Re: [RFC] io_uring CQ ring backpressure

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

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On 06/11/2019 19:21, Jens Axboe wrote:
> Currently we drop completion events, if the CQ ring is full. That's fine
> for requests with bounded completion times, but it may make it harder to
> use io_uring with networked IO where request completion times are
> generally unbounded. Or with POLL, for example, which is also unbounded.
> 
> This patch adds IORING_SETUP_CQ_NODROP, which changes the behavior a bit
> for CQ ring overflows. First of all, it doesn't overflow the ring, it
> simply stores backlog of completions that we weren't able to put into
> the CQ ring. To prevent the backlog from growing indefinitely, if the
> backlog is non-empty, we apply back pressure on IO submissions. Any
> attempt to submit new IO with a non-empty backlog will get an -EBUSY
> return from the kernel.
> 
> I think that makes for a pretty sane API in terms of how the application
> can handle it. With CQ_NODROP enabled, we'll never drop a completion
> event (well unless we're totally out of memory...), but we'll also not
> allow submissions with a completion backlog.
> 
> ---
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index b647cdf0312c..12e9fe2479f4 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -207,6 +207,7 @@ struct io_ring_ctx {
>  
>  		struct list_head	defer_list;
>  		struct list_head	timeout_list;
> +		struct list_head	cq_overflow_list;
>  
>  		wait_queue_head_t	inflight_wait;
>  	} ____cacheline_aligned_in_smp;
> @@ -414,6 +415,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>  
>  	ctx->flags = p->flags;
>  	init_waitqueue_head(&ctx->cq_wait);
> +	INIT_LIST_HEAD(&ctx->cq_overflow_list);
>  	init_completion(&ctx->ctx_done);
>  	init_completion(&ctx->sqo_thread_started);
>  	mutex_init(&ctx->uring_lock);
> @@ -588,6 +590,77 @@ static struct io_uring_cqe *io_get_cqring(struct io_ring_ctx *ctx)
>  	return &rings->cqes[tail & ctx->cq_mask];
>  }
>  
> +static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
> +{
> +	if (waitqueue_active(&ctx->wait))
> +		wake_up(&ctx->wait);
> +	if (waitqueue_active(&ctx->sqo_wait))
> +		wake_up(&ctx->sqo_wait);
> +	if (ctx->cq_ev_fd)
> +		eventfd_signal(ctx->cq_ev_fd, 1);
> +}
> +
> +struct cqe_drop {
> +	struct list_head list;
> +	u64 user_data;
> +	s32 res;
> +};

How about to use io_kiocb instead of new structure?
It already has valid req->user_data and occasionaly used
req->result. But this would probably take more work to do.

> +
> +static void io_cqring_overflow_flush(struct io_ring_ctx *ctx)
> +{
> +	struct io_rings *rings = ctx->rings;
> +	struct io_uring_cqe *cqe;
> +	struct cqe_drop *drop;
> +	unsigned long flags;
> +
> +	if (list_empty_careful(&ctx->cq_overflow_list))
> +		return;
> +	if (ctx->cached_cq_tail - READ_ONCE(rings->cq.head) ==
> +	    rings->cq_ring_entries)
> +		return;
> +
> +	spin_lock_irqsave(&ctx->completion_lock, flags);
> +
> +	while (!list_empty(&ctx->cq_overflow_list)) {
> +		drop = list_first_entry(&ctx->cq_overflow_list, struct cqe_drop,
> +						list);
> +		cqe = io_get_cqring(ctx);
> +		if (!cqe)
> +			break;
> +		list_del(&drop->list);
> +		WRITE_ONCE(cqe->user_data, drop->user_data);
> +		WRITE_ONCE(cqe->res, drop->res);
> +		WRITE_ONCE(cqe->flags, 0);
> +		kfree(drop);
> +	}
> +
> +	io_commit_cqring(ctx);
> +	spin_unlock_irqrestore(&ctx->completion_lock, flags);
> +	io_cqring_ev_posted(ctx);
> +}
> +
> +static void io_cqring_overflow(struct io_ring_ctx *ctx, u64 ki_user_data,
> +			       long res)
> +	__must_hold(&ctx->completion_lock)
> +{
> +	struct cqe_drop *drop;
> +
> +	if (!(ctx->flags & IORING_SETUP_CQ_NODROP)) {
> +log_overflow:
> +		WRITE_ONCE(ctx->rings->cq_overflow,
> +				atomic_inc_return(&ctx->cached_cq_overflow));
> +		return;
> +	}
> +
> +	drop = kmalloc(sizeof(*drop), GFP_ATOMIC);
> +	if (!drop)
> +		goto log_overflow;
> +
> +	drop->user_data = ki_user_data;
> +	drop->res = res;
> +	list_add_tail(&drop->list, &ctx->cq_overflow_list);
> +}
> +
>  static void io_cqring_fill_event(struct io_ring_ctx *ctx, u64 ki_user_data,
>  				 long res)
>  {
> @@ -601,26 +674,15 @@ static void io_cqring_fill_event(struct io_ring_ctx *ctx, u64 ki_user_data,
>  	 * the ring.
>  	 */
>  	cqe = io_get_cqring(ctx);
> -	if (cqe) {
> +	if (likely(cqe)) {
>  		WRITE_ONCE(cqe->user_data, ki_user_data);
>  		WRITE_ONCE(cqe->res, res);
>  		WRITE_ONCE(cqe->flags, 0);
>  	} else {
> -		WRITE_ONCE(ctx->rings->cq_overflow,
> -				atomic_inc_return(&ctx->cached_cq_overflow));
> +		io_cqring_overflow(ctx, ki_user_data, res);
>  	}
>  }
>  
> -static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
> -{
> -	if (waitqueue_active(&ctx->wait))
> -		wake_up(&ctx->wait);
> -	if (waitqueue_active(&ctx->sqo_wait))
> -		wake_up(&ctx->sqo_wait);
> -	if (ctx->cq_ev_fd)
> -		eventfd_signal(ctx->cq_ev_fd, 1);
> -}
> -
>  static void io_cqring_add_event(struct io_ring_ctx *ctx, u64 user_data,
>  				long res)
>  {
> @@ -859,8 +921,13 @@ static void io_put_req(struct io_kiocb *req, struct io_kiocb **nxtptr)
>  	}
>  }
>  
> -static unsigned io_cqring_events(struct io_rings *rings)
> +static unsigned io_cqring_events(struct io_ring_ctx *ctx)
>  {
> +	struct io_rings *rings = ctx->rings;
> +
> +	if (ctx->flags & IORING_SETUP_CQ_NODROP)
> +		io_cqring_overflow_flush(ctx);
> +
>  	/* See comment at the top of this file */
>  	smp_rmb();
>  	return READ_ONCE(rings->cq.tail) - READ_ONCE(rings->cq.head);
> @@ -1016,7 +1083,7 @@ static int __io_iopoll_check(struct io_ring_ctx *ctx, unsigned *nr_events,
>  		 * If we do, we can potentially be spinning for commands that
>  		 * already triggered a CQE (eg in error).
>  		 */
> -		if (io_cqring_events(ctx->rings))
> +		if (io_cqring_events(ctx))
>  			break;
>  
>  		/*
> @@ -2873,6 +2940,10 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  	int i, submitted = 0;
>  	bool mm_fault = false;
>  
> +	if ((ctx->flags & IORING_SETUP_CQ_NODROP) &&
> +	    !list_empty(&ctx->cq_overflow_list))
> +		return -EBUSY;
> +
>  	if (nr > IO_PLUG_THRESHOLD) {
>  		io_submit_state_start(&state, ctx, nr);
>  		statep = &state;
> @@ -2952,6 +3023,7 @@ static int io_sq_thread(void *data)
>  	timeout = inflight = 0;
>  	while (!kthread_should_park()) {
>  		unsigned int to_submit;
> +		int ret;
>  
>  		if (inflight) {
>  			unsigned nr_events = 0;
> @@ -3036,8 +3108,10 @@ static int io_sq_thread(void *data)
>  		}
>  
>  		to_submit = min(to_submit, ctx->sq_entries);
> -		inflight += io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm,
> -					   true);
> +		ret = io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm, true);
> +		if (ret < 0)
> +			continue;
> +		inflight += ret;
>  

After rebase could be simplified to 
if (ret >= 0)
	inflight += ret;


>  		/* Commit SQ ring head once we've consumed all SQEs */
>  		io_commit_sqring(ctx);
> @@ -3070,7 +3144,7 @@ static inline bool io_should_wake(struct io_wait_queue *iowq)
>  	 * started waiting. For timeouts, we always want to return to userspace,
>  	 * regardless of event count.
>  	 */
> -	return io_cqring_events(ctx->rings) >= iowq->to_wait ||
> +	return io_cqring_events(ctx) >= iowq->to_wait ||
>  			atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
>  }
>  
> @@ -3105,7 +3179,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>  	struct io_rings *rings = ctx->rings;
>  	int ret = 0;
>  
> -	if (io_cqring_events(rings) >= min_events)
> +	if (io_cqring_events(ctx) >= min_events)
>  		return 0;
>  
>  	if (sig) {
> @@ -4406,7 +4480,8 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
>  	}
>  
>  	if (p.flags & ~(IORING_SETUP_IOPOLL | IORING_SETUP_SQPOLL |
> -			IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE))
> +			IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE |
> +			IORING_SETUP_CQ_NODROP))
>  		return -EINVAL;
>  
>  	ret = io_uring_create(entries, &p);
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index f1a118b01d18..3d8517eb376e 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -56,6 +56,7 @@ struct io_uring_sqe {
>  #define IORING_SETUP_SQPOLL	(1U << 1)	/* SQ poll thread */
>  #define IORING_SETUP_SQ_AFF	(1U << 2)	/* sq_thread_cpu is valid */
>  #define IORING_SETUP_CQSIZE	(1U << 3)	/* app defines CQ size */
> +#define IORING_SETUP_CQ_NODROP	(1U << 4)	/* no CQ drops */
>  
>  #define IORING_OP_NOP		0
>  #define IORING_OP_READV		1
> 

-- 
Pavel Begunkov


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