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

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Date:   Tue, 17 Dec 2019 16:21:32 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] io_uring: batch getting pcpu references

On 12/17/19 3:28 PM, 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)
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
> 
> For notice: it could be done without @extra_refs variable,
> but looked too tangled because of gotos.
> 
> 
>  fs/io_uring.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index cf4138f0e504..6c85dfc62224 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -845,9 +845,6 @@ 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))
> @@ -3929,6 +3926,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  	struct io_submit_state state, *statep = NULL;
>  	struct io_kiocb *link = NULL;
>  	int i, submitted = 0;
> +	unsigned int extra_refs;
>  	bool mm_fault = false;
>  
>  	/* if we have a backlog and couldn't flush it all, return BUSY */
> @@ -3941,6 +3939,10 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  		statep = &state;
>  	}
>  
> +	if (!percpu_ref_tryget_many(&ctx->refs, nr))
> +		return -EAGAIN;
> +	extra_refs = nr;
> +
>  	for (i = 0; i < nr; i++) {
>  		struct io_kiocb *req = io_get_req(ctx, statep);
>  
> @@ -3949,6 +3951,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  				submitted = -EAGAIN;
>  			break;
>  		}
> +		--extra_refs;
>  		if (!io_get_sqring(ctx, req)) {
>  			__io_free_req(req);
>  			break;
> @@ -3976,6 +3979,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  		io_queue_link_head(link);
>  	if (statep)
>  		io_submit_state_end(&state);
> +	if (extra_refs)
> +		percpu_ref_put_many(&ctx->refs, extra_refs);

Might be cleaner to introduce a 'ret' variable, and leave submitted to be just
that, the number submitted. Then you could just do:

if (submitted != nr)
	percpu_ref_put_many(&ctx->refs, nr - submitted);

and not need that weird extra_refs.

-- 
Jens Axboe