Re: [PATCH 2/2] io_uring: add irq completion work to the head of task_list

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/8/24 下午8:57, Pavel Begunkov 写道:
> On 8/23/21 7:36 PM, Hao Xu wrote:
>> Now we have a lot of task_work users, some are just to complete a req
>> and generate a cqe. Let's put the work at the head position of the
>> task_list, so that it can be handled quickly and thus to reduce
>> avg req latency. an explanatory case:
>>
>> origin timeline:
>>      submit_sqe-->irq-->add completion task_work
>>      -->run heavy work0~n-->run completion task_work
>> now timeline:
>>      submit_sqe-->irq-->add completion task_work
>>      -->run completion task_work-->run heavy work0~n
> 
> Might be good. There are not so many hot tw users:
> poll, queuing linked requests, and the new IRQ. Could be
> BPF in the future.
async buffered reads as well, regarding buffered reads is
hot operation.
> 
> So, for the test case I'd think about some heavy-ish
> submissions linked to your IRQ req. For instance,
> keeping a large QD of
> 
> read(IRQ-based) -> linked read_pipe(PAGE_SIZE);
> 
> and running it for a while, so they get completely
> out of sync and tw works really mix up. It reads
> from pipes size<=PAGE_SIZE, so it completes inline,
> but the copy takes enough of time.
Thanks Pavel, previously I tried
direct read-->buffered read(async buffered read)
didn't see much difference. I'll try the above case
you offered.
> 
> One thing is that Jens specifically wanted tw's to
> be in FIFO order, where IRQ based will be in LIFO.
> I don't think it's a real problem though, the
> completion handler should be brief enough.In my latest code, the IRQ based tw are also FIFO,
only LIFO between IRQ based tw and other tw:
timeline: tw1 tw2 irq1 irq2
task_list: irq1 irq2 tw1 tw2
> 
>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>> ---
>>   fs/io-wq.h    |  9 +++++++++
>>   fs/io_uring.c | 21 ++++++++++++---------
>>   2 files changed, 21 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/io-wq.h b/fs/io-wq.h
>> index 308af3928424..51b4408fd177 100644
>> --- a/fs/io-wq.h
>> +++ b/fs/io-wq.h
>> @@ -41,6 +41,15 @@ static inline void wq_list_add_after(struct io_wq_work_node *node,
>>   		list->last = node;
>>   }
>>   
>> +static inline void wq_list_add_head(struct io_wq_work_node *node,
>> +				    struct io_wq_work_list *list)
>> +{
>> +	node->next = list->first;
>> +	list->first = node;
>> +	if (!node->next)
>> +		list->last = node;
>> +}
>> +
>>   static inline void wq_list_add_tail(struct io_wq_work_node *node,
>>   				    struct io_wq_work_list *list)
>>   {
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 8172f5f893ad..954cd8583945 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -2050,7 +2050,7 @@ static void tctx_task_work(struct callback_head *cb)
>>   	ctx_flush_and_put(ctx);
>>   }
>>   
>> -static void io_req_task_work_add(struct io_kiocb *req)
>> +static void io_req_task_work_add(struct io_kiocb *req, bool emergency)
>>   {
>>   	struct task_struct *tsk = req->task;
>>   	struct io_uring_task *tctx = tsk->io_uring;
>> @@ -2062,7 +2062,10 @@ static void io_req_task_work_add(struct io_kiocb *req)
>>   	WARN_ON_ONCE(!tctx);
>>   
>>   	spin_lock_irqsave(&tctx->task_lock, flags);
>> -	wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
>> +	if (emergency)
>> +		wq_list_add_head(&req->io_task_work.node, &tctx->task_list);
>> +	else
>> +		wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
>>   	running = tctx->task_running;
>>   	if (!running)
>>   		tctx->task_running = true;
>> @@ -2122,19 +2125,19 @@ static void io_req_task_queue_fail(struct io_kiocb *req, int ret)
>>   {
>>   	req->result = ret;
>>   	req->io_task_work.func = io_req_task_cancel;
>> -	io_req_task_work_add(req);
>> +	io_req_task_work_add(req, true);
>>   }
>>   
>>   static void io_req_task_queue(struct io_kiocb *req)
>>   {
>>   	req->io_task_work.func = io_req_task_submit;
>> -	io_req_task_work_add(req);
>> +	io_req_task_work_add(req, false);
>>   }
>>   
>>   static void io_req_task_queue_reissue(struct io_kiocb *req)
>>   {
>>   	req->io_task_work.func = io_queue_async_work;
>> -	io_req_task_work_add(req);
>> +	io_req_task_work_add(req, false);
>>   }
>>   
>>   static inline void io_queue_next(struct io_kiocb *req)
>> @@ -2249,7 +2252,7 @@ static inline void io_put_req_deferred(struct io_kiocb *req)
>>   {
>>   	if (req_ref_put_and_test(req)) {
>>   		req->io_task_work.func = io_free_req;
>> -		io_req_task_work_add(req);
>> +		io_req_task_work_add(req, false);
>>   	}
>>   }
>>   
>> @@ -2564,7 +2567,7 @@ static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
>>   		return;
>>   	req->result = res;
>>   	req->io_task_work.func = io_req_task_complete;
>> -	io_req_task_work_add(req);
>> +	io_req_task_work_add(req, true);
>>   }
>>   
>>   static void io_complete_rw_iopoll(struct kiocb *kiocb, long res, long res2)
>> @@ -4881,7 +4884,7 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
>>   	 * of executing it. We can't safely execute it anyway, as we may not
>>   	 * have the needed state needed for it anyway.
>>   	 */
>> -	io_req_task_work_add(req);
>> +	io_req_task_work_add(req, false);
>>   	return 1;
>>   }
>>   
>> @@ -6430,7 +6433,7 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
>>   	spin_unlock_irqrestore(&ctx->timeout_lock, flags);
>>   
>>   	req->io_task_work.func = io_req_task_link_timeout;
>> -	io_req_task_work_add(req);
>> +	io_req_task_work_add(req, false);
>>   	return HRTIMER_NORESTART;
>>   }
>>   
>>
>