Re: [PATCH] io_uring: don't take percpu_ref operations for registered files in IOPOLL mode

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> In io_file_get() and io_put_file(), currently we use percpu_ref_get() and
> percpu_ref_put() for registered files, but it's hard to say they're very
> light-weight synchronization primitives, especially in arm platform. In one
> our arm machine, I get below perf data(registered files enabled):
> Samples: 98K of event 'cycles:ppp', Event count (approx.): 63789396810
> Overhead  Command      Shared Object     Symbol
>     ...
>     0.78%  io_uring-sq  [kernel.vmlinux]  [k] io_file_get
> There is an obvious overhead that can not be ignored.
> 
> Currently I don't find any good and generic solution for this issue, but
> in IOPOLL mode, given that we can always ensure get/put registered files
> under uring_lock, we can use a simple and plain u64 counter to synchronize
> with registered files update operations in __io_sqe_files_update().
> 
> With this patch, perf data show shows:
> Samples: 104K of event 'cycles:ppp', Event count (approx.): 67478249890
> Overhead  Command      Shared Object     Symbol
>     ...
>     0.27%  io_uring-sq  [kernel.vmlinux]  [k] io_file_get
The above %0.78 => %0.27 improvements are observed in arm machine with 4.19 kernel.
In upstream mainline codes, since this patch "2b0d3d3e4fcf percpu_ref: reduce memory
footprint of percpu_ref in fast path", I believe the io_file_get's overhead would
be further smaller. I have same tests in same machine, in upstream codes with my patch,
now the io_file_get's overhead is %0.44.

This patch's idea is simple, and now seems it only gives minor performance improvement,
do you have any comments about this patch, should I continue re-send it?

Regards,
Xiaoguang Wang

> 
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>   fs/io_uring.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-------
>   1 file changed, 50 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ce69bd9b0838..186072861af9 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -195,6 +195,11 @@ struct fixed_file_table {
>   
>   struct fixed_file_ref_node {
>   	struct percpu_ref		refs;
> +	/*
> +	 * Track the number of reqs that reference this node, currently it's
> +	 * only used in IOPOLL mode.
> +	 */
> +	u64				count;
>   	struct list_head		node;
>   	struct list_head		file_list;
>   	struct fixed_file_data		*file_data;
> @@ -651,7 +656,10 @@ struct io_kiocb {
>   	 */
>   	struct list_head		inflight_entry;
>   
> -	struct percpu_ref		*fixed_file_refs;
> +	union {
> +		struct percpu_ref		*fixed_file_refs;
> +		struct fixed_file_ref_node	*fixed_file_ref_node;
> +	};
>   	struct callback_head		task_work;
>   	/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
>   	struct hlist_node		hash_node;
> @@ -1544,9 +1552,20 @@ static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx,
>   static inline void io_put_file(struct io_kiocb *req, struct file *file,
>   			  bool fixed)
>   {
> -	if (fixed)
> -		percpu_ref_put(req->fixed_file_refs);
> -	else
> +	struct io_ring_ctx *ctx = req->ctx;
> +
> +	if (fixed) {
> +		/* See same comments in io_sqe_files_unregister(). */
> +		if (ctx->flags & IORING_SETUP_IOPOLL) {
> +			struct fixed_file_ref_node *ref_node = req->fixed_file_ref_node;
> +			struct percpu_ref *refs = &ref_node->refs;
> +
> +			ref_node->count--;
> +			if ((ctx->file_data->cur_refs != refs) && !ref_node->count)
> +				percpu_ref_kill(refs);
> +		} else
> +			percpu_ref_put(req->fixed_file_refs);
> +	} else
>   		fput(file);
>   }
>   
> @@ -5967,8 +5986,21 @@ static int io_file_get(struct io_submit_state *state, struct io_kiocb *req,
>   		fd = array_index_nospec(fd, ctx->nr_user_files);
>   		file = io_file_from_index(ctx, fd);
>   		if (file) {
> -			req->fixed_file_refs = ctx->file_data->cur_refs;
> -			percpu_ref_get(req->fixed_file_refs);
> +			struct percpu_ref *refs = ctx->file_data->cur_refs;
> +
> +			/*
> +			 * In IOPOLL mode, we can always ensure get/put registered files under
> +			 * uring_lock, so we can use a simple and plain u64 counter to synchronize
> +			 * with registered files update operations in __io_sqe_files_update.
> +			 */
> +			if (ctx->flags & IORING_SETUP_IOPOLL) {
> +				req->fixed_file_ref_node = container_of(refs,
> +						struct fixed_file_ref_node, refs);
> +				req->fixed_file_ref_node->count++;
> +			} else {
> +				req->fixed_file_refs = refs;
> +				percpu_ref_get(refs);
> +			}
>   		}
>   	} else {
>   		trace_io_uring_file_get(ctx, fd);
> @@ -6781,7 +6813,12 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
>   		ref_node = list_first_entry(&data->ref_list,
>   				struct fixed_file_ref_node, node);
>   	spin_unlock(&data->lock);
> -	if (ref_node)
> +	/*
> +	 * If count is not zero, that means we're in IOPOLL mode, and there are
> +	 * still reqs that reference this ref_node, let the final req do the
> +	 * percpu_ref_kill job.
> +	 */
> +	if (ref_node && !ref_node->count)
>   		percpu_ref_kill(&ref_node->refs);
>   
>   	percpu_ref_kill(&data->refs);
> @@ -7363,7 +7400,12 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
>   	}
>   
>   	if (needs_switch) {
> -		percpu_ref_kill(data->cur_refs);
> +		struct fixed_file_ref_node *old_ref_node = container_of(data->cur_refs,
> +				struct fixed_file_ref_node, refs);
> +
> +		/* See same comments in io_sqe_files_unregister(). */
> +		if (!old_ref_node->count)
> +			percpu_ref_kill(data->cur_refs);
>   		spin_lock(&data->lock);
>   		list_add(&ref_node->node, &data->ref_list);
>   		data->cur_refs = &ref_node->refs;
>