Skip to content

Commit

Permalink
sbitmap: only queue kyber's wait callback if not already active
Browse files Browse the repository at this point in the history
Under heavy loads where the kyber I/O scheduler hits the token limits for
its scheduling domains, kyber can become stuck.  When active requests
complete, kyber may not be woken up leaving the I/O requests in kyber
stuck.

This stuck state is due to a race condition with kyber and the sbitmap
functions it uses to run a callback when enough requests have completed.
The running of a sbt_wait callback can race with the attempt to insert the
sbt_wait.  Since sbitmap_del_wait_queue removes the sbt_wait from the list
first then sets the sbq field to NULL, kyber can see the item as not on a
list but the call to sbitmap_add_wait_queue will see sbq as non-NULL. This
results in the sbt_wait being inserted onto the wait list but ws_active
doesn't get incremented.  So the sbitmap queue does not know there is a
waiter on a wait list.

Since sbitmap doesn't think there is a waiter, kyber may never be
informed that there are domain tokens available and the I/O never advances.
With the sbt_wait on a wait list, kyber believes it has an active waiter
so cannot insert a new waiter when reaching the domain's full state.

This race can be fixed by only adding the sbt_wait to the queue if the
sbq field is NULL.  If sbq is not NULL, there is already an action active
which will trigger the re-running of kyber.  Let it run and add the
sbt_wait to the wait list if still needing to wait.

Reviewed-by: Omar Sandoval <[email protected]>
Signed-off-by: David Jeffery <[email protected]>
Reported-by: John Pittman <[email protected]>
Tested-by: John Pittman <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
  • Loading branch information
David Jeffery authored and axboe committed Dec 20, 2019
1 parent 3b7995a commit df034c9
Showing 1 changed file with 1 addition and 1 deletion.
2 changes: 1 addition & 1 deletion lib/sbitmap.c
Original file line number Diff line number Diff line change
Expand Up @@ -650,8 +650,8 @@ void sbitmap_add_wait_queue(struct sbitmap_queue *sbq,
if (!sbq_wait->sbq) {
sbq_wait->sbq = sbq;
atomic_inc(&sbq->ws_active);
add_wait_queue(&ws->wait, &sbq_wait->wait);
}
add_wait_queue(&ws->wait, &sbq_wait->wait);
}
EXPORT_SYMBOL_GPL(sbitmap_add_wait_queue);

Expand Down

0 comments on commit df034c9

Please sign in to comment.