On Fri, Jun 22, 2018 at 04:51:26PM -0600, Jens Axboe wrote:

On 6/22/18 4:43 PM, Ming Lei wrote:

quoted

On Fri, Jun 22, 2018 at 01:26:10PM -0600, Jens Axboe wrote:

quoted

blk-wbt adds waiters to the tail of the waitqueue, and factors in the
task placement in its decision making on whether or not the current task
can proceed. This can cause issues for the lowest class of writers,
since they can get woken up, denied access, and then put back to sleep
at the end of the waitqueue.

Fix this so that we only utilize the tail add for the initial sleep, and
we don't factor in the wait queue placement after we've slept (and are
now using the head addition).

Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 4f89b28fa652..7beeabd05f4a 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c

@@ -550,7 +550,7 @@ static inline bool may_queue(struct rq_wb *rwb, struct rq_wait *rqw,
 	 * If the waitqueue is already active and we are not the next
 	 * in line to be woken up, wait for our turn.
 	 */
-	if (waitqueue_active(&rqw->wait) &&
+	if (wait && waitqueue_active(&rqw->wait) &&
 	    rqw->wait.head.next != &wait->entry)
 		return false;
 

@@ -567,16 +567,27 @@ static void __wbt_wait(struct rq_wb *rwb, enum wbt_flags wb_acct,
 	__acquires(lock)
 {
 	struct rq_wait *rqw = get_rq_wait(rwb, wb_acct);
+	struct wait_queue_entry *waitptr = NULL;
 	DEFINE_WAIT(wait);
 
-	if (may_queue(rwb, rqw, &wait, rw))
+	if (may_queue(rwb, rqw, waitptr, rw))
 		return;
 
+	waitptr = &wait;
 	do {
-		prepare_to_wait_exclusive(&rqw->wait, &wait,
+		/*
+		 * Don't add ourselves to the wq tail if we've already
+		 * slept. Otherwise we can penalize background writes
+		 * indefinitely.
+		 */

I saw this indefinite wbt_wait() in systemd-journal when running
aio-stress read test, but just once, not figured out one approach
to reproduce it yet, just wondering if you have quick test case for
reproducing and verifying this issue.

I've seen it in production, but I'm currently relying on someone else
to reproduce it synthetically. I'm just providing the patches for
testing.

quoted

quoted

+		if (waitptr)
+			prepare_to_wait_exclusive(&rqw->wait, &wait,
+							TASK_UNINTERRUPTIBLE);
+		else
+			prepare_to_wait(&rqw->wait, &wait,
 						TASK_UNINTERRUPTIBLE);

Could you explain a bit why the 'wait_entry' order matters wrt. this
issue? Since other 'wait_entry' still may come at the head meantime to
the same wq before checking in may_queue().

Let's say we have 10 tasks queued up. Each one gets added to the tail,
so when it's our turn, we've now reached the head. We fail to get a
queue token, so we go back to sleep. At that point we should add
back to the head, not the tail, for fairness purposes.

OK, it is reasonable to do it for fairness purpose, but seems we still
don't know how the wait forever in wbt_wait() is fixed by this way.

I guess the reason is in the check of 'rqw->wait.head.next != &wait->entry',
which is basically removed when the waiter is waken up.


Thanks,
Ming