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

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Date:   Sun, 22 Dec 2019 00:56:42 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] io_uring: batch getting pcpu references

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@...il.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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