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

Ming Lei
2018-06-22T22:43:02Z

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.

> + 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().

Can we remove 'rqw->wait.head.next != &wait->entry' from may_queue()?
I guess that should be one optimization, but seems quite dangerous since
'rqw->wait.head.next' may point to one freed stack variable.

Thanks,
Ming