchmark with submit and wait 128 non-linked nops showed ~5%
> performance gain. (7044 KIOPS vs 7423)
>

Hmm, this won't work. Please drop it.

> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>
> It's just becoming more bulky with ret for me, and would love to hear,
> hot to make it clearer. This version removes all error handling from
> hot path, though with goto.
>
> fs/io_uring.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 513f1922ce6a..b89a8b975c69 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1045,9 +1045,6 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
> gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
> struct io_kiocb *req;
>
> - if (!percpu_ref_tryget(&ctx->refs))
> - return NULL;
> -
> if (!state) {
> req = kmem_cache_alloc(req_cachep, gfp);
> if (unlikely(!req))
> @@ -4400,6 +4397,9 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
> return -EBUSY;
> }
>
> + if (!percpu_ref_tryget_many(&ctx->refs, nr))
> + return -EAGAIN;
> +
> if (nr > IO_PLUG_THRESHOLD) {
> io_submit_state_start(&state, nr);
> statep = &state;
> @@ -4408,16 +4408,22 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
> for (i = 0; i < nr; i++) {
> const struct io_uring_sqe *sqe;
> struct io_kiocb *req;
> + unsigned int unused_refs;
>
> req = io_get_req(ctx, statep);
> if (unlikely(!req)) {
> + unused_refs = nr - submitted;
> if (!submitted)
> submitted = -EAGAIN;
> +put_refs:
> + percpu_ref_put_many(&ctx->refs, unused_refs);
> break;
> }
> if (!io_get_sqring(ctx, req, &sqe)) {
> __io_free_req(req);
> - break;
> + /* __io_free_req() puts a ref */
> + unused_refs = nr - submitted - 1;
> + goto put_refs;
> }
>
> /* will complete beyond this point, count as submitted */
>

--
Pavel Begunkov

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