Messages in this thread | | | Subject | Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check | From | Holger Hoffstätte <> | Date | Thu, 20 May 2021 18:39:18 +0200 |
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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@hotmail.it> >> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> >> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@gmail.com> >> --- >> 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
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