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Message-Id: <20171009111123.2961-2-paolo.valente@linaro.org>
Date: Mon, 9 Oct 2017 13:11:23 +0200
From: Paolo Valente <paolo.valente@...aro.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
ulf.hansson@...aro.org, broonie@...nel.org, lee.tibbert@...il.com,
oleksandr@...alenko.name, angeloruocco90@...il.com,
philm@...jaro.org, Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX] 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.
Reported-by: Philip Müller <philm@...jaro.org>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
Signed-off-by: Angelo Ruocco <angeloruocco90@...il.com>
Tested-by: Philip Müller <philm@...jaro.org>
Tested-by: Oleksandr Natalenko <oleksandr@...alenko.name>
Tested-by: Lee Tibbert <lee.tibbert@...il.com>
---
block/bfq-iosched.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 57 insertions(+), 2 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0d7272a..8689b24 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3685,9 +3685,36 @@ 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);
@@ -4418,6 +4445,34 @@ 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);
}
--
2.10.0
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