Re: [PATCH v8] io_uring: batch completion in prior_task_list

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

On 12/8/21 05:21, Hao Xu wrote:
> In previous patches, we have already gathered some tw with
> io_req_task_complete() as callback in prior_task_list, let's complete
> them in batch while we cannot grab uring lock. In this way, we batch
> the req_complete_post path.

Works fine now, don't see any problem

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

> 
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
> 
> v4->v5
> - change the implementation of merge_wq_list
> 
> v5->v6
> - change the logic of handling prior task list to:
>    1) grabbed uring_lock: leverage the inline completion infra
>    2) otherwise: batch __req_complete_post() calls to save
>       completion_lock operations.
> 
> v6->v7
> - add Pavel's fix of wrong spin unlock
> - remove a patch and rebase work
> 
> v7->v8
> - the previous fix in v7 is incompleted, fix it.(Pavel's comment)
> - code clean(Jens' comment)
> 
>   fs/io_uring.c | 71 +++++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 60 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 21738ed7521e..92dc33519466 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2225,7 +2225,49 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx, bool *locked)
>   	percpu_ref_put(&ctx->refs);
>   }
>   
> -static void handle_tw_list(struct io_wq_work_node *node, struct io_ring_ctx **ctx, bool *locked)
> +static inline void ctx_commit_and_unlock(struct io_ring_ctx *ctx)
> +{
> +	io_commit_cqring(ctx);
> +	spin_unlock(&ctx->completion_lock);
> +	io_cqring_ev_posted(ctx);
> +}
> +
> +static void handle_prev_tw_list(struct io_wq_work_node *node,
> +				struct io_ring_ctx **ctx, bool *uring_locked)
> +{
> +	if (*ctx && !*uring_locked)
> +		spin_lock(&(*ctx)->completion_lock);
> +
> +	do {
> +		struct io_wq_work_node *next = node->next;
> +		struct io_kiocb *req = container_of(node, struct io_kiocb,
> +						    io_task_work.node);
> +
> +		if (req->ctx != *ctx) {
> +			if (unlikely(!*uring_locked && *ctx))
> +				ctx_commit_and_unlock(*ctx);
> +
> +			ctx_flush_and_put(*ctx, uring_locked);
> +			*ctx = req->ctx;
> +			/* if not contended, grab and improve batching */
> +			*uring_locked = mutex_trylock(&(*ctx)->uring_lock);
> +			percpu_ref_get(&(*ctx)->refs);
> +			if (unlikely(!*uring_locked))
> +				spin_lock(&(*ctx)->completion_lock);
> +		}
> +		if (likely(*uring_locked))
> +			req->io_task_work.func(req, uring_locked);
> +		else
> +			__io_req_complete_post(req, req->result, io_put_kbuf(req));
> +		node = next;
> +	} while (node);
> +
> +	if (unlikely(!*uring_locked))
> +		ctx_commit_and_unlock(*ctx);
> +}
> +
> +static void handle_tw_list(struct io_wq_work_node *node,
> +			   struct io_ring_ctx **ctx, bool *locked)
>   {
>   	do {
>   		struct io_wq_work_node *next = node->next;
> @@ -2246,31 +2288,38 @@ static void handle_tw_list(struct io_wq_work_node *node, struct io_ring_ctx **ct
>   
>   static void tctx_task_work(struct callback_head *cb)
>   {
> -	bool locked = false;
> +	bool uring_locked = false;
>   	struct io_ring_ctx *ctx = NULL;
>   	struct io_uring_task *tctx = container_of(cb, struct io_uring_task,
>   						  task_work);
>   
>   	while (1) {
> -		struct io_wq_work_node *node;
> +		struct io_wq_work_node *node1, *node2;
>   
> -		if (!tctx->prior_task_list.first &&
> -		    !tctx->task_list.first && locked)
> +		if (!tctx->task_list.first &&
> +		    !tctx->prior_task_list.first && uring_locked)
>   			io_submit_flush_completions(ctx);
>   
>   		spin_lock_irq(&tctx->task_lock);
> -		node= wq_list_merge(&tctx->prior_task_list, &tctx->task_list);
> -		if (!node)
> +		node1 = tctx->prior_task_list.first;
> +		node2 = tctx->task_list.first;
> +		INIT_WQ_LIST(&tctx->task_list);
> +		INIT_WQ_LIST(&tctx->prior_task_list);
> +		if (!node2 && !node1)
>   			tctx->task_running = false;
>   		spin_unlock_irq(&tctx->task_lock);
> -		if (!node)
> +		if (!node2 && !node1)
>   			break;
>   
> -		handle_tw_list(node, &ctx, &locked);
> +		if (node1)
> +			handle_prev_tw_list(node1, &ctx, &uring_locked);
> +
> +		if (node2)
> +			handle_tw_list(node2, &ctx, &uring_locked);
>   		cond_resched();
>   	}
>   
> -	ctx_flush_and_put(ctx, &locked);
> +	ctx_flush_and_put(ctx, &uring_locked);
>   }
>   
>   static void io_req_task_work_add(struct io_kiocb *req, bool priority)
> @@ -2759,7 +2808,7 @@ static void io_complete_rw(struct kiocb *kiocb, long res)
>   		return;
>   	req->result = res;
>   	req->io_task_work.func = io_req_task_complete;
> -	io_req_task_work_add(req, true);
> +	io_req_task_work_add(req, !!(req->ctx->flags & IORING_SETUP_SQPOLL));
>   }
>   
>   static void io_complete_rw_iopoll(struct kiocb *kiocb, long res)
> 

-- 
Pavel Begunkov