Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check

From: Holger Hoffstätte
Date: Thu May 20 2021 - 12:39:23 EST

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

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 =

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):

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.