Re: [PATCH 2/2] io_uring: do the sqpoll napi busy poll outside the submission block

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/30/24 22:10, Olivier Langlois wrote:
> there are many small reasons justifying this change.
> 
> 1. busy poll must be performed even on rings that have no iopoll and no
>     new sqe. It is quite possible that a ring configured for inbound
>     traffic with multishot be several hours without receiving new request
>     submissions
> 2. NAPI busy poll does not perform any credential validation
> 3. If the thread is awaken by task work, processing the task work is
>     prioritary over NAPI busy loop. This is why a second loop has been
>     created after the io_sq_tw() call instead of doing the busy loop in
>     __io_sq_thread() outside its credential acquisition block.

That patch should be first as it's a fix we care to backport.
It's also

Fixes: 8d0c12a80cdeb ("io-uring: add napi busy poll support")
Cc: stable@vger.kernel.org

And a comment below

> 
> Signed-off-by: Olivier Langlois <olivier@trillion01.com>
> ---
>   io_uring/napi.h   | 9 +++++++++
>   io_uring/sqpoll.c | 7 ++++---
>   2 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/io_uring/napi.h b/io_uring/napi.h
> index 88f1c21d5548..5506c6af1ff5 100644
> --- a/io_uring/napi.h
> +++ b/io_uring/napi.h
> @@ -101,4 +101,13 @@ static inline int io_napi_sqpoll_busy_poll(struct io_ring_ctx *ctx)
>   }
>   #endif /* CONFIG_NET_RX_BUSY_POLL */
>   
> +static inline int io_do_sqpoll_napi(struct io_ring_ctx *ctx)
> +{
> +	int ret = 0;
> +
> +	if (io_napi(ctx))
> +		ret = io_napi_sqpoll_busy_poll(ctx);
> +	return ret;
> +}
> +
>   #endif
> diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
> index cc4a25136030..ec558daa0331 100644
> --- a/io_uring/sqpoll.c
> +++ b/io_uring/sqpoll.c
> @@ -195,9 +195,6 @@ 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);
>   
> -		if (io_napi(ctx))
> -			ret += io_napi_sqpoll_busy_poll(ctx);
> -
>   		if (to_submit && wq_has_sleeper(&ctx->sqo_sq_wait))
>   			wake_up(&ctx->sqo_sq_wait);
>   		if (creds)
> @@ -322,6 +319,10 @@ static int io_sq_thread(void *data)
>   		if (io_sq_tw(&retry_list, IORING_TW_CAP_ENTRIES_VALUE))
>   			sqt_spin = true;
>   
> +		list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
> +			if (io_do_sqpoll_napi(ctx))
> +				sqt_spin = true;

io_do_sqpoll_napi() returns 1 as long as there are napis in the list,
iow even if there is no activity it'll spin almost forever (60s is
forever) bypassing sq_thread_idle.

Let's not update sqt_spin here, if the user wants it to poll for
longer it can pass a larger SQPOLL idle timeout value.


> +		}
>   		if (sqt_spin || !time_after(jiffies, timeout)) {
>   			if (sqt_spin) {
>   				io_sq_update_worktime(sqd, &start);

-- 
Pavel Begunkov