On 2021-05-20 09:15, Holger Hoffstätte wrote:
On 2021-05-18 12:43, Luca Mariotti wrote:When attempting to schedule a merge of a given bfq_queue with the currently in-service bfq_queue or with a cooperating bfq_queue among the scheduled bfq_queues, delayed stable merge is checked for rotational or non-queueing devs. For this stable merge to be performed, some conditions must be met. If the current bfq_queue underwent some split from some merged bfq_queue, one of these conditions is that two hundred milliseconds must elapse from split, otherwise this condition is always met. Unfortunately, by mistake, time_is_after_jiffies() was written instead of time_is_before_jiffies() for this check, verifying that less than two hundred milliseconds have elapsed instead of verifying that at least two hundred milliseconds have elapsed. Fix this issue by replacing time_is_after_jiffies() with time_is_before_jiffies(). Signed-off-by: Luca Mariotti <mariottiluca1@xxxxxxxxxx> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@xxxxxxxxx> --- block/bfq-iosched.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index acd1f881273e..2adb1e69c9d2 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -2697,7 +2697,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq, if (unlikely(!bfqd->nonrot_with_queueing)) { if (bic->stable_merge_bfqq && !bfq_bfqq_just_created(bfqq) && - time_is_after_jiffies(bfqq->split_time + + time_is_before_jiffies(bfqq->split_time + msecs_to_jiffies(200))) { struct bfq_queue *stable_merge_bfqq = bic->stable_merge_bfqq;Not sure why but with this patch I quickly got a division-by-zero in BFQ and complete system halt. Unfortunately I couldn't capture the exact stack trace, but it read something like bfq_calc_weight() or something ike that. I looked through the code and found bfq_delta(), so maybe weight got reduced to 0?
Tried again, another boom. This time I managed to capture a stack trace (scrolled out at the top, but it's the same as before and easily reproducible): https://imgur.com/a/sU1pDaF This is a heavily patched 5.10.x, but it's been perfectly stable so far until I added this last patch; the one before was avoid-circular-stable-merges. Maybe an unintentional side effect? In any case all I see is bfq_delta() inlined into bfq_calc_finish() and exploding since entity->weight is apparently 0. Hope this helps. -h
