Re: [PATCH v8 5/7] io-uring: add sqpoll support for napi busy poll

[Jens Axboe <axboe@kernel.dk>]

On 2/9/23 4:01?PM, Stefan Roesch wrote:
> This adds the sqpoll support to the io-uring napi.

This should also have a bit more of an explanation of _why_ this
is needed and being done.

> diff --git a/io_uring/napi.c b/io_uring/napi.c
> index c9e2afae382d..038957b46a0e 100644
> --- a/io_uring/napi.c
> +++ b/io_uring/napi.c
> @@ -278,4 +278,29 @@ void io_napi_end_busy_loop(struct io_ring_ctx *ctx, struct io_wait_queue *iowq,
>  		io_napi_merge_lists(ctx, napi_list);
>  }
>  
> +/*
> + * io_napi_sqpoll_busy_poll() - busy poll loop for sqpoll
> + * @ctx: pointer to io-uring context structure
> + * @napi_list: pointer to head of napi list
> + *
> + * Splice of the napi list and execute the napi busy poll loop.
> + */
> +int io_napi_sqpoll_busy_poll(struct io_ring_ctx *ctx, struct list_head *napi_list)
> +{
> +	int ret = 0;
> +
> +	spin_lock(&ctx->napi_lock);
> +	list_splice_init(&ctx->napi_list, napi_list);
> +	spin_unlock(&ctx->napi_lock);
> +
> +	if (!list_empty(napi_list) &&
> +	    READ_ONCE(ctx->napi_busy_poll_to) > 0 &&
> +	    io_napi_busy_loop(napi_list, ctx->napi_prefer_busy_poll)) {
> +		io_napi_merge_lists(ctx, napi_list);
> +		ret = 1;
> +	}
> +
> +	return ret;

Should 'ret' be a bool and the return value of the function too?

> diff --git a/io_uring/napi.h b/io_uring/napi.h
> index 0672592cfb79..23a6df32805f 100644
> --- a/io_uring/napi.h
> +++ b/io_uring/napi.h
> @@ -23,6 +23,7 @@ void io_napi_adjust_busy_loop_timeout(struct io_ring_ctx *ctx,
>  			struct timespec64 *ts);
>  void io_napi_end_busy_loop(struct io_ring_ctx *ctx, struct io_wait_queue *iowq,
>  			struct list_head *napi_list);
> +int io_napi_sqpoll_busy_poll(struct io_ring_ctx *ctx, struct list_head *napi_list);
>  
>  #else
>  
> @@ -43,6 +44,7 @@ static inline void io_napi_add(struct io_kiocb *req)
>  #define io_napi_setup_busy_loop(ctx, iowq, napi_list) do {} while (0)
>  #define io_napi_adjust_busy_loop_timeout(ctx, iowq, napi_list, ts) do {} while (0)
>  #define io_napi_end_busy_loop(ctx, iowq, napi_list) do {} while (0)
> +#define io_napi_sqpoll_busy_poll(ctx, napi_list) (0)

This should be:

#define io_napi_sqpoll_busy_poll(ctx, napi_list)
{(
)}
do { } while (0)

>  
>  #endif
>  
> diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
> index 0119d3f1a556..90fdbd87434a 100644
> --- a/io_uring/sqpoll.c
> +++ b/io_uring/sqpoll.c
> @@ -15,6 +15,7 @@
>  #include <uapi/linux/io_uring.h>
>  
>  #include "io_uring.h"
> +#include "napi.h"
>  #include "sqpoll.h"
>  
>  #define IORING_SQPOLL_CAP_ENTRIES_VALUE 8
> @@ -168,6 +169,7 @@ static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
>  {
>  	unsigned int to_submit;
>  	int ret = 0;
> +	NAPI_LIST_HEAD(local_napi_list);

In general, keep these roughly in inverse xmas tree.

>  	to_submit = io_sqring_entries(ctx);
>  	/* if we're handling multiple rings, cap submit size for fairness */
> @@ -193,6 +195,8 @@ static int __io_sq_thread(struct io_ring_ctx *ctx, bool cap_entries)
>  			ret = io_submit_sqes(ctx, to_submit);
>  		mutex_unlock(&ctx->uring_lock);
>  
> +		ret += io_napi_sqpoll_busy_poll(ctx, &local_napi_list);
> +
>  		if (to_submit && wq_has_sleeper(&ctx->sqo_sq_wait))
>  			wake_up(&ctx->sqo_sq_wait);
>  		if (creds)

Not clear to me what this ret += ... does here? We're currently
returning number of sqes submitted. Maybe this is fine and just means
'we did some work', but if so, should add a comment for that.

-- 
Jens Axboe