linux-kernel - Re: [PATCH 2/2] io_uring: Create define to modify a SQPOLL parameter

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Date:   Tue, 22 Jun 2021 21:58:33 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Olivier Langlois <olivier@...llion01.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] io_uring: Create define to modify a SQPOLL parameter

On 6/22/21 7:45 PM, Olivier Langlois wrote:
> The magic number used to cap the number of entries extracted from an
> io_uring instance SQ before moving to the other instances is an
> interesting parameter to experiment with.

Not particularly related to this patch, but the problem with this
capping is that there is no reliable way to do request linking
using shared sqpoll. It may break a link in half (or in N parts for
long links) and submit them separately and in parallel.


> A define has been created to make it easy to change its value from a
> single location.
> 
> Signed-off-by: Olivier Langlois <olivier@...llion01.com>
> ---
>  fs/io_uring.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 02f789e07d4c..3f271bd7726b 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -89,6 +89,7 @@
>  
>  #define IORING_MAX_ENTRIES	32768
>  #define IORING_MAX_CQ_ENTRIES	(2 * IORING_MAX_ENTRIES)
> +#define IORING_SQPOLL_CAP_ENTRIES_VALUE 8
>  
>  /*
>   * Shift of 9 is 512 entries, or exactly one page on 64-bit archs
> @@ -6797,8 +6798,8 @@ static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
>  
>  	to_submit = io_sqring_entries(ctx);
>  	/* if we're handling multiple rings, cap submit size for fairness */
> -	if (cap_entries && to_submit > 8)
> -		to_submit = 8;
> +	if (cap_entries && to_submit > IORING_SQPOLL_CAP_ENTRIES_VALUE)
> +		to_submit = IORING_SQPOLL_CAP_ENTRIES_VALUE;
>  
>  	if (!list_empty(&ctx->iopoll_list) || to_submit) {
>  		unsigned nr_events = 0;
> 

-- 
Pavel Begunkov