[PATCH 3/8] io_uring: add a limited tw list for irq completion work

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

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 to a new tw list which has a
higher priority, 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

One thing to watch out is sometimes irq completion TWs comes
overwhelmingly, which makes the new tw list grows fast, and TWs in
the old list are starved. So we have to limit the length of the new
tw list. A practical value is 1/3:
    len of new tw list < 1/3 * (len of new + old tw list)

In this way, the new tw list has a limited length and normal task get
there chance to run.

Tested this patch(and the following ones) by manually replace
__io_queue_sqe() to io_req_task_complete() to construct 'heavy' task
works. Then test with fio:

ioengine=io_uring
thread=1
bs=4k
direct=1
rw=randread
time_based=1
runtime=600
randrepeat=0
group_reporting=1
filename=/dev/nvme0n1

Tried various iodepth.
The peak IOPS for this patch is 314K, while the old one is 249K.
For avg latency, difference shows when iodepth grow:
depth and avg latency(usec):
	depth      new          old
	 1        22.80        23.77
	 2        23.48        24.54
	 4        24.26        25.57
	 8        29.21        32.89
	 16       53.61        63.50
	 32       106.29       131.34
	 64       217.21       256.33
	 128      421.59       513.87
	 256      815.15       1050.99

95%, 99% etc more data in cover letter.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
---
 fs/io_uring.c | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 8317c360f7a4..9272b2cfcfb7 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -461,6 +461,7 @@ struct io_ring_ctx {
 	};
 };
 
+#define MAX_EMERGENCY_TW_RATIO	3
 struct io_uring_task {
 	/* submission side */
 	int			cached_refs;
@@ -475,6 +476,9 @@ struct io_uring_task {
 	spinlock_t		task_lock;
 	struct io_wq_work_list	task_list;
 	struct callback_head	task_work;
+	struct io_wq_work_list	prior_task_list;
+	unsigned int		nr;
+	unsigned int		prior_nr;
 	bool			task_running;
 };
 
@@ -2132,12 +2136,16 @@ static void tctx_task_work(struct callback_head *cb)
 	while (1) {
 		struct io_wq_work_node *node;
 
-		if (!tctx->task_list.first && locked)
+		if (!tctx->prior_task_list.first &&
+		    !tctx->task_list.first && locked)
 			io_submit_flush_completions(ctx);
 
 		spin_lock_irq(&tctx->task_lock);
-		node = tctx->task_list.first;
+		wq_list_merge(&tctx->prior_task_list, &tctx->task_list);
+		node = tctx->prior_task_list.first;
 		INIT_WQ_LIST(&tctx->task_list);
+		INIT_WQ_LIST(&tctx->prior_task_list);
+		tctx->nr = tctx->prior_nr = 0;
 		if (!node)
 			tctx->task_running = false;
 		spin_unlock_irq(&tctx->task_lock);
@@ -2166,7 +2174,7 @@ static void tctx_task_work(struct callback_head *cb)
 	ctx_flush_and_put(ctx, &locked);
 }
 
-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;
@@ -2178,7 +2186,13 @@ 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 && tctx->prior_nr * MAX_EMERGENCY_TW_RATIO < tctx->nr) {
+		wq_list_add_tail(&req->io_task_work.node, &tctx->prior_task_list);
+		tctx->prior_nr++;
+	} else {
+		wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
+	}
+	tctx->nr++;
 	running = tctx->task_running;
 	if (!running)
 		tctx->task_running = true;
@@ -2202,9 +2216,12 @@ static void io_req_task_work_add(struct io_kiocb *req)
 	}
 
 	spin_lock_irqsave(&tctx->task_lock, flags);
+	tctx->nr = tctx->prior_nr = 0;
 	tctx->task_running = false;
-	node = tctx->task_list.first;
+	wq_list_merge(&tctx->prior_task_list, &tctx->task_list);
+	node = tctx->prior_task_list.first;
 	INIT_WQ_LIST(&tctx->task_list);
+	INIT_WQ_LIST(&tctx->prior_task_list);
 	spin_unlock_irqrestore(&tctx->task_lock, flags);
 
 	while (node) {
@@ -2241,19 +2258,19 @@ 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)
@@ -2373,7 +2390,7 @@ 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_work;
-		io_req_task_work_add(req);
+		io_req_task_work_add(req, false);
 	}
 }
 
@@ -2693,7 +2710,7 @@ 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)
@@ -5256,7 +5273,7 @@ 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;
 }
 
@@ -5934,7 +5951,7 @@ static enum hrtimer_restart io_timeout_fn(struct hrtimer *timer)
 	spin_unlock_irqrestore(&ctx->timeout_lock, flags);
 
 	req->io_task_work.func = io_req_task_timeout;
-	io_req_task_work_add(req);
+	io_req_task_work_add(req, false);
 	return HRTIMER_NORESTART;
 }
 
@@ -6916,7 +6933,7 @@ 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;
 }
 
@@ -8543,6 +8560,7 @@ static int io_uring_alloc_task_context(struct task_struct *task,
 	task->io_uring = tctx;
 	spin_lock_init(&tctx->task_lock);
 	INIT_WQ_LIST(&tctx->task_list);
+	INIT_WQ_LIST(&tctx->prior_task_list);
 	init_task_work(&tctx->task_work, tctx_task_work);
 	return 0;
 }
-- 
2.24.4