Re: [RFC 3/3] io_uring: try to batch poll request completion

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

hi,


> On 9/22/21 1:34 PM, Xiaoguang Wang wrote:
>> For an echo-server based on io_uring's IORING_POLL_ADD_MULTI feature,
>> only poll request are completed in task work, normal read/write
>> requests are issued when user app sees cqes on corresponding poll
>> requests, and they will mostly read/write data successfully, which
>> don't need task work. So at least for echo-server model, batching
>> poll request completion properly will give benefits.
>>
>> Currently don't find any appropriate place to store batched poll
>> requests, put them in struct io_submit_state temporarily, which I
>> think it'll need rework in future.
>>
>> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
>> ---
>>   fs/io_uring.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 63 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 6fdfb688cf91..14118388bfc6 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -321,6 +321,11 @@ struct io_submit_state {
>>   	 */
>>   	struct io_kiocb		*compl_reqs[IO_COMPL_BATCH];
>>   	unsigned int		compl_nr;
>> +
>> +	struct io_kiocb		*poll_compl_reqs[IO_COMPL_BATCH];
>> +	bool			poll_req_status[IO_COMPL_BATCH];
>> +	unsigned int		poll_compl_nr;
>> +
>>   	/* inline/task_work completion list, under ->uring_lock */
>>   	struct list_head	free_list;
>>   
>> @@ -2093,6 +2098,8 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx, bool *locked)
>>   	percpu_ref_put(&ctx->refs);
>>   }
>>   
>> +static void io_poll_flush_completions(struct io_ring_ctx *ctx, bool *locked);
>> +
>>   static void tctx_task_work(struct callback_head *cb)
>>   {
>>   	bool locked = false;
>> @@ -2103,8 +2110,11 @@ static void tctx_task_work(struct callback_head *cb)
>>   	while (1) {
>>   		struct io_wq_work_node *node;
>>   
>> -		if (!tctx->task_list.first && locked && ctx->submit_state.compl_nr)
>> +		if (!tctx->task_list.first && locked && (ctx->submit_state.compl_nr ||
>> +		    ctx->submit_state.poll_compl_nr)) {
> io_submit_flush_completions() shouldn't be called if there are no requests... And the
> check is already inside for-next, will be
>
> if (... && locked) {
> 	io_submit_flush_completions();
> 	if (poll_compl_nr)
> 		io_poll_flush_completions();

OK, thanks for pointing this, and I have dropped the poll request 
completion batching patch, since

it shows performance fluctuations, hard to say whether it's useful.


Regards,

Xiaoguang Wang


> }
>
>>   			io_submit_flush_completions(ctx);
>> +			io_poll_flush_completions(ctx, &locked);
>> +		}
>>   
>>   		spin_lock_irq(&tctx->task_lock);
>>   		node = tctx->task_list.first;
>> @@ -5134,6 +5144,49 @@ static inline bool io_poll_complete(struct io_kiocb *req, __poll_t mask)
>>   static bool __io_poll_remove_one(struct io_kiocb *req,
>>   				 struct io_poll_iocb *poll, bool do_cancel);
>>   
>> +static void io_poll_flush_completions(struct io_ring_ctx *ctx, bool *locked)
>> +	__must_hold(&ctx->uring_lock)
>> +{
>> +	struct io_submit_state *state = &ctx->submit_state;
>> +	struct io_kiocb *req, *nxt;
>> +	int i, nr = state->poll_compl_nr;
>> +	bool done, skip_done = true;
>> +
>> +	spin_lock(&ctx->completion_lock);
>> +	for (i = 0; i < nr; i++) {
>> +		req = state->poll_compl_reqs[i];
>> +		done = __io_poll_complete(req, req->result);
> I believe we first need to fix all the poll problems and lay out something more intuitive
> than the current implementation, or it'd be pure hell to do afterwards.
>
> Can be a nice addition, curious about numbers as well.
>
>