block, bfq: fix unbalanced decrements of burst size

The commit "block, bfq: decrease burst size when queues in burst
exit" introduced the decrement of burst_size on the removal of a
bfq_queue from the burst list. Unfortunately, this decrement can
happen to be performed even when burst size is already equal to 0,
because of unbalanced decrements. A description follows of the cause
of these unbalanced decrements, namely a wrong assumption, and of the
way how this wrong assumption leads to unbalanced decrements.

The wrong assumption is that a bfq_queue can exit only if the process
associated with the bfq_queue has exited. This is false, because a
bfq_queue, say Q, may exit also as a consequence of a merge with
another bfq_queue. In this case, Q exits because the I/O of its
associated process has been redirected to another bfq_queue.

The decrement unbalance occurs because Q may then be re-created after
a split, and added back to the current burst list, *without*
incrementing burst_size. burst_size is not incremented because Q is
not a new bfq_queue added to the burst list, but a bfq_queue only
temporarily removed from the list, and, before the commit "bfq-sq,
bfq-mq: decrease burst size when queues in burst exit", burst_size was
not decremented when Q was removed.

This commit addresses this issue by just checking whether the exiting
bfq_queue is a merged bfq_queue, and, in that case, not decrementing
burst_size. Unfortunately, this still leaves room for unbalanced
decrements, in the following rarer case: on a split, the bfq_queue
happens to be inserted into a different burst list than that it was
removed from when merged. If this happens, the number of elements in
the new burst list becomes higher than burst_size (by one). When the
bfq_queue then exits, it is of course not in a merged state any
longer, thus burst_size is decremented, which results in an unbalanced
decrement.  To handle this sporadic, unlucky case in a simple way,
this commit also checks that burst_size is larger than 0 before
decrementing it.

Finally, this commit removes an useless, extra check: the check that
the bfq_queue is sync, performed before checking whether the bfq_queue
is in the burst list. This extra check is redundant, because only sync
bfq_queues can be inserted into the burst list.

Fixes: 7cb04004fa37 ("block, bfq: decrease burst size when queues in burst exit")
Reported-by: Philip Müller <philm@manjaro.org>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Tested-by: Philip Müller <philm@manjaro.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 57 insertions, 2 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 261f98695910..889a8549d97f 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3726,9 +3726,36 @@ void bfq_put_queue(struct bfq_queue *bfqq)
         if (bfqq->ref)
                 return;
 
-        if (bfq_bfqq_sync(bfqq) && !hlist_unhashed(&bfqq->burst_list_node)) {
+        if (!hlist_unhashed(&bfqq->burst_list_node)) {
                 hlist_del_init(&bfqq->burst_list_node);
-                bfqq->bfqd->burst_size--;
+                /*
+                 * Decrement also burst size after the removal, if the
+                 * process associated with bfqq is exiting, and thus
+                 * does not contribute to the burst any longer. This
+                 * decrement helps filter out false positives of large
+                 * bursts, when some short-lived process (often due to
+                 * the execution of commands by some service) happens
+                 * to start and exit while a complex application is
+                 * starting, and thus spawning several processes that
+                 * do I/O (and that *must not* be treated as a large
+                 * burst, see comments on bfq_handle_burst).
+                 *
+                 * In particular, the decrement is performed only if:
+                 * 1) bfqq is not a merged queue, because, if it is,
+                 * then this free of bfqq is not triggered by the exit
+                 * of the process bfqq is associated with, but exactly
+                 * by the fact that bfqq has just been merged.
+                 * 2) burst_size is greater than 0, to handle
+                 * unbalanced decrements. Unbalanced decrements may
+                 * happen in te following case: bfqq is inserted into
+                 * the current burst list--without incrementing
+                 * bust_size--because of a split, but the current
+                 * burst list is not the burst list bfqq belonged to
+                 * (see comments on the case of a split in
+                 * bfq_set_request).
+                 */
+                if (bfqq->bic && bfqq->bfqd->burst_size > 0)
+                        bfqq->bfqd->burst_size--;
         }
 
         kmem_cache_free(bfq_pool, bfqq);
@@ -4460,6 +4487,34 @@ static struct bfq_queue *bfq_get_bfqq_handle_split(struct bfq_data *bfqd,
                 else {
                         bfq_clear_bfqq_in_large_burst(bfqq);
                         if (bic->was_in_burst_list)
+                                /*
+                                 * If bfqq was in the current
+                                 * burst list before being
+                                 * merged, then we have to add
+                                 * it back. And we do not need
+                                 * to increase burst_size, as
+                                 * we did not decrement
+                                 * burst_size when we removed
+                                 * bfqq from the burst list as
+                                 * a consequence of a merge
+                                 * (see comments in
+                                 * bfq_put_queue). In this
+                                 * respect, it would be rather
+                                 * costly to know whether the
+                                 * current burst list is still
+                                 * the same burst list from
+                                 * which bfqq was removed on
+                                 * the merge. To avoid this
+                                 * cost, if bfqq was in a
+                                 * burst list, then we add
+                                 * bfqq to the current burst
+                                 * list without any further
+                                 * check. This can cause
+                                 * inappropriate insertions,
+                                 * but rarely enough to not
+                                 * harm the detection of large
+                                 * bursts significantly.
+                                 */
                                 hlist_add_head(&bfqq->burst_list_node,
                                                &bfqd->burst_list);
                 }