Re: [PATCH 3/3] io_uring: briefly loose locks while reaping events

[Jens Axboe <axboe@kernel.dk>]

On 7/6/20 8:14 AM, Pavel Begunkov wrote:
> It's not nice to hold @uring_lock for too long io_iopoll_reap_events().
> For instance, the lock is needed to publish requests to @poll_list, and
> that locks out tasks doing that for no good reason. Loose it
> occasionally.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 020944a193d0..568e25bcadd6 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2059,11 +2059,14 @@ static void io_iopoll_reap_events(struct io_ring_ctx *ctx)
>  
>  		io_iopoll_getevents(ctx, &nr_events, 1);
>  
> +		/* task_work takes the lock to publish a req to @poll_list */
> +		mutex_unlock(&ctx->uring_lock);
>  		/*
>  		 * Ensure we allow local-to-the-cpu processing to take place,
>  		 * in this case we need to ensure that we reap all events.
>  		 */
>  		cond_resched();
> +		mutex_lock(&ctx->uring_lock);
>  	}
>  	mutex_unlock(&ctx->uring_lock);
>  }

This would be much better written as:

if (need_resched()) {
	mutex_unlock(&ctx->uring_lock);
	cond_resched();
	mutex_lock(&ctx->uring_lock);
}

to avoid shuffling the lock when not needed to. Every cycle counts for
polled IO.

-- 
Jens Axboe