Re: [PATCH] blk-wbt: fix indefinite background writeback sleep

[Jens Axboe <axboe@kernel.dk>]

On 6/22/18 5:11 PM, Ming Lei wrote:
> On Fri, Jun 22, 2018 at 04:51:26PM -0600, Jens Axboe wrote:
>> On 6/22/18 4:43 PM, Ming Lei wrote:
>>> On Fri, Jun 22, 2018 at 01:26:10PM -0600, Jens Axboe wrote:
>>>> 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.
>>
>>>> +		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.

It's not, it's just removing a class of unfairness. You should not go
to the back of the queue if you fail, you should remain near the top.

> 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.

Plus at that point it's useless, since we are the head of queue. The
check only makes sense if we tail add.

-- 
Jens Axboe