From 99fead8d38e5302b1be9403d4de815ce9174a3df Mon Sep 17 00:00:00 2001 From: Paolo Valente Date: Mon, 9 Oct 2017 13:11:23 +0200 Subject: block, bfq: fix unbalanced decrements of burst size MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Paolo Valente Signed-off-by: Angelo Ruocco Tested-by: Philip Müller Tested-by: Oleksandr Natalenko Tested-by: Lee Tibbert Signed-off-by: Jens Axboe --- block/bfq-iosched.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 57 insertions(+), 2 deletions(-) (limited to 'block/bfq-iosched.c') diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 261f98695910f..889a8549d97f3 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); } -- cgit v1.2.3